HiHighhli h ght s 20220 © 2020. The Company of Biologists Ltd. Contents

3 Falcons’ vision up to speed for fast 28 Zebra finches adapt to cope well with lifestyle extreme conditions 4 Leaping small fish out-power breaching 29 Parrots discard dowdy pigments in whales favour of own brand 5 Antarctic bald notothens use spleen 30 Potassium leak short circuits trout heart scuba tank to keep down blood at high temperatures viscosity 31 Surfing behind rocks costs trout dear 6 Evolution built men to pack a punch when feeding 7 Australian jack jumper ant foragers 32 Pygmy mice whistle for the audience have no need for mental map 33 Cabbage whites have a unique take on 8 Weakly electric fishes’ secret social polarized light lives revealed 34 Chilly rattlesnakes strike slower, but not 9 Mantis shrimp pull punches in air for as slow as expected self-preservation 35 Minute mecysmaucheniid spider triggers 10 Stressed chickadees get hot under the fastest trap-jaws collar to save energy 36 Knuckle-walking chimpanzees go 3-D 11 Cameras do not lie: elephant seals with ‘Avatar’ technology prefer fish 37 Tobacco hornworms change stride when 12 African pygmy mouse upgrades the going gets different mitochondria to compensate for size 38 Flexible sea butterflies embrace to thrust 13 Fat loss triggers ant lifestyle change 39 Puffin hearing unaffected by 14 First two weeks crucial for white-nose amphibious lifestyle syndrome survivors 40 Hefty shells help hermit crabs cling on 15 Fish maintain tissue pH despite CO2 blast in surf 16 Hot limpets can’t hang on as tight as 41 Resilient aquifer stoneflies handle low cold ones oxygen well 17 Forest protects Heliconius butterflies 42 Intrepid lice survive extreme pressure from climate extremes when hitching rides on elephant seals 18 Two percent change switches motor 43 Catfish keep head flat when gulping muscle to brake 44 Lizards pant to keep cool 19 Simulated larvae reveal why fish fry lose their dinner 45 Cuvier’s beaked whales take record- 20 Cyber-frog leg leaps out of reality breaking dives in their stride 21 Feisty squid and fish flash back to 46 Lesser long-nosed bats have finely dazzle predatory elephant seals tuned sweet tooth 22 Swallow mums push metabolic limits 47 Champion annual killifish embryos when they can keep cool survive more than 16 months out of water 23 It’s cold out, but whale sharks stay warm within 48 Serotonin key for trap-jaw ant aggression 24 Testosterone soups up golden-collared 49 Colour is key when female chameleons manakin roll-snap at expense of choose Mr Right endurance 50 How whale-surfing remoras stay in 25 Hot minnows could struggle to navigate touch with their steeds as temperatures rise 51 Goby fins have fingertip sensitivity 26 Dinosaur eels build up their fin bones 52 Fiddler crabs ignore near misses when for life on land threatened from all sides 27 Why bark beetles roam near and far All articles written by Kathryn Knight

1 Are you an early-career researcher interested in: • gaining new research skills • expanding your research network • testing novel hypotheses with a team of collaborators?

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biologists.com/travelling-fellowships Falcons’ vision up to speed for fast lifestyle

Few people are lucky enough to transform their passion from a hobby into a career, but Simon Potier from Lund University has done exactly that. ‘Falconry flows in my veins’, says Potier, who was immersed in the ancient sport from an early age. ‘My father is the main falconer at the Les Ailes de l’Urga falconry park’, says Potier adding that his dad owns more than 60 birds. Working with some of the fastest animals on the planet, it was inevitable that Simon Potier Potier would become fascinated by the birds’ vision. ‘Some of them have the highest spatial resolving power known in the animal kingdom’, he says, allowing the swift hunters to target prey with meticulous precision. However, Potier, Michael Pfaff and Almut Kelber realised that the raptors’ high-definition vision was only part of the equation. Closing in on fleeing victims at speeds in excess of 320 km h−1, raptors must be able to see events that are so fast that we would be oblivious to them. The question that Potier and colleagues wanted to ask was how fast could the birds see? Returning home to Normandy, where the birds of prey are based, with experienced falconer Margaux Lieuvin, Potier set up a room equipped with two LED lamps mounted on the wall. One lamp appeared to be on constantly [flashing 1000 Hz (times per second)], while the flashing rate of the second could be adjusted from 10 Hz until the bird could no longer distinguish the flicker. Positioning a perch in front of each lamp and setting the adjustable lamp to flash at the slowest rate, Potier and Lieuvin trained three Harris’s hawks (Parabuteo unicinctus), two peregrine falcons (Falco peregrinus) and a saker falcon (Falco cherrug) to fly and land on the perch in front of the constantly on lamp, rewarding the birds with a morsel of tasty chicken when they chose correctly. ‘We were quite surprised how fast the falcons learned to fly to one perch … [but] this is the advantage of using birds from falconry; they are trained to fly every day with humans’, says Potier. Then, the duo began gradually increasing the flicker rate of the adjustable lamp until the birds could no longer distinguish between the two lamps and began choosing their resting perch randomly. After months of patiently working with the animals, it was clear that the record-breaking peregrine falcon has the fastest visual response of the three species; they were able to distinguish lights flashing up to 129 Hz, more than twice as fast as humans. In contrast, the saker falcon was able to see flickers up to 102 Hz and the Harris’s hawks could only distinguish lights flashing at 81 Hz. Potier also realised that the birds’ high-speed vision fits with their different hunting strategies. Peregrine falcons need high-speed vision to pursue agile aviators on the wing, while Harris’s hawks, which specialise in dive-bombing slower rodents on the ground, can make do with slightly slower vision. However, he points out that chickens can detect flashing lights as well as Harris’s hawks can, and the vision of pied flycatchers – which respond to lights flashing up to 146 Hz – is even faster than that of peregrine falcons. It seems that birds that could be on the menu might need to see as well as the hunters that prey on them. In addition, Potier warns that lights that appear to be on continuously for us could be distressing for birds of prey that are sensitive to flickering. ‘Our study provides evidence that bright artificial illumination flickering at 100 Hz (common in Europe) or 120 Hz (in the USA) may not be suitable in enclosures for raptors, specifically falcons’, he says. 10.1242/jeb.219493 Potier, S., Lieuvin, M., Pfaff, M. and Kelber, A. (2020). How fast can raptors see? J. Exp. Biol. 223, jeb209031. doi:10.1242/jeb.209031

3 Leaping small fish out-power breaching whales

When Emmett Johnston from Queen’s University, Belfast, UK, attached a motion sensing tag to a basking shark off the Irish coast, he expected the piece of kit to have a sedate ride. Nothing prepared Johnston and colleagues for the rollercoaster that the sensor went on in practice. ‘Having retrieved it, we watched the video and, to our amazement, the movie ended with the shark suddenly Gregory ‘Slobirdr’ Smith accelerating up through the water [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]. column, hitting the water surface at 5 m s−1 and breaching for about a second’, says Lewis Halsey from the University of Roehampton, London, UK. The massive fish was behaving more like cavorting gray whales or leaping salmon than the lethargic filter feeder it was meant to be. The team calculated that each leap could use up to 1/17th of the animal’s daily metabolic budget (doi:10.1098/rsbl.2018.0537). However, after reviewing movies of breaching basking sharks filmed by observers on land, Halsey suspected that it might be possible to calculate the amount of energy an aquatic animal requires to make it into the air from the footage. ‘I thought there must be lots of video available of other aquatic species breaching’, says Halsey. Could he find a way to calculate how much power it takes for fish, whales and dolphins to surge out of the water from movies shot by citizen scientists? Halsey turned to the millennials’ TV channel of choice. ‘I kept searching YouTube with names of fish and cetaceans and various words for jumping until I couldn’t find any more’, he chuckles, recalling how, eventually, he ended up with almost 30 clips of species ranging from a 20 cm long African tetra to a 13 m long humpback whale. Then Halsey teamed up with Gil Iosilevskii from the Technion, Israel, to calculate the animals’ sheer power and speed as they burst out of the water. ‘Gil did the hardcore maths’, recalls Halsey, adding, ‘He is one of the few people around with the capacity to develop the equations we used to estimate power output from the videos’. Halsey then estimated the animals’ sizes, measured the length of time each animal was airborne and determined several other values before plugging them into Iosilevskii’s equations. Amazingly, the common bottlenose dolphins and spinner dolphins (both ~2 m long) hit the highest take-off speeds of ~10.7 m s−1 (38.5 km h−1), with the smaller species achieving progressively lower speeds; the slowest leaps were performed by the diminutive African tetra, which took off at only 4.4 m s−1. Meanwhile, the largest creatures, including the basking shark, great white shark and massive humpback whale hit speeds ranging from 5.8 to 9.1 m s−1; they didn’t come close to challenging the dolphins’ speed. However, when the duo compared the peak muscle power of the animals, it was clear that the smaller species – including mackerel, mullet and the common bottlenose dolphin – generated the highest muscle powers of over 50 W per kg of muscle when jetting out of the waves. In contrast, the heftier great white shark, orca and humpback whale, managed less than 10 W per kg of muscle. Halsey suspects that the smaller species’ powerful departures ‘represents an estimate of a universal upper limit to power output’, adding that their pound-for-pound power was very similar to that of top human athletes performing right at their physical limits. ‘When these animals are breaching at their fastest, they might well be performing at maximum effort’, says Halsey. 10.1242/jeb.219436 Halsey, L. G. and Iosilevskii, G. (2020). The energetics of ‘airtime’: estimating swim power from breaching behaviour in fishes and cetaceans. J. Exp. Biol. 223, jeb216036. doi:10.1242/jeb.216036

4 Antarctic bald notothens use spleen scuba tank to keep down blood viscosity

Michael Axelsson Trapped in the frigid currents surrounding Antarctica with body temperatures that virtually never exceed 0°C, icefish and rockcod live at temperatures that would freeze most species solid. To survive, the fish pack their body fluids with antifreeze proteins to ensure that they remain liquid. However, pumping protein-packed thick blood around the body is hard work for the heart and some fish have taken drastic measures to overcome the pressure. For example, icefish have entirely done away with red blood cells to thin the blood and reduce viscosity. But Michael Axelsson, Jeroen Brijs and Albin Gräns from the University of Gothenburg and the Swedish University of Agricultural Sciences wondered whether the spleen could hold the key to the survival of another Antarctic species, bald notothens (Pagothenia borchgrevinki). Explaining that the spleen usually stores red blood cells ready for use during exertion – the organ has even been referred to as the Weddell seal’s natural scuba tank – Brijs and colleagues wondered whether bald notothens depend on their spleens to store red blood cells and reduce blood viscosity in readiness for a workout? After travelling south to the McMurdo Research Station in Antarctica, Axelsson, Gräns and their colleagues Malin Rosengren and Fredrik Jutfelt went fishing through the sea ice for the hardy creatures. ‘Encountering the frosty wastelands of Antarctica was a very memorable experience, despite the extremely low temperatures and imminent risk of getting caught in a snow-storm’, says Gräns. Then the team set about finding how many red blood cells were circulating in the fish’s blood when the animals were swimming hard or when they were digesting dinner, which is another energetic process for cold-blooded creatures. Dividing the fish into four groups, the team fed one group before chasing them around the tank for 10 min, while the second set of fed fish had a rest. Meanwhile, the third group had a 10 min workout on an empty stomach while the hungry fourth group rested. The scientists then took blood samples and discovered that fish that were simply digesting a meal had more than doubled the quantity of red blood cells in their circulation, while the unfed fish that had been chased around the tank had more than tripled their red blood cell count. The spleen seemed to be helping out the fish when they were exerting themselves by boosting their red blood cell supply. But what would this increase mean for the fish in practice? Would the additional red blood cells supply more oxygen to fuel their exertions and how would their hearts cope with the raised blood viscosity? The team recorded how much oxygen the fish consumed while they were exercising and then operated on other fish to tie off their spleens – to prevent them from releasing red blood cells – to find out how well they fared during exertion. The fish that could no longer use blood cells from their spleens increased their metabolic rate by ~70%, but the fish that still had use of their spleen scuba tanks raised their metabolic rate by an impressive ~150%. However, the blood pressure of the exercising fish rose by 12% and their hearts were working 30% harder, thanks to the additional circulating red blood cells. ‘There is a considerable energetic and physiological cost associated with transporting the highly viscous blood’, says Gräns. Bald notothens are able to capitalise on their ability to hold red blood cells in reserve, ready for an additional burst of energy, without incurring the costs of pumping viscous blood around the body continually. Brijs says, ‘This strategy is most likely advantageous for bald notothens when hunting their prey or escaping their predators’. 10.1242/jeb.221549 Brijs, J., Axelsson, M., Rosengren, M., Jutfelt, F. and Gräns, A. (2020). Extreme blood-boosting capacity of an Antarctic fish represents an adaptation to life in a sub-zero environment. J. Exp. Biol. 223, jeb218164. doi:10.1242/jeb.218164

5 Evolution built men to pack a punch

Jeremy Morris The differences between males and females can be extreme. Tiny triplewart seadevil males simply fuse themselves for life to their relatively large female partners while female rusty tussock moths are almost immobile egg carriers for their more elaborate males. Although the differences between male and female humans are far less dramatic, men can have up to 90% more upper body strength than women. ‘[The] difference between males and females often reveals how evolution has shaped the bodies of males and females in different ways’, says Jeremy Morris from Wofford College, USA, who was curious to find out why men are so much more powerful than women. The possibility that male hand-to-hand combat was a driving force in human evolution has long intrigued Morris’s thesis advisor, David Carrier at the University of Utah, USA, so Morris decided to check out whether coming to blows could have driven men to build up. But Morris, Carrier, Jenna Link and James Martin, also from the University of Utah, had to come up with an alternative way of testing how much power people pack in punches. ‘People that are not trained in martial arts are hesitant to punch a punching bag with much force because there is a risk of injury’, says Morris. However, after putting their heads together the team came up with the idea of sitting volunteers down and asking them to power their arms forward by simply cranking a flywheel. ‘The advantage of using arm cranking … is that we can get maximum performance from people that have little or no experience punching’, says Morris. Recruiting almost 40 reasonably fit adult volunteers from friends and colleagues around the campus, Morris and Link asked them to warm up gently before driving the crank wheel as hard as possible over the top third of a full rotation to simulate a punching action. ‘We encouraged the participants, yelling “Go, go, go! Come on!” the same way that a physical trainer would at a gym to get the maximum performance’, recalls Morris. Calculating the amount of power each participant threw into spinning the heavy flywheel, the chaps came out streets ahead of the women, packing almost three times more power on average into their wheel spins (282 W) than the females (108 W). However, when the team tested how much effort the adults could put into pulling the wheel in reverse, the differences were less pronounced; the men only just doubled the amount of power exerted by the women. In addition, when the team compared how much more force the male volunteers were able to invest in overarm throwing – an alternative theory for the differences between the physical builds of men and women – the men only invested twice as much power as the women. ‘The results of this study support the hypothesis that evolution has selected for physical traits in males that make them better at inflicting damage by punching with the fist’, says Morris. And he is keen to find out how hard male and female trained athletes can throw a punch, to answer an important question: is punching the physical activity with the greatest disparity between men and women? 10.1242/jeb.221135 Morris, J. S., Link, J., Martin, J. C. and Carrier, D. R. (2020). Sexual dimorphism in human arm power and force: implications for sexual selection on fighting ability. J. Exp. Biol. 223, jeb212365. doi:10.1242/jeb.212365

6 Australian jack jumper ant foragers have no need for mental map

Ajay Narendra Before embarking on their careers as fully fledged foragers, novice ants take a few tours of their neighbourhood to familiarise themselves with the lay of the land. ‘Much evidence has accumulated to show that ants form visual memories of how the scene looks at their nests and along routes’, says Jochen Zeil from the Australian National University. However, it wasn’t clear how ants use these visual snapshots to help them navigate. Do they build a mental map that allows them to keep track of their location at all times, or are they using a simpler navigation system, where they prefer to walk toward some views while actively avoiding others? Wondering how the intrepid explorers negotiate their surroundings, Trevor Murray, Zoltán Kócsi, Ajay Narendra and Zeil decided to find out how Australian jack jumper ants (Myrmecia croslandi) behave at various locations around their natural foraging range in the hope of bamboozling the insects into revealing the key to their navigational success. Intercepting 28 foragers either as they emerged from their nest or when they arrived at a tree that was popular for foraging, the researchers then set the detainees trotting on a mobile treadmill (constructed by Hansjürgen Dahmen at Tübingen University, Germany) at several locations in the area – including a site midway between the tree and nest on the foraging route, one 6 m west of the nest and one directly above the nest – to find out in which directions they preferred to scamper. Recording the direction of each ant stride, the team realised that the ants that were positioned on their foraging route and west of the nest seemed to know in which direction they should be heading. They always swivelled toward their intended goal, even when the treadmill was off course at a location that they had never visited before. However, it was clear that the ants weren’t keeping track of the distance that they had travelled, as they continued scampering long after they had covered the distance to their goal. ‘They knew where to go but did not know where they were’, says Murray. In addition, when the team set ants that had just returned to the nest upon the treadmill directly above the nest, they seemed to recognise where they were, scurrying in random directions in search of their home, while the ants that had been intercepted at the tree turned consistently in the direction where they expected to find home. They ants seemed to know where they were, but was that all there was to the story? Meanwhile in France, Floret Le Möel and Antoine Wystrach from the University Paul Sabatier had been asking the same question and wondering whether a simple rule based on the insects’ view of their surrounding landscape could explain their navigation. Were the ants simply compelled to run toward views that were similar to their memories of the landscape surrounding the nest, while heading away from views that lay in directions away from home? Based on this idea, the pair built a computer-simulated ant and when they compared the cyber-ants’ navigation with that of the Canberra ants, Murray says that they were ‘stunned by the similarities between the two’. By simply following the rule directing them toward attractive views and away from repellent views, the simulated insects performed exactly like the biological ants on Dahmen’s treadmill. Ants have come up with a simple strategy that helps them to find home, no matter where they have roamed, and Zeil concludes, ‘Ants know their way around the world without having a map’. 10.1242/jeb.221127 Murray, T., Kócsi, Z., Dahmen, H., Narendra, A., Le Möel, F., Wystrach, A. and Zeil, J. (2020). The role of attractive and repellent scene memories in ant homing (Myrmecia croslandi). J. Exp. Biol. 223, jeb210021. doi:10.1242/jeb.210021

7 Weakly electric fishes’ secret social lives revealed

There’s a lot of chit-chat going on in the rivers of Panama, but most creatures can’t even tune in. That’s because the conversations are literally electric. ‘Weakly electric fish continuously generate an electric field’, says Jörg Henninger from Eberhard Karls University of Tübingen, Germany, explaining that each individual produces a unique electrical signature which they use for identification, communication and when navigating after Rüdiger Krahe dark. But PI Jan Benda and Henninger realised that the fish’s charged communications could also provide them with an unprecedented glimpse inside the furtive animals’ social lives. ‘Most outdoor studies require loggers or tags to be mounted on the animals’, says Henninger, explaining that the tracking devices allow scientists to follow social interactions between individuals. However, the electric signatures produced by each fish are effectively individual tags, if only the scientists could build a system to disassemble the electric babble. Initially, Benda, Henninger and Rüdiger Krahe from Humbolt Universität zu Berlin, Germany, built a grid of 54 electrodes distributed across an area of 3.6 m2 that could be dropped into a river to record the fish’s electric fields. Then they teamed up with Fabian Sinz, also from Eberhard Karls University of Tübingen, Germany, to develop complex computer algorithms to interpret the intricate electric fields. Benda, Henninger and Krahe then travelled to Quebrada La Hoya creek in Panama to try out the grid for real. ‘In the early stages, many things broke, partly because of high humidity. Once, we even lost a grid because of an unexpected and sudden rise of water level’, recalls Henninger. However, after successfully recording 130 h of electric cacophony, Henninger eventually focused on one 25 h period of uninterrupted dialogue. Analysing the electric field frequencies, the team identified three species occupying the same stretch of river: Eigenmannia humboldtii, producing electric fields between 200 and 580 Hz, Sternopygus dariensis with electric field frequencies lower than 220 Hz and Apteronotus rostratus generating electric fields with frequencies ranging from 580 Hz to over 1000 Hz. ‘We were very surprised and lucky to find that the species and individuals were separated so clearly’, says Henninger. Charting the fish’s manoeuvres over the course of 24 h, the team noticed that the fish mainly migrated upstream at speeds of up to 0.2 m s−1 in the hours after dark, returning downstream towards the end of the night at speeds of almost 0.3 m s−1, with some potentially covering distances of up to 3 km each night. In addition, the fish preferred swimming close to the river bank packed with tree roots, where the dragonfly larvae and midges upon which they dine shelter. Most E. humboldtii also seemed to prefer travelling solo (61 out of 65) rather than in shoals and when the team analysed the strength of individual A. rostratus electric signals, some were actively courting and trying to attract mates. ‘This suggests that a substantial number of fish disperse from their group during the night either to forage on their own or to find mating partners’, says Benda. Pointing out how few of these observations could have been possible using conventional tagging approaches, Krahe adds, ‘The multispecies community we describe here hints at the complexity of signals weakly electric fish are actually facing’. However, for the time being, the residents of Quebrada La Hoya creek are probably content continuing their crackling conversations until more electronic eavesdroppers come to visit. 10.1242/jeb.222455 Henninger, J., Krahe, R., Sinz, F. and Benda, J. (2020). Tracking activity patterns of a multispecies community of gymnotiform weakly electric fish in their neotropical habitat without tagging. J. Exp. Biol. 223, jeb206342. doi:10.1242/jeb.206342

8 Mantis shrimp pull punches in air for self-preservation

Kate Feller Mantis shrimp (Squilla mantis) don’t take kindly to captivity. ‘They have a general baseline of being angry’, chuckles Kate Feller, currently at the University of Minnesota, USA, recalling how the contrary stomatopods are particularly keen to lash out when exposed to air. ‘I had developed a means of holding mantis shrimp with their striking appendages out of water while their gills [arranged beneath the tail] remained submerged for an electrophysiology study I was conducting’, recalls Feller. However, when Greg Sutton from the University of Lincoln, UK, wandered past her University of Cambridge (UK) lab bench, he noticed the exposed crustaceans and commented that it would be interesting to measure their hammer blows in air. ‘No one had done that’, explains Feller. Knowing that the animals launch their ballistic appendages at the speed of a bullet in dense water, it seemed likely that they could even exceed those eye-watering speeds in thinner air. Feller set up Paloma Gonzalez-Bellido’s high speed camera to catch the angry animals in the act. ‘We stimulated them to strike by poking them in the abdomen with a blunt stick. It was like poking them in the tummy, which they hated’, she says, adding that the manoeuvre was not without risk. ‘I have a pretty epic photo of my bleeding hand over a white sink when one stabbed me during this process’, she smiles. And at first glance, the animals’ blows looked every bit as punchy as those beneath water. However, when Feller analysed the spring-loaded air-blows, she was puzzled; the manoeuvre was only half as fast: ‘The air strikes only averaged about 18 km h−1, which is pretty pitiful even for a slow striker like S. mantis’, she says. In fact, when she and Sutton compared the mantis shrimp blows with another spring-loaded manoeuvre – grasshopper leaps – it turned out that the crustaceans were only packing 0.42 W of power into their 1.2 g hammers, the same as the leaping grasshoppers, even though they are capable of propelling their hammers in water with ten times more power (4 W). Puzzled by the mantis shrimps’ lacklustre performance, Feller and Sutton initially wondered if the animals’ spring-loaded propulsion mechanism was simply unable to transmit as much power in air as in water. But then, a paper by Sheila Patek’s lab was published (jeb198085) showing that the crustaceans can fine-tune their blows depending on the context. Feller and Sutton realised that the exposed mantis shrimp were more likely downgrading their blows in the thinner material. But why were the crustaceans pulling their punches when they could have taken the opportunity to make even more of a mess of Feller’s fingers? ‘My hypothesis is that this may be related to how mantis shrimp dissipate the excess energy of their strike’, she says. Locusts and other leaping insects have energy-absorbing structures at the back of the limb to protect joints from damage, whereas the explosive energy released by submerged mantis shrimps is usually dissipated by their tough opponents and the hard snail shells they target. ‘In air, not only are the forces of drag from water absent, but the entire sensory experience is messed up, so maybe – in the absence of a perceived target – the animals don’t give it the full pow so they don’t blow out their joint’, Feller suggests. In other words, the crustaceans’ pulled air-punches might simply be a matter of self-preservation. 10.1242/jeb.222604 Feller, K. D., Sutton, G. P. and Gonzalez-Bellido, P. T. (2020). Medium compensation in a spring-actuated system. J. Exp. Biol. 223, jeb208678. doi:10.1242/jeb.208678

9 Stressed chickadees get hot under the collar to save energy

Feeling stressed can make some animals’ blood run cold, while others feel hot under the collar and Joshua Robertson, Gabriela Mastromonaco and Gary Burness, from Trent University and Toronto Zoo, Canada, explain that ‘the functional value of this phenomenon is poorly understood’. One possibility was that a drop in surface temperature could be attributed to terrified creatures shunting blood away from the skin to minimise blood loss in the event of an injury; although it wasn’t clear why other animals run warm. However, the Canadian team had a hunch that fluctuating skin temperatures could provide a mechanism for saving energy. Robertson says, ‘Birds usually dissipate heat by evaporative cooling, which is more energetically- and resource-demanding than dry heat dissipation’. The researchers wondered if stressed birds avoid wasting the energy that they would otherwise need to expend panting by dumping the heat generated by their anxiety through their hot skins instead. In addition, the trio suspected that birds may shunt blood flow away from the skin when air temperatures turn cold, to capitalise on the heat generated by the birds’ state of anxiety to keep warm at no extra cost. To test the idea, the trio filmed black-capped chickadees with a temperature-sensing camera under a range of stressful situations – ranging from being harassed by a stuffed hawk to the presence of humans – from late spring to early summer to find out how the temperature of the patch of skin around their eyes varied. And when the team compared the chickadees’ temperature variation with the air temperature, they found that the stressed birds’ skin temperatures rose in sizzling conditions when they would otherwise have been expending energy in order to remain cool. And when the conditions were chilly, the anxious birds diverted blood away from their surface, cooling the skin around their eyes to conserve the warmth produced by the tense situation and save energy. The stressed chickadees seemed to be adjusting the blood flow to their skin to reduce their energy consumption depending on the prevailing weather, and Robertson adds, ‘it does seem risky to enhance perfusion of blood to the skin during stress exposure, but our results suggest that perhaps it is a risk chickadees are willing to take’. 10.1242/jeb.222968 Robertson, J. K., Mastromonaco, G. and Burness, G. (2020). Evidence that stress-induced changes in surface temperature serve a thermoregulatory function. J. Exp. Biol. 223, jeb213421. doi:10.1242/jeb.213421 10 Cameras do not lie: elephant seals prefer fish

Kaori Yoshino Modern teenagers think eking out a few more minutes from their dying mobile phone battery is a chore, but spare a thought for Akinori Takahashi from The Graduate University for Advances Studies and Yasuhiko Naito from the National Institute of Polar Research, both in Japan. Battery life can be a major obstacle for researchers tracking the behaviour of animals migrating through oceans. ‘The duration of video recordings is limited to several hours or days’, says Takahashi. So, when Naito and Daniel Costa from University of California Santa Cruz, USA, wanted to find out which species elephant seals (Mirounga angustirostris) dine upon during their 2.5–7.5 month foraging odysseys, by attaching a video camera to the mammals’ heads, they knew they would have to crack the battery problem. Even though the solution seems straightforward – only switch on the camera and infrared lights when needed – Naito had to figure out a three-stage trigger that recognised when a seal was on the verge of catching a snack. Knowing that it takes the voyagers several weeks to reach their feeding grounds, the team first fitted a timer that only activated the system once the seals were out at sea. Then they added a depth meter, so that the equipment was primed once seals had reached the depths at which they forage (400–800 m). Finally, the researchers included an accelerometer to activate the camera and infrared flash as the seals surged forward, ready for the kill. Having assembled the camera tag, the team headed to Año Nuevo State Park on the Northern California coast, USA, to attach the camera and sensors to the head or jaws of female elephant seals before they embarked on their epic odyssey. ‘We always needed to be careful of big male seals around us, to avoid them smashing our gear’, says Takahashi, adding ‘We needed to anaesthetize the females because they are large and we needed them to stay still while we carefully adjusted the angle of the cameras on their heads… to take good footage of prey capture’. Releasing the first seals on their annual voyage in 2013, Takahashi admits that he was anxious as he waited for them to return 3 months later. And when the team eventually downloaded the first submerged video clips, they were excited to see fish and squid loom into view before vanishing into the hungry seals’ jaws. Eventually, after deploying 15 cameras over 5 years, the team retrieved almost 50 h of footage, capturing almost 1500 dives across the eastern North Pacific as the elephant seal females feasted on almost 700 animals, including lantern fish and squid, over depths ranging from 239 to 1167 m in chilly 3.2–7.4°C waters. And while some of the intercepted animals seemed to be caught unawares by the impending attack, others made a valiant dash for safety; some squid even produced defiant bioluminescent flashes, which appeared to captivate the seals. However, when the team analysed which species the seals preferred to dine upon, they were surprised that fish accounted for at least 78% of the diet, with squid only contributing up to 10%. ‘Our data offer a contrasting view to previous studies based on stomach contents’, says Takahashi, which had suggested that the seals dine predominantly on squid. However, the team attributes the discrepancy to the squids’ tough beaks. They probably remain longer in the seals’ stomachs than other more easily digested morsels, giving the impression that elephant seals prefer squid, when, in reality, fish is far higher up the menu. 10.1242/jeb.222976 Yoshino, K., Takahashi, A., Adachi, T., Costa, D. P., Robinson, P. W., Peterson, S. H., Hückstädt, L. A., Holser, R. R. and Naito, Y. (2020). Acceleration-triggered animal-borne videos show a dominance of fish in the diet of female northern elephant seals. J. Exp. Biol. 223, jeb212936. doi:10.1242/jeb.212936

11 African pygmy mouse upgrades mitochondria to compensate for size

Laurana Serres-Giardi licensed under the Creative There’s a reason why so few warm blooded Commons Attribution-Share Alike 3.0 Unported. (endothermic) animals are super tiny. It’s simply too costly for them to keep their internal central heating turned up. This is because the tiny powerhouses (mitochondria) that produce the energy (ATP) required for warmth and activity become less efficient as animals scale down and their metabolic rates increase. The few mammals that have miniaturised tend to conserve energy by hibernating and dropping their body temperature while inactive. However, Mélanie Boël, Damien Roussel and Yann Voituron from the Université de Lyon, France, wondered whether the mitochondria of these minute mammals could have improved their efficiency, by producing less waste heat to augment their ATP supply. The team decided to compare the performance of mitochondria from three species of mice – two members of the smallest mouse family, African pygmy mice Mus mattheyi (~5 g) and Mus minutoides (~7 g), and larger house mice, Mus musculus (~22 g) – to find out whether any of the minuscule mammals benefit from boosted mitochondria. First, Boël recorded the animals’ oxygen consumption and carbon dioxide production rate for 4 days to calculate their metabolic rate as they scampered around their individual cages. Not surprisingly, the heavyweight house mice had the lowest active metabolic rate (~0.03 W g−1) compared with those of the pygmy mice, which tipped the scales at ~0.04 W g−1 (M. minutoides) and ~0.06 W g−1 (M. mattheyi). But would their mitochondria prove to be as inefficient as those of other diminutive warm-blooded creatures? After collecting samples of the animals’ foreleg muscles, Boël measured the respiration rate of muscle fibres before painstakingly collecting mitochondria from the muscles and liver, which together account for almost half of an animal’s basal metabolic rate. Then, she recorded their oxygen consumption and determined how much ATP and waste heat the mitochondria produce. ‘To perform our study, we needed quite a large amount of isolated mitochondria’, says Boël, who had to alter the method used by most researchers to collect sufficient mitochondria from the minute (0.3 g) muscles. Sure enough, the mitochondria of the bulkier house mice and one of the pygmy mice (M. minutoides) performed exactly as they expected based on the animals’ respective sizes; the mitochondria of M. minutoides were relatively inefficient compared with those of the house mice. However, when Boël analysed the mitochondria of the smallest mouse on the planet (M. mattheyi), the team was astonished. The muscle and liver mitochondria were as efficient as those of the house mice. Instead of generating large amounts of waste heat while producing ATP, the mitochondria of M. mattheyi were able to produce as much ATP per molecule of oxygen as the house mice. ‘The hypothesis of a mitochondrial adaptation, by which extremely small species could avoid … associated energy wastage in order to maintain their cellular energy homeostasis, is thus verified in two tissues of M. mattheyi’, says Boël. Even though minute M. mattheyi can hibernate to conserve energy, they have also improved the efficiency of their mitochondria to reduce energy expenditure. And, Boël is eager to discover whether this minuscule titan is unique, ‘in order to know if the improvement of mitochondrial efficiency in species below 7 g is a physiological adaptation limiting the high energy cost to maintain their body temperature’, she says. 10.1242/jeb.223693 Boël, M., Romestaing, C., Duchamp, C., Veyrunes, F., Renaud, S., Roussel, D. and Voituron, Y. (2020). Improved mitochondrial coupling as a response to high mass-specific metabolic rate in extremely small mammals. J. Exp. Biol. 223, jeb215558. doi:10.1242/jeb.215558

12 Fat loss triggers ant lifestyle change

Life in Platythyrea punctata ant nests is all about the pecking order: from the dominant egg layers down to their lowlier sisters, which toil on behalf of their nestmates, and the humblest foragers. ‘Young workers establish dominance hierarchies and rank orders’, says Abel Bernadou from University of Regensburg, Germany, adding that the stay-at-home workers seemed to carry more fat than the leaner foragers, in addition to living longer. ‘However, it wasn’t clear how the body fat levels of P. punctata workers play a role in Abel Bernadou regulating the transition from inside to outside nest activities’, says Bernadou. The team also knew that it was possible that other factors, such as the ants’ fertility, could be responsible for their abrupt lifestyle change. So, Bernadou, Elisabeth Hoffacker, Julia Pable and Jürgen Heinze decided to find out how much fat corpulent nest workers and lithe foragers carry. After collecting ants at different stages of life, from the youngest nest workers to elderly foragers, and measuring the amount of fat they were carrying, it was clear that the youngest housekeepers were the fattest. They had body fat levels around 20%, while the oldest housekeepers’ fat content fell to ~10% and that of the elderly foragers crashed to only 5%. But had the slim foragers’ fat loss triggered their change of role, or were other factors at play? Wondering whether the ants’ ovaries might contribute to their lifestyle change, Hoffacker diligently followed the foraging antics of 90 workers, ranging in age from newly emerged adults to elderly foragers, for several days. Analysing the activity levels of the fattest ants, the condition of their ovaries (whether they were well or poorly developed) had no effect. The fattest ants spent little time foraging, regardless of their ovary development, so the ovaries do not trigger the ant’s transformation from nest worker to forager. However, when Bernadou plotted the amount of time the ants spent out-and-about against their thorax fat content, there was a clear switch point. Ants with a thorax fat content above 4% barely ever left the nest, while the slimmer ants below the 4% threshold rarely spent any time at home. Corpulence, not fertility, seemed to be the key factor in the move to an outdoor lifestyle, but is body fat the trigger that converts an indoor worker to an intrepid outdoor forager? To answer this question, Pable collected young adults that had just emerged from their pupae and slimmed down half of them, by only feeding them once in 10 days, while feeding up the remaining ants on a daily diet of Drosophila. Then, she paired up a slim and a tubby youngster in a tiny nest and kept an eye on their activities to identify which of the two spent most time outdoors. During the first 3 days, the skinny youngsters spent twice as long outside as their well-fed sisters. Even though the under-fed ants were much younger than normal foragers, they were as keen to venture out as the elderly thin ants. In other words, body fat is the key to the ants’ transition from worker to forager, and Bernadou says, ‘Leanness is not a consequence of foraging activity but has a causal effect on the transition from duties inside the nest to outside tasks’. The team adds that the fat switch makes sense on several levels. Only sending out the slimmest members of the nest protects the collective fat reserves, as the foragers may not return, in addition to ensuring that the youngest workers are well upholstered to invest energy in producing the eggs that ensure the nest’s future. 10.1242/jeb.224329 Bernadou, A., Hoffacker, E., Pable, J. and Heinze, J. (2020). Lipid content influences division of labour in a clonal ant. J. Exp. Biol. 223, jeb219238. doi:10.1242/jeb.219238

13 First two weeks crucial for white-nose syndrome survivors

Nate Fuller With only a few grams of fat to sustain them through their 9-month winter hibernation, there is little flexibility in little brown bat (Myotis lucifugus) energy budgets. So, when white- nose syndrome strikes – a disease caused by the Pseudogymnoascus destructans fungus – the consequences can be dire. ‘Over 90% of hibernating populations have been lost at some sites’, says Nate Fuller from Texas Tech University, USA, adding that the starving survivors, which are left with tattered wings, are vulnerable when they attempt to build themselves up again. But, little was known about the length of time it takes the mammals to heal and the long-term impact of their winter infection. To begin understanding how bats survive white-nose syndrome, Fuller and Liam McGuire set out on a 1300 mile road trip between Ottawa and Winnipeg in the weeks before the hibernating bats emerge, to collect infected animals from their overwintering sites in order to chart their road to recovery. ‘The most difficult aspect of the study was transporting the bats in a battery-operated refrigerator … on a 24 h, nonstop drive around Lake Superior’, says Fuller, recalling encounters with moose, wolves and snow along the way. The bats were also in bad shape thanks to the infection when they arrived at Craig Willis’s University of Winnipeg lab. Housing the sick animals in a comfy roost with access to a spacious flight chamber, Fuller and Heather Mayberry handfed the creatures initially on fat mealworms, until they got the hang of feeding themselves. Fuller also monitored the grumpy animals’ recovery for 40 days, collecting samples from their wings, as well as keeping track of their mass and overall health. Initially, the bats’ wings appeared quite healthy, but they began deteriorating markedly 5 days later, with holes and sores appearing. In addition, he knew that the bats’ mass had fallen from ~12 g at the start of hibernation to ~6 g when collected from their roosts; however, the animals soon began gaining mass as they recovered. ‘Their individual personalities started to become apparent’, chuckles Fuller, who was also impressed by how well the wings then healed. ‘I have numerous scars from injuries I received years ago, but bat wings rarely reflect large holes and injuries from the previous month, let alone years past’, he adds. Cataloguing other details of the bats’ recoveries with McGuire, Mayberry, Evan Pannkuk, Todd Blute and Catherine Haase, Fuller realised that the bats often dropped their body temperature significantly (18°C) when roosting, in a process known as torpor, early in their recovery to conserve energy. However, the bats seemed to depend less on torpor during the second week, only dropping their body temperature by a few degrees while they invested heavily in repairing the damage to their wings. In addition, the waterproofing lipids at the surface of the wing, which are likely to protect the bats from dehydration and infection, improved during their recovery. And, when the team tracked the extent of the bats’ infection, most of the animals were almost completely fungus-free within 10 days, although they were concerned when they realised that some of the bats became reinfected several weeks later. ‘It appears that there is a critical 2-week window during which surviving bats undergo a period of intense healing. If the bats make it through this process, their chance of survival goes way up’, says Fuller. But, he is concerned that recovery takes a great deal out of the vulnerable animals, and he adds, ‘We hope this will give us some indication of why some bat populations have not disappeared despite repeated seasonal infection’. 10.1242/jeb.223669 Fuller, N. W., McGuire, L. P., Pannkuk, E. L., Blute, T., Haase, C. G., Mayberry, H. W., Risch, T. S. and Willis, C. K. R. (2020). Disease recovery in bats affected by white-nose syndrome. J. Exp. Biol. 223, jeb211912. doi:10.1242/jeb.211912

14 Fish maintain tissue pH despite CO2 blast

Tino Straus, GNU Free Documentation License, Version 1.2 or Even though lake and river CO2 levels any later version published by the Free Software Foundation. never near the fizzy heights found inside a can of soda, CO2 can vary drastically between bodies of water, even within a matter of meters. ‘For example, CO2 levels in tropical environments may be as high as 8 kPa, which is 27 times higher than the average global freshwater level of 0.3 kPa’, says Ryan Shartau from the University of British Columbia, Canada. Sudden blasts of CO2 can send the physiology of fish haywire, swiftly driving down the pH of their blood before they topple over and asphyxiate. Although most fish are able to rebalance their blood pH swiftly enough to maintain their equilibrium when CO2 shifts are mild, a select few seem to be able to deal with potentially fatal build-ups of CO2 by stabilising the pH within tissues, regardless of the pH of their circulating blood. ‘Prior to 2008, only three fish species, marbled swamp eel, armoured catfish and white sturgeon, were known to maintain a stable pH of their tissues during high CO2’, says Shartau. However, he and his thesis advisor, Colin Brauner, wondered how unique these extraordinary fish are, so they tested how white sturgeon, which deal well with high CO2, and rainbow trout, which do not, coped as they increased the CO2 in the fishes’ water. −1 Raising the CO2 concentration at rates ranging from 1 up to 4 kPa h , Shartau found that the white sturgeon fared far better than the rainbow trout. The sturgeon only toppled over at 22 kPa CO2 compared with the rainbow trout, which lost their balance at 5.5 kPa CO2. Then Shartau set his sights on more exotic fish from waters that can experience naturally high pulses of CO2. Visiting Dane Crossley’s lab at the University of North Texas, he and Zac Kohl caught spotted gar and channel catfish from nearby lakes and rivers, and settled them into the lab before increasing the CO2 levels in their water to 1.5, 3 or 6 kPa CO2 for 3 h. Then the pressure was on to measure the pH of the fish’s blood and to collect specimens of the fish’s heart, muscle, liver and brain without the internal pH altering. ‘We rapidly collected the tissues, wrapped them in aluminium foil and flash-froze them in liquid nitrogen to stop metabolic activity’, says Shartau. However, he recalls underestimating how much time it would take to process all of the tissues. ‘I was seriously short on time, so I worked solid around the clock, with only a short break for Thanksgiving dinner at Dane’s, before going back in the lab and finishing an hour before I headed to the airport’, he laughs. Over the next 4 years, Shartau visited Adalberto Val at the Brazilian National Institute for Research of the Amazon and Peter Allen at Mississippi State University, USA, to check out the responses of fish ranging from American paddlefish and alligator gar to matrinxã and the most resilient of them all, tambaqui. When the team finally compared the pH values of the fishes’ tissues against those of their blood, they were astonished. Although the fish blood pH values fell dramatically as the CO2 level increased, their tissue pH remained stable and some even raised the pH of the heart. ‘We were surprised by the number of fish species able to maintain a stable pH inside their tissues during those high CO2 exposures’, says Shartau, adding, ‘Going into this study, we were concerned we might not find any’. He and Brauner are now keen to find out exactly how fish pull off this feat and whether aquatic invertebrates are also capable of defending themselves from a blast of CO . 10.1242/jeb.224543 2 Shartau, R. B., Baker, D. W., Harter, T. S., Aboagye, D. L., Allen, P. J., Val, A. L., Crossley, D. A., Kohl, Z. F., Hedrick, M. S., Damsgaard, C. and Brauner, C. J. (2020). Preferential intracellular pH regulation is a common trait amongst fishes exposed to high environmental CO2. J. Exp. Biol. 223, jeb208868. doi:10.1242/jeb.208868

15 Hot limpets can’t hang on as tight as cold ones

Dick Daniels (http://carolinabirds.org/) CC BY-SA Recent reports make it clear that there are (https://creativecommons.org/ licenses/by-sa/3.0) going to be many losers if we continue pumping climate-changing gases into the atmosphere. Yet, as delicate ecosystem balances shift, some species may be able to take advantage of other’s setbacks. Although limpets are well known for their tenacious grip when exposed on rocky seashores, Rachel Pound, a Master’s student from California State University Fullerton, USA, and colleagues wondered how well the resolute molluscs are able to cling on as temperatures rise and, more to the point, how hungry seabirds might fare as they attempt to prise their lunches loose. But first, Pound, Luke Miller, Felicia King and PI Jennifer Burnaford decided to get a handle on how hot under the shell limpets get as the tides come and go. Dismembering iButton temperature sensors and reassembling them inside empty limpet shells, the team distributed their robolimpets on the rocks of Dana Point in Southern California to keep track of mollusc body temperatures. Over a 2-year period, the hottest limpets logged impressive top temperatures over 40°C in January, and the team noticed that limpets exposed on flat rocks and vertical southern faces tended to experience the highest temperatures. Having pinned down how hot the limpets can become, Pound and colleagues turned their attention to how hard hungry oystercatchers have to strike limpets to get them to loosen their grip, before popping them off. This time, the team attached a limpet shell to a force transducer and enticed a captive black oystercatcher, known as ‘Squeakers’, at the Living Coast Discovery Center, USA, to have a go at wrestling the tasty morsel free. ‘For a bird that didn’t grow up eating limpets, Squeakers became fairly adept at attacking them’, says Miller, from San Diego State University, USA, who was impressed that Squeakers could exert pecks of up to 37.4 N, five to six times his body weight. Then the scientists wanted to know how hard oystercatchers have to peck at real exposed limpets on the seashore to wrench them free. After designing a mechanical oystercatcher constructed from a spring scale with a 3D printed beak to peck at limpets anchored at cool (~15°C) and hot (~30°C) temperatures, the team found that the mechanobird picked off four times as many warm limpets as cold limpets, with the warmer limpets falling free after just five 14 N pecks. And, when the team tested how vulnerable the limpets were to a hungry Squeakers, it took between 0.07 and 21.01 s for him to detach a limpet from its hold, with the cooler limpets hanging on six times longer than their warmer counterparts, although Squeakers eventually detached all of the limpets successfully. ‘This was a very simple study, but our findings suggest that the body temperature of limpets in the field could affect their likelihood of being removed from the rock and eaten by oystercatchers’, says Burnaford. And she adds that even though Squeakers was eventually able to dislodge all of the clinging limpets, he was able to remove the warm limpets far faster than the cooler ones, suggesting that oystercatchers could pick off many more molluscs on warmer days than on cooler occasions. ‘If rising temperatures affect prey and their ability to avoid being eaten more than they affect the predators, we might see dramatic changes in this ecosystem in the years to come’, says Burnaford. 10.1242/jeb.224923 Pound, R. J., Miller, L. P., King, F. A. and Burnaford, J. L. (2020). Temperature affects susceptibility of intertidal limpets to bird predation. J. Exp. Biol. 223, jeb213595. doi:10.1242/jeb.213595

16 Forest protects Heliconius butterflies from climate extremes

For many species, mountains aren’t just physical barriers. As the altitude increases, the air temperature falls and humidity can change, preventing cold- blooded species (ectotherms) from crossing into adjacent valleys. And the situation is even more intricate for insects that make their homes in tropical mountain forests. Gabriela Montejo-Kovacevich from the University of Cambridge, UK, explains that the climate beneath the forest canopy rarely resembles the measurements made by local weather stations. Gabriela Montejo-Kovacevich ‘The forest understoreey is often more than 2°C cooler than the canopy and can span an 11% difference in humidity’, says Montejo- Kovacevich. In addition, tropical species are also more vulnerable to climbing temperatures, as they never experience the drastic seasonal changes that make temperate species more hardy. Knowing that tropical species are likely to be more susceptible to climate change, thanks to seasonal stability, Montejo-Kovacevich and colleagues from the Universities of Cambridge and Sheffield, and the Universidad Regional Amazónica de Ikiam, Ecuador, decided to get a handle on just how the climate within mountain forests differs from meteorologists’ climate predictions. Travelling to high (~1200 m) and low (~450 m) locations on the eastern and western flanks of the Ecuadorian Andes, Montejo-Kovacevich, Simon Martin, Chris Jiggins and Nicola Nadeau suspended temperature loggers and humidity detectors in the forest canopy (~11 m) and nearer the ground (~1 m). ‘We used fishing weights attached to a line and threw them up into the canopy, hoping they would go over our chosen branches’, says Montejo-Kovacevich. And, when they returned 12 months later, she recalls that the forest had reclaimed all 56 data loggers: ‘there were ants’ and wasps’ nests, fungus, moss and all sorts of things growing on them’, she chuckles. Then the team compared the forest climate with weather predictions and it was clear that the forest had a major effect on the temperature within. Near to the ground at low altitudes, the vegetation offered relief (temperatures of ~25°C) from the predicted temperatures of 27–29°C, while the temperatures edged up around 30°C in the canopy. However, the effects of the forest seemed to be less dramatic at high altitude, with a temperature difference of ~1°C between the canopy and ground cover. But how much of an effect would these differences have on the insects that dwell within? Montejo-Kovacevich and her colleagues armed themselves with butterfly nets and went hunting for Heliconius butterflies in the Ecuadorian Andes at low (~500 m) and high (~1400 m) altitudes. Having collected 11 members of the family, Montejo-Kovacevich set up impromptu laboratories, sometimes in the bathrooms of remote hostels, to warm the insects while recording how long it took for them to topple over, to find out how resilient they were. Not surprisingly, the butterflies from the foothills coped better, managing to stay upright for ~18 min, in contrast to the butterflies from high altitude, which succumbed after ~5 min. But were these differences hardwired into the populations, or were the butterflies becoming acclimatised to their individual environments and could they cope better if reared in a different location? Collecting fertilised Heliconius erato lativitta females from high and low locations, the team reared the offspring at the Universidad Regional Amazónica Ikaim to find out how the youngsters coped with high temperatures. Remarkably, the offspring of the vulnerable high- altitude insects were significantly more robust than their parents, almost matching the tolerance of the low-altitude descendants. It seems that temperatures experienced by insects beneath the canopy may be less extreme than meteorologists’ climate predictions, and insects may have the ability to cope as climate change takes grip. 10.1242/jeb.225102 Montejo-Kovacevich, G., Martin, S. H., Meier, J. I., Bacquet, C. N., Monllor, M., Jiggins, C. D. and Nadeau, N. J. (2020). Microclimate buffering and thermal tolerance across elevations in a tropical butterfly. J. Exp. Biol. 223, jeb220426. doi:10.1242/jeb.220426 17 Two percent change switches motor muscle to brake

Simon Sponberg and Travis Tune Although some things look similar superficially, scratch beneath the surface and there can be significant differences. Travis Tune and Simon Sponberg, from the Georgia Institute of Technology, USA, explain that two muscles in the cockroach leg – the dorsal and ventral femoral extensors – appear identical at first glance; their contraction characteristics are indistinguishable. Yet, the ventral muscle functions as a brake, absorbing energy as the insects scamper about, while the dorsal muscle works like a motor powering the insect’s manoeuvres. ‘It is difficult to explain why these muscles’ contraction characteristics are so similar but have very different functions during running’, says Tune. Yet the duo realised that minute differences in the structure of the two muscles on the nanometer scale (0.000000001 m), could account for their radically different performances. To figure out what was going on, Tune and Sponberg needed to visit one of the world’s largest X-ray microscopes, the Advanced Photon Source (APS), USA, to peek inside the muscles as they contracted. But why did the duo have to use such a powerful microscope? It turns out that muscles have a lot in common with a grain of sugar. The sliding filaments packed together in a muscle are arranged a little like tightly packed candy sticks in a box, or sugar molecules in a grain of sugar. Shining X-rays on a grain of sugar tells you how the molecules are arranged within and how tightly they are packed, so the X-ray microscope could reveal whether there are any differences in how the filaments in the two muscles are arranged on this minute scale. ‘The APS facility is a kilometre-long apparatus that feels enormous’, says Sponberg, and Tune adds, ‘Working there can be stressful though, since beam time is limited and you need to work round the clock to collect enough data’. The duo X-rayed each muscle before re- creating the electrical nerve signals that stimulate the tissues to contract as if the cockroach was running. ‘We had to take X-ray images several hundred times a second’, says Sponberg. Then, the duo with Weikang Ma and Thomas Irving (Illinois Institute of Technology) analysed how the muscle filaments in the static muscles pack together, only to discover that the arrangements were essentially identical. The differences in the braking and driving muscles were not down to the way in which the fibres are arranged in the muscle. However, when they measured how tightly the filaments are packed, the fibres in the brake muscle were arranged more loosely (52 nm apart) compared with the motor muscle fibres, which were packed 51 nm apart. ‘A single nanometer in the myofilament lattice is the first structural difference detected in these otherwise identical muscles’, says Tune. But, when Tune and Sponberg triggered the muscles to contract, the spacing between the motor muscle fibres increased slightly (1 nm), until it matched that of the more loosely packed brake muscles. Even this tiny (2%) change in the packing distance between muscle filaments could have a dramatic effect on how the actin and myosin filaments slide past each other to generate force. ‘The brake-like muscle is like a morning person’, says Sponberg; ‘as soon as it’s activated, it’s ready to go. However, the motor-like muscle has to stretch to get to the same lattice spacing. This expansion can shift both when and how much force it produces during a stride and, potentially, account for its difference from the braking muscle’, he concludes. 10.1242/jeb.225383 Tune, T. C., Ma, W., Irving, T. and Sponberg, S. (2020). Nanometer-scale structure differences in the myofilament lattice spacing of two cockroach leg muscles correspond to their different functions. J. Exp. Biol. 223, jeb212829. doi:10.1242/jeb.212829

18 Simulated larvae reveal why fish fry lose their dinner

Roi Holzman It almost sounds like a punishment worthy of Hades: most morsels slurped up by tiny hungry fish fry are ripped from their lips at the final instant. ‘For example, when 8 day old seabream fry attempt to capture their prey, they need five or six strikes to capture one prey item’, says Roi Holzman from Tel Aviv University, Israel. Yet, no one knew exactly how the unfortunate chain of events that snatches food from the youngsters’ mouths unfolds. Intrigued by the mystery, Holzman and Krishnamoorthy Krishnan, Asif Nafi and Roi Gurka from Costal Carolina University, USA, knew that there was only one way of finding out: by using a computer simulation of the events occurring within the fry’s miniscule mouths. But first, the team needed to understand how the tiny fish larvae move their jaws and flare open their gill covers as they slurp in water through their mouths when attempting to gulp down a titbit. Turning to Victor China’s movies of gilthead seabream (Sparus aurata) larvae – ranging in age from 7 to 37 days post-hatching (dph) – attempting to feed on one of their favourite snacks (0.16 mm long rotifers), Holzman detailed how the fry opened their mouths and gills as they slurped in water and how that changed as they grew. Over a month, the diameter of the fry’s mouths doubled to 0.5 mm and their mouths also grew longer (0.7 mm to 2 mm). In addition, the larvae were eventually able to open their gill covers almost 1 mm wide. The team then designed a simulated fish larva mouth based on the measurements of the growing larvae, shaped like an elongated rugby ball, which could open wide at the front as the mouth drew in water. In addition, the team added simulations that took account of how long it took the larva to open its mouth, how the stickiness of the water reduced as each larva grew and the way the water surged forward as the larva’s mouth gaped wide, to get a better sense of how water flowed through the minute fry’s mouths. After months of patient computer programming, the team was relieved when the first simulations successfully recreated the water flow through the cyberlarva’s mouth and out of the gill slits at the back. Also, as the cyberlarva grew, the speed of the simulated water flow through the mouth increased from 28.3 mm s−1 in the youngest larva up to 136.2 mm s−1 in the oldest larva as it swallowed water at a rate of 5.9 mm3 s−1. However, when the youngest cyberlarva closed its mouth at the end of a gulp, the team was astonished to see approximately one-tenth of the water gush back out of its mouth. This unexpected turn of events perfectly explained why food fragments that should have been swallowed suddenly popped back out of the frustrated fry’s mouth. Fortunately, the inconvenient backwash vanished when the larvae grew older (23 dph); however, if the larger fish only took a leisurely gulp, the problem came back and they too could lose their dinner. Puzzled by the young larvae’s misfortune, the team took a closer look at the trajectory of the water surging into the mouths of the tiniest simulated fry and realised the strongest outwash was generated when the mini fish opened their mouths slowly. It seems that opening their mouths fast is essential for famished gilthead seabream larvae to fill their mini bellies. 10.1242/jeb.225979 Krishnan, K., Nafi, A. S., Gurka, R. and Holzman, R. (2020). The hydrodynamic regime drives flow reversals in suction-feeding larval fishes during early ontogeny. J. Exp. Biol. 223 jeb214734. doi:10.1242/jeb.214734

19 Cyber-frog leg leaps out of reality

The epoch of genetic engineering has provided curious scientists with a previously unimaginable toolkit to begin unravelling the secrets of life. Yet, tinkering with DNA can only take us so far. When it comes to understanding how tissues such as muscle propel animals through the world, researchers need other tools, which stretch the tissue while simulating the nerve signals that drive contractions, to learn how they deliver force. But these experiments are far from the real-world experiences of muscles powering limbs. ‘We can’t currently measure how an isolated muscle would interact with the skeleton and other muscles’, says Chris Richards from The Royal Veterinary College, UK. However, when Richards came across a paper by Benjamin Robertson and Greg Sawicki from Georgia Tech, USA (doi:10.1073/pnas.1500702112), describing how they had coupled the calf muscle from an American bullfrog with a computer simulation of the leg’s movement, Richards realised that this type of virtual reality body could help him understand how frogs jump. But first, he and student Enrico Eberhard would have to construct a cyber-frog leg. First they needed a computer simulation of the limb’s movement that could perform the calculations faster than the movement itself. Richards says, ‘The speed is crucial because the simulation needs to update quickly enough for the muscle to “feel” as if it’s interacting with the simulation’. Fortunately, David Borton at Brown University had told Richards about a piece of software called MuJoCo, which calculates how a group of objects moves faster than the actual movement, so Richards knew he could simulate the movement of the frog’s leg fast enough in real time. The duo based the motion of the simulated leg on the movements of red-legged running frogs (Kassina maculata) as they leapt, substituting an African clawed frog (Xenopus laevis) calf muscle for the running frog muscle in the cyber-frog leg. ‘Most difficult was getting the muscle hardware to communicate with the simulation software’, says Richards, describing how Eberhard had to tightly coordinate the electronics recording the strength of the muscle’s contraction with the MuJoCo leg movement simulation. Once all of the components had been assembled, Richards and Eberhard were ready to throw the switch to set their in vitro virtual reality frog leaping, and calculate how the limb moved in response to each tiny step in the calf muscle contraction to recreate a single 100 ms leap. In addition, they ran a series of test leaps that a real frog could never attempt, ranging from a leg with no muscle to a muscle that just behaved like an elastic band. In addition, Richards and Eberhard investigated how well the frog pushed off when the muscle only began generating force half-way through the leap, as well as slightly altering the anatomy of their simulated frog, by shifting where the top of the calf muscle attached higher up the thigh. Not surprisingly, the cyber-frog fell flat on its face when it had no muscle to push off with and the simulation where the muscle behaved like an elastic band didn’t fare much better. However, the simulated amphibian did manage a mini-leap when the muscle kicked in half-way through the launch. Impressively, the cyber-frog produced an elegant hop when the muscle contracted normally while the modified simulated amphibian with the shifted muscle performed the highest and longest bound. ‘I was extremely happy when I saw the results’, says Richards, who is also excited about the opportunities that his new virtual reality amphibian present. ‘I plan to use this to study how evolutionary changes in skeletal morphology affect performance’, he says. 10.1242/jeb.225748 Richards, C. T. and Eberhard, E. A. (2020). In vitro virtual reality: an anatomically explicit musculoskeletal simulation powered by in vitro muscle using closed-loop tissue–software interaction. J. Exp. Biol. 223, jeb210054. doi:10.1242/jeb.210054 20 Feisty squid and fish flash back to dazzle predatory elephant seals

Martıń Ignacio Brogger After weeks marooned on beaches nursing their pups, southern elephant seal mothers have only one thing on their mind: getting back into the ocean to feast on lantern fish and squid. ‘We know how far they go, for how long, how deep they dive and that they target currents and boundaries between oceans where they find prey in large numbers’, says Pauline Goulet from the University of St Andrews, UK. However, no one was sure how the ravenous predators locate victims in the inky depths, although Goulet and her PI Mark Johnson suspected that the eerie bioluminescent glow produced by many creatures in the deep ocean might have something to do with it. ‘Bioluminescent organisms are the main source of light (80%) in waters deeper than 500 m’, says Goulet. Explaining that these animals produce two forms of light – a continual dim glow for camouflage from beneath and dazzling flashes, possibly to distract predators – the duo wondered whether the ravenous seals might capitalise on the creatures revealing themselves. Or, could the feisty prey buy themselves time by dazzling their attackers to make a getaway? Curious to know how these games of cat and mouse play out beneath the waves Goulet, Christophe Guinet from the Centre d’Etudes Biologiques de Chizé, France, and Johnson decided to catch elephant seals in the act while pursuing their pyrotechnic victims. Together, Goulet and Johnson assembled a tag that could log the seals’ movements as they hunted, in addition to recording flashes of light when the mammals encountered bioluminescent snacks. ‘Because the bioluminescent flashes are so short, typically less than a second, the tags required a very fast light sensor’, Goulet explains. Then, Guinet travelled to the Kerguelen Islands in the Southern Ocean to attach the new tags and GPS trackers to five elephant seal mothers, with the help of Julie Mestre and Hassen Hallegue. ‘There is always one person on watch for other seals when you are equipping them, because you are completely focused on what you are doing and unaware of an aggressive individual coming to bite you’, he recalls. In addition, Johnson joined Guinet and Julieta and Claudio Campagna in Argentina to tag two more seals. Retrieving four tags when the seals returned 2 months later, the team could see that most of the animals had headed off on a 3000 km odyssey deep into regions of the ocean packed with fish. However, one intrepid Argentinian seal circumnavigated Cape Horn, eventually travelling 2300 km before locating fish off the coast of Chile. Then, after months of scrutinising the seals’ manoeuvres while painstakingly analysing more than 2000 bioluminscent flashes over depths ranging from 79 to 719 m, Goulet and Johnson realised that the flashing animals were trying to scare off their attackers. ‘The prey always emits a flash the second the seal launches an attack, which suggests that the flash is a defensive reaction when the prey realises it is being attacked’, says Goulet. In addition, the seals quickly snapped up fish that failed to light up while they had a harder time capturing dinner when their snack dazzled them unexpectedly. However, one seal seemed to have turned the tables on its daring diet by tricking its victims into giving themselves away with a subtle twitch of the head that triggered a revealing flash. It seems that bioluminescent fish fight back by attempting to startle their elephant seal pursuers, but their attackers can also learn to exploit their prey’s bioluminescent betrayal. Goulet and Johnson also hope to identify which species are on the seal’s menu from the animals’ distinctive flashes when they next return to the Southern Ocean. 10.1242/jeb.226548 Goulet, P., Guinet, C., Campagna, C., Campagna, J., Tyack, P. L. and Johnson, M. (2020). Flash and grab: deep-diving southern elephant seals trigger anti-predator flashes in bioluminescent prey. J. Exp. Biol. 223, jeb222810. doi:10.1242/jeb.222810

21 Swallow mums push metabolic limits when they can keep cool

Mykola Swarnyk Parenting is not for the faint hearted. [CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0/)] Dedicated tree swallow mums and dads commit themselves to 16 h of exertion each day, snapping up insects for their ravenous offspring. And they maintain this herculean effort for up to 22 days until their young are ready to leave the nest. It seems there are no limits to the lengths these parents will go. But Simon Tapper and Gary Burness from Trent University, Canada, with Joseph Nocera from the University of New Brunswick, Canada, weren’t so sure. Tapper explains that the amount of energy a warm-blooded animal can invest in rearing its young – which is the most energetically demanding time of any animal’s life – seems to be restricted by the amount of heat it can dispose of. However, most of the evidence supporting the idea is based on the experiences of new mouse mothers. Also, it wasn’t clear how much of an effect lifestyle might have on an animal’s ability to remain cool. Thinking about parent tree swallows, Tapper and colleagues wondered whether the industrious mothers might be able to labour even harder while raising their young if they were able to lose more heat through a bald patch in their feathers. Staking out nesting boxes in two nearby nature reserves, Tapper waited until clutches of tree swallow eggs hatched before briefly capturing the new mothers and gently snipping off the feathers covering the patch of warm skin that they use to incubate their eggs. He also inserted a minute tag into the necks of both parents to record when they returned to feed their demanding brood, in addition to keeping track of the weather conditions and monitoring the hatchlings as they grew. Initially, it looked as though the trimmed mothers weren’t working any harder than females with an intact plumage; both visited their offspring roughly every 5.2 min. It was only when the team factored in the effect of the weather that the trimmed mums seemed to have an advantage. They were able to push themselves 25% harder than the untrimmed females on hot days, making an average of 43 more foraging trips per day to feed their young, although the trimmed mothers struggled to keep up with the untrimmed mothers’ feeding rate on chillier days. In addition, the trimmed mums were able to make ~3 more visits per hour to their nests than the untrimmed females when feeding large broods of 7 nestlings. Most impressively, the offspring of the trimmed mums piled on the grams faster than the hatchlings of untrimmed mothers, weighing 1.82 g more when they departed, and they had better chances of survival after leaving the nest. It seems that the exertions of tree swallow mothers are restricted by the amount of heat that they can lose from their bodies, explaining why mothers that had had feathers removed were able to forage more on hot days than those with a full set of feathers that could overheat. However, the team suspects that the boosted growth of the trimmed mothers’ chicks could have more to do with the extra warmth available through their mother’s bald patch. They are also concerned that rising temperatures caused by climate change could place chicks at greater risk, as their frenetic parents may be unable to meet their relentless demands as a consequence of heat exhaustion, making it harder for the chicks to grow swiftly to join their parents on the wing. 10.1242/jeb.227082 Tapper, S., Nocera, J. J. and Burness, G. (2020). Heat dissipation capacity influences reproductive performance in an aerial insectivore. J. Exp. Biol. 223, jeb222232. doi:10.1242/jeb.222232

22 It’s cold out, but whale sharks stay warm within

Okinawa Churaumi Aquarium, Japan Most so-called ‘cold-blooded’ animals (ectotherms) are anything but. Deriving heat from the surroundings and their own muscles, ectothermic animals are usually quite warm to the touch. However, retaining heat in water can be more of a challenge. ‘Since fish live in water with high thermal conductivity, it has been assumed that they lose most of their metabolic heat through their gills’, says Itsumi Nakamura from Nagasaki University, Japan. And body size can also affect how quickly fish lose warmth to their surroundings, with tiny species (<10 g) losing heat much more rapidly than medium sized creatures (0.1–5 kg). But Nakamura and Katsufumi Sato from The University of Tokyo, Japan, were curious to find out how much heat the largest fish of all, whale sharks (Rhincodon typus), lose while plumbing the depths. ‘They can dive to depths exceeding 1000 m’, says Nakamura, where the temperature can be 20°C lower than at the surface (~27°C). So, when Nakamura and Sato heard from Rui Matsumoto of Okinawa Churaumi Aquarium, Japan, that the facility planned to release two giant whale sharks that had been in captivity for almost 20 years, their chance had come. However, before the huge animals (~1600 kg and ~1500 kg) could be released, the scientists needed to attach a data logging tag that would record the animals’ movements, seawater temperature and the temperature of the muscle behind the head. Manoeuvring each shark into a massive container filled with seawater, the team anaesthetised the animals before gently securing the tags; ‘it took about 5–10 minutes to attach the device’, says Nakamura. Then the team carefully towed the container to a release site in the East China Sea. ‘I prayed that they would return safely to the wild’, says Nakamura, who admits that he was relieved to see that the first shark had been swimming vigorously when its tag bobbed safely to the surface 7 days later. Back in the lab after retrieving two additional tags, one from a smaller wild whale shark caught in a net off Okinawa and the other from the second captive shark, the team began to unpick how the massive animals’ body temperatures fluctuated as they swept through the water. Despite descending regularly to depths of ~100 m, where the water was 20°C, the whale shark body temperatures barely varied from 27°C. However, the first shark had plunged to 390 m immediately after release, remaining submerged for 12 h. Yet, even at this depth – where the water was 14°C – the animal’s muscle temperature only declined slowly, eventually falling to 19°C. On other occasions, the sharks plunged even further, with the first shark setting a record descent to 1427 m at a water temperature of 3.4°C. But how did the whale shark’s heat loss rates compare with those of other fish? It turns out that whale sharks don’t top up their temperature with warmth generated by their muscles and they lose heat far more slowly than any other species. Plotting the heat losses of fish ranging in size from 0.3 g bluefin tuna youngsters up to the colossal whale sharks, the team found that the whale sharks lost heat at ~0.0022°C min−1 in contrast to much smaller species that lost heat rapidly at ~1°C min−1. Nakamura says, ‘The large body size of whale sharks aids in preventing a decrease in body temperature during deep excursions’, explaining why the massive animals can dive safely to depths that smaller species never dare reach. 10.1242/jeb.227900 Nakamura, I., Matsumoto, R. and Sato, K. (2020). Body temperature stability in the whale shark, the world’s largest fish. J. Exp. Biol. 223, jeb210286. doi:10.1242/jeb.210286

23 Testosterone soups up golden-collared manakin roll-snap at expense of endurance

Golden-collared manakin (Manacus vitellinus) males like to put on a flamboyant show for the ladies. Snapping their wings together almost as fast as bees to produce a distinctive mechanical buzz – known as a roll-snap – the dainty males bound to and fro between saplings during the mating season in an attempt to outshine other nearby suitors. But the diminutive birds pay a price for their ostentatious demonstration; they can’t continue snapping their wings for long before the scapulohumeralis caudalis muscle powering the whirlwind display tires. ‘A faster roll-snap results in a display with fewer total snaps’, says Matthew Fuxjager from Brown University, USA. Yet it wasn’t clear whether the trade-off between roll-snap speed and duration occurs because the muscle is working at its physical limits, losing endurance as it contracts faster, or whether the muscle adapts in the run up to the breeding season to reduce the trade-off. Intrigued by the flirtatious little creatures, Fuxjager, Daniel Tobiansjy, Meredith Miles and Franz Goller headed to Panama to find out how the extraordinarily fast muscle performs when the hormone testosterone – which builds up the muscle in preparation for their show – can’t work its magic. The team reasoned that if physical limits were impeding the muscle’s ability to contract fast, blocking testosterone would reduce the impact of the trade-off, reducing how quickly the muscle tired at the fastest contraction rates. Giving three male golden-collared manakins a drug that prevented testosterone from having an effect for a week, the team then compared how the birds’ roll-snap muscle contractions compared with the muscle contractions of birds that still benefited from their testosterone surge. They discovered that the muscle of the birds that could respond to testosterone began tiring significantly at wingbeats faster than 67 beats s−1; they traded-off endurance for speed. However, when the team tested the muscle of the birds treated with testosterone blocker the muscle contracted more slowly; testosterone was speeding up the contraction resulting in the speed/endurance trade-off. Fuxjager says, ‘Our results suggest that testosterone speeds up the muscle so that it can produce a sexy display, but in doing so reduces the muscle’s endurance. In this way, testosterone enhances one element of the bird’s courtship signal, while impeding another at the same time’. 10.1242/jeb.227017 Tobiansky, D. J., Miles, M. C., Goller, F. and Fuxjager, M. J. (2020). Androgenic modulation of extraordinary muscle speed creates a performance trade-off with endurance. J. Exp. Biol. 223, jeb222984. doi:10.1242/jeb.222984

24 Hot minnows could struggle to navigate as temperatures rise

Climate change is going to get uncomfortable for many, but for ectotherms (so-called ‘cold blooded’ animals), which depend on their surroundings to maintain a healthy body temperature, the consequences could be disastrous. For example, warmer temperatures raise the metabolic rate for basic life support, leaving less spare energy for animals to go about their daily activities, in addition to impacting the energy use of the brain. But no one knows how these changes might affect the ability of ectotherms to negotiate their surroundings. For example, could higher temperatures produce bold super-smart fish or timid dunderheads that struggle with the simplest task? Libor Závorka from Shaun Killen’s lab at the University of Glasgow, UK, nipped to the nearby River Kelvin to catch young minnows (Phoxinus phoxinus) before raising them at different temperatures (14 and 20°C) for 8 months to find out how climate change might affect their ability to navigate their surroundings. Measuring the fish’s oxygen consumption while they lazed about and after a chase in the tank (to get their heart racing) it was clear that the fish that had grown up in extreme heat (20°C) used more energy to keep their bodies ticking over. Yet, they were able to increase the amount of energy they consumed when exercising hard, so they had as much spare energy as fish currently residing at current River Kelvin temperatures (14°C) for day-to-day activities. But when the team tested the minnows’ ability to home in on a bloodworm snack secluded in a simple maze, they realised that the fish that had been raised at 20°C struggled to secure their tasty reward, despite having larger brains. The hot fish raised in conditions that could be more common in the future simply weren’t as good at navigating as modern- day fish, even though their larger brains could have made them smarter. All in all, it seems probable that minnows will be able to keep active as temperatures rise in the future, but Killen and his colleagues are concerned that they may struggle to explore their surroundings successfully, ‘probably affecting fitness and ecological interactions’, says Závorka. 10.1242/jeb.228593 Závorka, L., Koeck, B., Armstrong, T. A., Sog˘anci, M., Crespel, A. and Killen, S. S. (2020). Reduced exploration capacity despite brain volume increase in warm-acclimated common minnow. J Exp. Biol. 223, jeb223453. doi:10.1242/jeb.223453

25 Dinosaur eels build up their fin bones for life on land

The name ‘dinosaur eel’ – otherwise known as Polypterus senegalus – sounds impressive, but these fish are not actually dinosaurs or eels, although they are genuinely extraordinary. Equipped with a pair of lungs in addition to gills, tough armour-plated scales and a stegosaurus-like dorsal fin, dinosaur eels look rather prehistoric propelling themselves slowly through water with their front fins. And the exotic creature’s idiosyncrasies don’t end there. ‘Polypterus can survive on land for Antoine Morin long periods and use their front pectoral fins to move around’, says Trina Du from the University of Ottawa, Canada; which makes them extremely intriguing for scientists wanting to understand how the first animals dragged themselves out of the primordial swamp onto land. ‘Water and land are drastically different environments requiring different strategies for animals to breathe, eat and move’, says Du. As dinosaur eels still face many of the challenges that the first land pioneers must have encountered, Trina Du and Emily Standen, also from the University of Ottawa, decided to investigate how the pectoral fins of dinosaur eels stand up to the forces of gravity when they resettle on land. After purchasing some of the fascinating creatures from a local pet supplier, the duo placed one group in a tank while the rest got to try out their land fins under two different scenarios. In one situation, the fish were provided with a small dry area, so they could clamber out of the water when they wished, in addition to being encouraged once a day to propel themselves with their fins 30 cm along a smooth surface to return to the water. In the other, the fish were provided with a gravel surface in a shallow pool (3 mm) of water, and spent 5 weeks carrying their weight on their fins. Then, Du and Standen took a close look at the effect that gravity had on the bones in the fish’s fins. Comparing the fish that had spent 5 weeks residing on gravel with the fish that had no access to land, the duo could see that the front fin bone (the propterygium, which attaches to the shoulder) of the landlocked fish was longer and stronger than that of fish that never left the water. This bone-strengthening response was also a reversal of that of birds and mammals – which strengthen bone by exercising – although the duo suspects that this difference could be down to the distinctive structures of fish and mammal bones. In addition, two of the bones (metapterygia and propterygia) were thicker after the fish lived out of water and the fine bones that fan out the fin (medial radials) had also grown longer. However, practising a 30 cm stroll each day had a different effect on the fins from 5 weeks of standing out of the water. The fish that took daily exercise only built up the medial radial fin bones, making them longer and stronger. ‘This localised response may be due to the position of the pectoral fins contacting the ground while walking’, says Standen, describing how the fin splays out as the fish presses down to propel itself forward. Dinosaur eels are able to build up their fin bones when they move from a weightless existence to life on land, although taking exercise has a different effect from simply standing around. And Du and Standen suspect that this adaptability could have been a key factor in the success of the first explorers that dragged themselves out onto the shore 380 million years ago. 10.1242/jeb.228510 Du, T. Y. and Standen, E. M. (2020). Terrestrial acclimation and exercise lead to bone functional response in Polypterus senegalus pectoral fins. J. Exp. Biol. 223, jeb217554. doi:10.1242/jeb.217554

26 Why bark beetles roam near and far

Bark beetles are the forester’s nemesis. Burrowing beneath the bark of trees to feed and lay their eggs, the beetles weed out weakened trees naturally. However, in the rolling lodgepole pine forests of North America, epidemics of mountain pine beetles (Dendroctonus ponderosae) doom stands of trees to a slow death. Kelsey Jones from the University of Alberta, Canada, explains that some female bark beetles fly tens of kilometres in search of fresh trees, while Kelsey Jones others fly only a few metres before settling and summoning backup with pheromone scents. Yet it wasn’t clear what advantage the combination of long- and short-range strategies bestows on the pests. Jones and Maya Evenden teamed up with Nadir Erbilgin, Rahmatollah Rajabzadeh and Guncha Ishanguyyeva, also from the University of Alberta, to find out how the distance a bark beetle flies affects their selection of a new home and the pheromones they produce, to understand why the insect population produces roamers and others that remain closer to home. First, the team had to get their hands on some bark beetles. Driving to remote Alberta locations during the autumn, Jones, Antonia Musso and Victor Shegelski searched for trees covered in the tell-tale balls of solidified sap produced when a tree is under attack, before felling and cutting 50 cm long sections from the trunks. Back in Edmonton, the team kept the logs in cold storage through the winter, before allowing them to warm in spring and collecting the beetles as they emerged. Jones then tested the females’ endurance as they embarked on their maiden flights by attaching each beetle to the rotating arm of a flight mill and recording the number of circuits completed over a 23 h period, discovering that, on average, the insects covered a distance of 4 km, ranging from 2 m to over 22 km. Then, she weighed the females, to find out how much body mass they lost during their odyssey, before offering them a fresh log to burrow into. Timing how quickly the beetles began burrowing – an indication of how much energy they had in reserve to set up home – Jones then inserted a funnel into the mouth of the freshly carved beetle gallery to collect the pheromone (trans- verbenol) that the female produced during the first day. Finally, she tethered males in the flight mill for a day, to simulate their pursuit, before uniting each with a female in her channel and collecting the pheromones, trans-verbenol and the male pheromone exo- brevicomin, over the following 4 days. Jones discovered that the distance flown by the female beetles had little bearing on their interest in burrowing into new logs. However, the females that had lost the least mass (less than 10%) seemed the most enthusiastic about boring into the log, although they also produced the least trans-verbenol pheromone. In contrast, the females that flew the furthest and lost the most mass produced the most pheromone, advertising their arrival. Meanwhile, the heaviest males at departure and the males that flew the furthest produced the most exo-brevicomin, regardless of how much mass they lost during flight. So, it seems that using a large amount of energy when venturing afar sets back pioneering females, favouring females that have more energy to provide their eggs after remaining closer to home. However, females that embark on a long-haul flight produce more pheromone; they have a better chance of attracting males from distant populations, improving the genetic health of their offspring. ‘Both flight strategies are beneficial to mountain pine beetle reproduction in different ways. As short- and long-distance fliers reproduce successfully, both strategies will remain in the population, maintaining genetic variability’, says Jones. 10.1242/jeb.227991 Jones, K. L., Rajabzadeh, R., Ishangulyyeva, G., Erbilgin, N. and Evenden, M. L. (2020). Mechanisms and consequences of flight polyphenisms in an outbreaking bark beetle species. J. Exp. Biol. 223, jeb219642. doi:10.1242/jeb.219642

27 Zebra finches adapt to cope well with extreme conditions

Simon Griffith Cossetted domestic zebra finches have a cushy lot compared with their rugged wild cousins in the Australian outback. ‘Maximum daily air temperatures routinely exceed 40°C and intense sun adds to the heat load the birds must endure while foraging and drinking’, says Christine Cooper from Curtin University, Australia. But even these robust little creatures could begin to succumb to the heat as climate change takes hold. ‘Extremely hot weather can cause large numbers of birds to die’, explains Cooper. However, it wasn’t clear whether the diminutive aeronauts have sufficient physiological versatility to protect themselves from the extremes that may yet come. As adaptability is going to be key to the survival of many species, Cooper and her colleagues Laura Hurley and Simon Griffith from Macquarie University, Australia, and Pierre Deviche from Arizona State University, USA, wondered how the gregarious finches cope after riding out a heatwave, on average summer days and when the weather is scorching. ‘Fowler’s Gap is a research and sheep station in a remote, dry region of Australia, with huge expanses of low, sparse vegetation’, says Cooper, who travelled there to investigate the free-ranging zebra finches. ‘Before the study, the birds had experienced a long, hot dry period of 2 years without rain’, says Cooper; however, the staff had previously installed a drinking trough, which was popular with the finches. Keeping an eye on the weather conditions between December 2018 and February 2019, the duo caught visiting zebra finches when the temperature had been very high (above 39°C) for 3 days, or relatively mild (between 27 and 36.3°C). Holding the birds in captivity briefly, Cooper and Hurley weighed them, measured their body temperature, metabolic rate and water loss rate at 30°C and 40°C, as well as collecting tiny blood samples to find out how well the birds dealt with stress after periods of hot and milder weather. Despite the different conditions that the birds had experienced in the run up, they all weighed around 11 g. They were not dehydrated – even though some had experienced temperatures in excess of 45°C – presumably because they were regularly gulping down water at the trough. The tiny birds also managed to protect themselves from dangerous water loss by letting their body temperatures rise from ~38°C to ~41.5°C at 40°C. However, the finches that had been captured after a cooler period were less well prepared for 40°C heat than the birds that had just ridden out a heat wave; the cooler birds lost 1.2% body water per hour compared with the heatwave birds, which only lost 0.97% body water per hour. In addition, the birds that had just experienced a heatwave were able to reduce their metabolism and the consequent heat production when the temperature was high. In short, the birds that had just emerged from a heat wave were better prepared for hot conditions than their cooler counterparts. And, when the duo checked for signs of stress in the birds’ blood, there was no evidence that the zebra finches that had just passed through a heatwave were struggling any more than their cooler counterparts. The stress hormone levels in their blood were essentially identical, regardless of the conditions, meaning that the birds didn’t seem to get too bothered about being hot. At the moment, thanks to their versatility, wild zebra finches cope well with the extreme temperatures that come their way during sizzling Australian summers and Cooper is keen that scientists take this ability to adjust into account when predicting how species will cope as climate change sends the mercury higher. 10.1242/jeb.228981 Cooper, C. E., Hurley, L. L., Deviche, P. and Griffith, S. C. (2020). Physiological responses of wild zebra finches (Taeniopygia guttata) to heatwaves. J. Exp. Biol. 223, jeb225524. doi:10.1242/jeb.225524 28 Parrots discard dowdy pigments in favour of own brand

Bedecked with a palette of colours spanning every shade of the rainbow, with black and white thrown in for good measure, parrots are some of the gaudiest creatures on the planet. Ismeal Galván, from the Doñana Biological Station, Spain, explains that most ostentatious creatures depend on a family of pigments known as melanins to put on a vivid show. However, parrots appear to have branched out, evolving their own specialised pigments, psittacofulvins, which produce shades ranging from red and orange to yellow. These alternative pigments correspond to the shades produced by one particular melanin pigment, pheomelanin, so Galván and Ana Neves, from the Federal University of Rio Grande do Norte, Brazil, wondered whether the lurid birds may have done away with the slightly dowdier pheomelanin pigments altogether to turn up their brightness. After teaming up with Dirk Van den Abeele, from Ornitho-Genetics VZW, to locate parrots ranging from the Nymphicus hollandicus cockatiel to the massive kea (Nestor notabilis) from New Zealand, the trio asked the birds’ owners to send one or two feathers from dull patches of yellow, orange, red or brown plumage, as they realised that these duller shades could be produced by either pheomelanin pigments or psittacofulvins. Then, Galván shone a specialised laser on the feathers, recording the reflected light, looking for the unique signal that would tell him whether the pigments creating the colour within each feather were pheomelanins or psittacofulvins. However, after analysing feathers from 28 different species, none contained any pheomelanin at all. The parrots had switched completely from using pheomelanin to colour their feathers to the psittacofulvin pigments, although they continue to produce another melanin pigment, eumelanin, responsible for shades of black and dark brown feathers. The team also explains that parrots are unable to make use of another group of red-yellow pigments – carotenoids, which also protect animals from metabolic toxins in their blood – to colour their feathers. Parrots have discarded many of the conventional pigments favoured by less vivid species in favour of their own more brilliant psittacofulvin palette, even though they still carry carotenoid pigments in their blood for protection. 10.1242/jeb.229591 Neves, A. C. d. O., Galván, I. and Van den Abeele, D. (2020). Impairment of mixed melanin-based pigmentation in parrots. J. Exp. Biol. 223, jeb225912. doi:10.1242/jeb.225912

29 Potassium leak short circuits trout heart at high temperatures

Liquid Art CC BY-SA Some fish could be facing the fight of (https://creativecommons.org/licenses/by-sa/4.0). their lives as temperatures rise. Jaakko Haverinen and Matti Vornanen from the University of Eastern Finland explain that there may come a point when their hearts simply can’t take the heat any longer and give out. But what brings about this fatal heart failure at high temperatures? Scientists have known for almost a century that the heart ventricle fails before the atrium at high temperatures, but it wasn’t clear whether the pacemaker that drives the beating atrium fails, causing the heart to beat dangerously slowly, or if the ventricle becomes less sensitive to the electrical currents that keep the heart ticking over. To discover which option sends trout hearts into collapse when the mercury rises, Haverinen and Vornanen began recording ECGs as they gradually increased the fish’s temperature from a comfortable 12°C up to 27°C. At normal temperatures (12°C), the bursts of electrical current in the two heart chambers (the atrium and the ventricle) that maintain the heart’s rhythm were perfectly synchronised and remained so even as the temperature increased. But when the temperature hit 25°C, things began to go wrong. The coordination between the two heart chambers disintegrated: the atrium beat rate rocketed to 188 beats min−1 at 27°C while the ventricle failed to keep pace as its beat rate dipped to 111 beats min−1. ‘The heart has two separate beating rates at critically high temperatures, one for the atrium and another for the ventricle’, say Haverinen and Vornanen. They realised that the ventricle was becoming less sensitive to the electrical currents that drive the heartbeat. Next, the duo focused on the currents flowing into and out of heart muscle cells and tracked the failure back to ion channels that control the entry and exit of the sodium and potassium ions carrying the electrical currents that drive contractions. Placing minute pipettes filled with either potassium or sodium ions on the surface of muscle cells, they were able to record potassium ions leaking from the muscle at 25°C, making it impossible for sufficient sodium to flood into the cell to trigger a contraction. The researchers conclude that the ventricle is the weak point that fails at high temperatures when potassium leaks from the muscle cells, preventing it from contracting. They add, ‘the sequence of events from the level of ion channels to the cardiac function in vivo provides a mechanistic explanation for the depression of cardiac output in fish at critically high temperature’. 10.1242/jeb.230169 Haverinen, J. and Vornanen, M. (2020). Reduced ventricular excitability causes atrioventricular block and depression of heart rate in fish at critically high temperatures. J. Exp. Biol. 223, jeb225227. doi:10.1242/jeb.225227

30 Surfing behind rocks costs trout dear when feeding

Despite appearances, life in the fast lane may not be as tumultuous as it might seem at first. Many fish species choose to reside in fast- flowing sections of river as the turbulence provides them with an almost free ride when sheltering downstream of rocks in the flow. ‘Prevailing theory suggests that Jimmy Liao many species exploit hydrodynamic refuges to minimise the cost of locomotion while foraging’, says Jimmy Liao from the University of Florida, USA. But he was curious to find out how much exertion it takes for fish sheltering behind rocks to catch a snack. Explaining that a lifetime of reading fly-fishing magazines had convinced him that fish use less energy when foraging in choppy water than in more sluggish sections, Liao, with Jacob Johansen and Otar Akanyeti, put the anglers’ perceived wisdom to the test. Slowly increasing the water flow in an artificial stream in the lab to 68 cm s−1, Liao and Johansen provided individual trout with a D-shaped column to shelter behind while measuring how much oxygen they consumed. Then they tricked the hungry trout into nipping out from behind the obstacle to snap at a tasty treat drifting past on a line, before wrenching it from the surprised fish’s lips; ‘They would sometimes chase it upstream’, he chuckles. However, when Liao compared the fish’s oxygen consumption with that of fish swimming freely in an unobstructed flow, he realised that the fish that were ensconced behind the D-shaped obstacle had to work 65% harder when snapping at the lure than fish that were feeding in a freely flowing stream. And, when Liao and Johansen monitored the fish’s reactions as tempting food pellets drifted toward the animals, the sheltering fish struggled to intercept their treats as the flow increased, barely capturing any pellets at the fastest flow (84 cm s−1), compared with the unobstructed fish, which still captured 60% of the pellets. Even though trout save energy when surfing in the wake of an obstacle, venturing out of the wake to feed requires more effort and their success rate plummets in faster flowing sections of water. Taking these lessons into account, Akanyeti and Liao built a computer model of the energetics of a fish feeding in a river to help them understand when it is best for fish to take advantage of shelter and when the benefits of refuging no longer outweigh the expense of foraging from behind a rock. Running the simulated river at flows ranging from still water up to 84 cm s−1, the team found that sheltering behind a rock was more beneficial in slower flows, while freely swimming fish appeared to have the upper hand in torrents of faster water; ‘Our model predicts that individuals living in flows greater than 50 cm s−1 should avoid refuges while foraging’, says Akanyeti. The simulation also suggested that toggling between hiding behind rocks for part of the day and venturing into faster smooth water at other times to feed benefits the fish most, allowing them to take advantage of both worlds. Although it seems that life is more costly for refuging trout than anglers had presumed, Liao suspects that the shrewd fish also make the most of other energy-saving shortcuts that we have yet to discover. He hopes that engineers designing slipways alongside dams for migrating fish and ecologists restoring river beds can learn from the life experiences of trout. ‘I’d like to see us more often asking the fish themselves what they want; which means ground-truthing designs based on fish behaviour’, he says. 10.1242/jeb.230086 Johansen, J. L., Akanyeti, O. and Liao, J. C. (2020). Oxygen consumption of drift-feeding rainbow trout: the energetic tradeoff between locomotion and feeding in flow. J. Exp. Biol. 223, jeb220962. doi:10.1242/jeb.220962

31 Pygmy mice whistle for the audience

It might just be squeaking to you and me, but to the ears of northern pygmy mice (Baiomys taylori), the shrill cries of their own can be a romantic serenade. Describing how the feisty little rodents rear up on their hindlegs to summon others, Bret Pasch from Northern Arizona University, USA, quotes from a research paper dating back to 1941, saying, ‘The staccato-like song of pygmy mice is described as a “high-pitched, barely Bret Pasch audible squeal” produced with the head “thrust forward and upward, stretching the throat”’. But there is an additional ultrasonic component of the rodent’s ardent aria, stretching well beyond the limits of our hearing. Explaining that many small rodents use either their vocal chords to produce lower pitched calls or whistles to hit ear-splitting highs, Tobias Riede from Midwestern University, USA, and Pasch, teamed up to find out how the most diminutive rodent in North America produces its extraordinary vocal range. ‘Pygmy mice are relatively rare’, says Pasch, describing how the tiny animals live in the desert grasslands of Arizona and New Mexico, and recalling how he often encountered rattlesnakes when retrieving the mini rodents from traps. Once the mice were safe in the Northern Arizona University lab, it was simply a case of recording the obliging creatures’ serenades. ‘Mice routinely sing spontaneously or if they encounter the voices or odour of a potential mate’, says Pasch. However, when the duo analysed the pygmy mouse’s vocal range, they were surprised that the voices of the 10 g rodents were much deeper than they had expected, ranging from 16 to 40 kHz. ‘Traditionally, our understanding is that vocal pitch is driven by the size of the vocal organ’, says Riede, explaining that the voices of smaller animals are always squeakier than the voices of larger creatures. Yet, the highest tones in the pygmy mouse’s range were only as high as those of lab rats, which are 40 times larger. Then, the team played a trick on the squeaky pygmy mice, replacing the air in their cages with heliox – a mixture of oxygen and helium inhaled by deep-sea divers – which shifts the pitch of whistles without altering the pitch of calls produced by vibrating vocal chords, to distinguish the origins of different tones in the rodent’s repertoire. And when the mice switched to breathing heliox, every tone in their vocal range became squeakier. The duo realised that instead of the voluble rodents using their vocal chords to make calls over the lower range of their repertoire, every syllable they articulated was a whistle. In addition, each syllable of the serenade became shorter, probably because the flow of the lighter gas through the animal’s tiny voice box is different from that of air, making the puffs of gas they exhale to whistle less effective. But how were the bijou animals able to produce such deep whistles when their frames are so tiny? Riede examined the structure of the tiny creature’s voice box. Dissecting the minute vocal organs, which are the smallest that Riede has ever investigated, he discovered that pygmy mice have an unexpectedly large pouch that expands the size of the voice box, allowing the tiny creatures to produce the deeper tones (~16 kHz) by whistling, instead of using their short vocal chords like other rodents. So, pygmy mice produce ultrasonic squeals by whistling, to evade the hearing of predators. Having discovered the secret of the maverick mammal’s relatively deep voices, Pasch and Riede are eager to discover how the mouse’s voice compares with those of other rodents to learn about the evolution of their communication. 10.1242/jeb.229427 Riede, T. and Pasch, B. (2020). Pygmy mouse songs reveal anatomical innovations underlying acoustic signal elaboration in rodents. J. Exp. Biol. 223, jeb223925. doi:10.1242/jeb.223925

32 Cabbage whites have a unique take on polarized light

Adam Blake Cabbage white butterfly females are choosy. They aren’t happy to deposit their eggs on any old bit of foliage; for them, it’s members of the Brassicaceae family or nothing. And, when Adam Blake and colleagues from Simon Fraser University, Canada, tried to interest the picky insects in other pieces of vegetation, it turned out that the degree of polarized light reflected by leaves played a large part in the choices of the butterfly mums-to-be. The selective females prefer to settle on leaves that reflected 25–35% of linearly polarized light, like tasty cabbage leaves, but turn up their noses at potato plants that reflect 45–55% linearly polarized light. But how were the keen insects’ eyes able to distinguish between the degree of linearly polarized light reflected by cabbage leaves and that reflected by the verdant foliage of potato plants? Intrigued by the pernickety females, Blake and PI Gerhard Gries set the insects a TV quiz to get to the bottom of how they perceive the difference between potato leaves and cabbages. Having already discovered that cabbage white butterflies prefer pictures of cabbage- shaped plants, Gina Hahn, Hayley Grey, Shelby Kwok, Deby McIntosh and Blake set about adjusting the colours and degree of linear polarization present in the light emitted from a TV screen showing a shot of a cabbage plant to find out how the changes affected the insects’ preferences. However, Blake explains, ‘Despite using identical monitors controlled by the same hardware and software, it took a great deal of effort to get both monitors to output similar colours (as measured by their spectra) when showing identical images’. Eventually, after months of painstakingly tinkering to synchronize the screens’ colours, the team was able to offer the butterflies a choice between two cabbage images – one with colours that had been tweaked – to begin to understand how light polarization affects their perception of colour. The team removed red or blue shades from the cabbage images, in addition to altering the degree of light polarization, and, as the cabbages’ colours changed, the butterflies’ preferences began shifting all over the place. In one case, the butterflies consistently preferred bluer cabbages (lacking a hint of red) with 51% polarization over bluer cabbages with 31% polarization, while they preferred yellower cabbages (with no blue in their spectrum) when the light was horizontally polarized with 31% polarization, but switched preference to 51% polarization when the light was vertically polarized. The team realised that instead of depending on one, or a combination of two, types of light receptors in their retina – tuned to either red, green or blue light – to detect polarized light, the cabbage whites must be using all three colour-tuned photoreceptors. In addition, the insects were perceiving the polarized light differently, depending on the shade and brightness of the colour. And when the team compared the cabbage white’s colour preferences with those of other butterflies, it was apparent that the impact of polarization on the cabbage white’s perception of colour is unique. Blake explains that horizontal polarization makes shades of green brighter for swallowtail butterflies; however, the cabbage white’s preferences revealed that the effects of light polarization did not make any particular colour brighter in their eyes. ‘Cabbage white butterflies respond to polarized light differently from not just other butterflies, but from all of the other polarization-sensitive animals studied thus far’, he says. 10.1242/jeb.230441 Blake, A. J., Hahn, G. S., Grey, H., Kwok, S. A., McIntosh, D. and Gries, G. (2020). Polarized light sensitivity in Pieris rapae is dependent on both color and intensity. J. Exp. Biol. 223, jeb220350. doi:10.1242/jeb.220350

33 Chilly rattlesnakes strike slower, but not as slow as expected

Grace Freymiller You know that feeling when it’s simply too hot to move; the heat is so oppressive that all you can do is sit indoors with a cool drink? Surprisingly, rattle snakes also struggle with heat in the summer, only they can’t resort to the cool drink. ‘The habitats of many rattlesnake species become too hot during the day’, says Malachi Whitford from San Diego State University and the University of California, Davis, USA, so the reptiles’ solution for the problem is to become nocturnal and hunt at night, but that results in an alternative drawback. ‘Night time temperatures can get quite cold’, says Whitford, explaining that lower body temperatures could slow the snakes’ lightning fast reactions. Which made Grace Freymiller, Timothy Higham, Rulon Clark and Whitford wonder how much of an effect chilly conditions might have on the snakes’ abilities to catch dinner. ‘Safety for the snake and for us is very important…we never put ourselves within the strike range of the snake’, says Whitford, describing how he and his colleagues corralled the vipers with snake hooks and tongs when collecting Mohave rattlesnakes (Crotalus scutulatus) in New Mexico and western rattlesnakes (Crotalus oreganus helleri) in southern California. Back at San Diego State University, Whitford waved an inflated balloon in front of the animals in an attempt to get them to land a bite. ‘We wanted something that would not injure their fangs, which are fairly delicate’, he says, adding that the snakes seem unperturbed when the balloon exploded. ‘Snakes do not have the best hearing…so the pop of the balloon did not startle them – although it made me jump on more than a few occasions’, he laughs. Filming the snakes striking the balloon at temperatures ranging from 15 to 35°C, the team could see that the warmer snakes were definitely the liveliest. ‘One Mojave rattlesnake, nicknamed Hulk, was renowned for being quite intimidating. At 35°C, he would strike very quickly at anything that moved and constantly try to escape the enclosure’, Whitford recalls. Sure enough, when the team the measured the speed of the snakes’ movements, the chilliest snakes were more sluggish than the hottest, with the coldest snakes striking at speeds of up to 3.73 m s−1, while the hottest snakes launched themselves at the balloon as fast as 5.26 m s−1. The coolest reptiles also opened their mouths more slowly and were less likely to have a go at a balloon victim. However, when the team compared the impact of temperature on the snakes’ attack speeds, the coldest snakes were moving faster than the team would anticipate if their speed was set by body temperature alone, suggesting that the chilly animals also rely on springy tendons to help catapult themselves at a potential snack. In addition, the hottest snakes landed fewer successful strikes than the colder animals, although Whitford suspects that the hot animals might have been pulling back a little, in order to scare off potential threats, instead of trying to plunge their fangs in. Even though cold rattlesnakes are more vulnerable, Whitford cautions that the feisty animals are still lethal. ‘A cold rattlesnake is still a very dangerous animal, so don’t try to pick it up!’ he warns. 10.1242/jeb.230938 Whitford, M. D., Freymiller, G. A., Higham, T. E. and Clark, R. W. (2020). The effects of temperature on the defensive strikes of rattlesnakes. J. Exp. Biol. 223, jeb223859. doi:10.1242/jeb.223859

34 Minute mecysmaucheniid spider triggers fastest trap-jaws

When Hannah Wood first encountered tiny mecysmaucheniid spiders scurrying through the leaf litter in Chile, she had no idea that some of the minute arachnids would have more in common with a family of ants – trap-jaw ants – that attack their victims with spring-loaded jaws. Instead of sitting enthroned in a web, the feisty spiders grasp their quarry in a vice-like grip with a pair of fierce-looking chelicerae – jaws tipped with fangs – which snap closed to ensnare their prey. However, some members of the family performed the manoeuvre so fast that it could not be powered by muscle alone. Knowing that trap-jaw ants depend on springy structures held open by a latch prior to slamming their jaws shut, Wood travelled to New Zealand in search of other members of the mecysmaucheniid family, to discover how the spiders bring their jaws crashing together. After sifting the leaf litter through a fine mesh to collect the 2–4 mm long spiders, Wood goaded the animals into clamping their jaws shut by prodding them with an eyelash attached to a pin as she filmed the chelicerae slam closed at speeds of up to 100,000 frames s−1. Focusing on two of the closest members of the family, Zearchaea spp. and Aotearoa magna, Wood realised that Zearchaea clamped their jaws shut at blistering speeds of up to 18.2 m s−1. ‘This is the fastest arachnid movement ever documented’, she says. In contrast, Aotearoa closed their jaws more sedately, only reaching average speeds of around 0.09 m s−1. Then, Wood calculated the power necessary to produce Zearchaea’s eye-watering accelerations and realised that the spiders would require a muscle that could produce 670,000 W kg−1 of power – which is completely impossible – to close their jaws in 0.07 ms, although Aotearoa jaw muscles should be more than capable of powering their slower closure. Knowing that Zearchaea must depend on a spring-loaded catapult to provide the enormous power required to clamp their jaws shut, Wood CT scanned the minute spider’s heads with Dula Parkinson at the Lawrence Berkeley National Lab Advance Light Source synchrotron to reveal how the jaws of both species connected to the carapace through muscles and ligaments. The slower Aotearoa jaws were equipped with nine pairs of muscles – one pair to pull the jaws open while the remaining eight muscle pairs clamp them shut. Zearchaea, in contrast, seem to have done away with three of the pairs of closure muscles. Focusing on the muscles that lever the jaws open in both species, Wood noticed that they rotate the jaws upwards before opening them wide in preparation for an attack; but there the similarities ended. Although she could see that four of the pairs of Aotearoa muscles were required to squeeze the chelicerae together, Zearchaea jaws seem to be slammed shut by a powerful catapult mechanism. By building a 3D printed model of the spider’s head, Wood realised that a pair of springy ligaments linking Zearchaea’s chelicerae to the carapace could store energy like a catapult when they are stretched as the jaws open, before being locked in place by a piece of the exoskeleton close to the jaw bases. Wood suspects that this latch is only released when muscles attached to the hinge linking the two chelicerae yank it back, allowing the stretched ligaments to recoil, slamming the jaws together. While Zearchaea’s spring-loaded jaws definitely earn them a place in the arachnid record book, Wood is also keen to understand why the close relatives evolved such different strategies in the relatively brief time since they last shared a common ancestor. 10.1242/jeb.231407 Wood, H. M. (2020). Morphology and performance of the ‘trap-jaw’ cheliceral strikes in spiders (Araneae, Mecysmaucheniidae). J. Exp. Biol. 223, jeb219899. doi:10.1242/jeb.219899

35 Knuckle-walking chimpanzees go 3-D with ‘Avatar’ technology

Although the majority of babies crawl on all fours, some shuffle on their bottoms and others crawl like commandos; but they never crawl like our nearest relatives. ‘Knuckle-walking is a strange hand posture that gorillas and chimpanzees use when walking quadrupedally’, says Nathan Thompson, from the New York Institute of Technology, USA. Yet, no one really knows how this unusual mode of walking evolved. One idea is that the long fingers of ape ancestors got in the way when they descended to the ground, so they simply Nathan Thompson rolled their fingers into a ball and walked on the knuckles instead. But what really intrigues Thompson is the possibility that the earliest humans may also have descended from the same ancestors, making knuckle-walking part of our history too. However, before we can begin searching the fossils of our ancient ancestors for evidence that they walked on their knuckles, we need to understand how knuckle-walking chimpanzees differ from other primates today, and how this impacts their skeletons to identify key features that could resolve whether our ancestors knuckle-walked. Thompson turned to 3-D motion capture to record every detail of the chimpanzees’ movements. He painted non-toxic dots on the joints of two animals’ hands and feet, and on their limbs, and then arranged four synchronised high-speed cameras around the runway as the chimpanzees knuckle-walked at their own speed. ‘It’s basically the low-budget version of “Avatar”’, he says. In addition, Thompson filmed two macaques – which walk either with their palms and fingers flat, or with the fingers flat and the palm tilted up – for comparison with the rolled knuckle movements of the chimpanzees. Then he painstakingly reconstructed how each bone and joint moved. Looking at the position of the arm and hand as the chimpanzees placed their knuckles on the ground, Thompson realised that the chimpanzee’s hands and forearms were rotated inward more than the macaques’, positioning the chimpanzees’ hands parallel to their bodies as their knuckles contacted the floor, while the macaques’ hands pointed forward. He also noticed that the chimpanzees tended to bend their wrists sideways (ulnar deviation) more than the macaques. ‘The amount of ulnar deviation used seems to be one of the differences between macaques and knuckle-walking chimpanzees’, he says. However, when scrutinising the animals’ wrists, Thompson realised that the positions were often more similar than had been appreciated previously, especially when the chimpanzees were knuckle-walking and the macaques were tilting the palm up while holding their weight on their flat fingers. In addition, Thompson was surprised by how far back the macaques bent their fingers while walking with flat palms (the fingers bend back as they lift the palm off the ground) and when walking on flat fingers with the palm tilted. Some primates, including chimpanzees and gorillas, have a bony ridge at the base of their knuckles (the metacarpophalangeal joint, where the fingers join the palm), which was thought to help stabilize the joint when walking with the palm off the ground. But because the macaque’s fingers bend back the same regardless of how the hand is positioned, Thompson realised that the ridge cannot be used as a feature to diagnose whether our fossil ancestors walked on their knuckles. ‘The presence of this bony ridge might not have a straightforward relationship to knuckle-walking or other hand postures’, he says. Having identified which aspects of the animals’ movements distinguish knuckle-walkers from conventional primates to help identify skeletal structures that could be signatures of knuckle-walking, Thompson is eager to learn how chimpanzees use their muscles while striding on their knuckles in the hope of getting to the crux of why apes adopted this unconventional way of getting about. 10.1242/jeb.231860 Thompson, N. E. (2020). The biomechanics of knuckle-walking: 3-D kinematics of the chimpanzee and macaque wrist, hand and fingers. J. Exp. Biol. 223, jeb224360. doi:10.1242/jeb.224360 36 Tobacco hornworms change stride when the going gets different

Barry Trimmer Unrestrained by a skeleton, tobacco hornworms are remarkably agile pests, rippling their bodies to scale tomato and tobacco plants, ready to gorge in preparation for their miraculous transformation into adult moths. The adaptable insects are capable of contorting themselves to remain in contact with almost any surface, ranging from stable ground to fluttering leaves, while manoeuvring between dining opportunities. But Barry Trimmer from Tufts University, USA, explains that the moth larvae’s squidgy bodies pose a unique set of challenges as they shuffle along. ‘Soft, deformable animals face very different biomechanical challenges compared with articulated animals’, he says. For example, animals equipped with skeletons often change stride when they encounter a new surface or switch to climbing a hill, which made Trimmer wonder whether the versatile caterpillars also adapt their movements when encountering a new situation. Together with Cinzia Metallo and Ritwika Mukherjee, Trimmer began investigating how the distinctive caterpillars crawled on soft and hard, vertical and horizontal surfaces, to find out whether they adjust their stride. After building a caterpillar-sized treadmill and constructing a super-springy Dragon Skin® silicone drive belt for the insects to saunter along, the trio set three hornworms strolling horizontally before flipping the treadmill upright and setting the caterpillars crawling upward and downward as they filmed the insects’ footwork. They also tested the insects’ manoeuvres as they scaled and strolled along a more rigid rubber walkway, to find out whether the creepy-crawlies adjusted their stride. Then, the scientists focused on the four prolegs – which bear the rear end of the 70 mm long insects – as the caterpillars crawled horizontally on the most rigid treadmill belt. The majority of the time, the plump caterpillars concertinaed the body forward at the start of a stride, lifting the rear-most proleg (the sixth) first, before the fifth, fourth and third rippled up. Then, once all four prolimbs were off the treadmill, the caterpillar lowered its sixth proleg back down as it extended its body, swinging each proleg forward and placing it in turn on the treadmill belt. However, on 20% of occasions, the smoothly rippling legs fell slightly out of sync, with the sixth proleg touching down on the treadmill before the third proleg was raised. ‘This change in rhythm is reminiscent of the way other animals change gait at different speeds or to accommodate a new terrain’, says Trimmer. The team also scrutinised the caterpillars ascending the treadmill vertically, noticing that the caterpillars resorted to this alternative footfall pattern more often, replacing the sixth proleg before raising the third 51% of the time, while the descending caterpillars preferred the alternative gait 65% of the time. And, when they provided the caterpillars with the springier Dragon Skin® treadmill belt, the insects relied much more on the stability of replacing the sixth proleg before the third lifted up, resorting to it 46% of the time on the flat, 73% when climbing and almost 80% when descending. ‘Our results show that caterpillars change their stepping patterns when they climb in different directions or when they are on different surfaces’, says Trimmer, adding that the caterpillars seemed to prefer stride patterns with more prolegs in contact with the treadmill for stability when the going was soft and when tackling an ascent or descent. The results also suggest that caterpillars are able to sense differences in the materials over which they are moving and adjust their stride accordingly. ‘These findings make us wonder how the stepping patterns are controlled by the nervous system and how caterpillars can tell that they are walking on different surfaces’, says Trimmer, who is curious to understand how the larvae feel these differences. 10.1242/jeb.230854 Metallo, C., Mukherjee, R. and Trimmer, B. A. (2020). Stepping pattern changes in the caterpillar Manduca sexta: the effects of orientation and substrate. J. Exp. Biol. 223, jeb220319. doi:10.1242/jeb.220319 37 Flexible sea butterflies embrace to thrust

Ferhat Karakas and David Murphy Remarkable as it may seem, some snails – sea butterflies – have more in common with the insects buzzing in your garden than the molluscs munching on your brassicas. Limacina helicina, a species of sea butterfly, waft through the Pacific Ocean much like flying insects, flapping their pseudopod wings in the same figure-of-eight wingbeat pattern. But David Murphy from the University of South Florida, USA, explains that sea butterfly wings are far more flexible than the rigid wings of insects. To find out whether this flexibility affects how some sea butterflies swim, Leocadio Blanco-Bercial, from the Bermuda Institute for Ocean Sciences (BIOS) and Ferhat Karakas, from the University of South Florida, set sail in the tropical ocean off Bermuda in search of members of the sea butterfly family to discover more about their unconventional swimming styles. ‘We have a lobster pot hauler that we use to deploy and retrieve the net’, says zooplankton ecologist Amy Maas, also at BIOS, describing how the net sinks down 100 m before they haul it back to the surface. Back on shore, Maas identified each of the 30 or so sea butterflies retrieved from the depths, before rushing the molluscs to Murphy and Karakas in the lab to film the animals’ manoeuvres in 3D. Placing several of the 0.05–1 cm long animals in a tiny tank, Murphy trained two high-speed cameras on the centre of the tank and hoped that some of the molluscs would swim into view. ‘There were high fives all around’, says Murphy, recalling the moment when their patience was rewarded and the first Cuvierina atlantica wobbled into shot, swinging back and forth like a pendulum, swimming at 3.5 cm s–1. Slowing the movies and reconstructing the flapping motion of the wings in 3D, Murphy and Karakas realised that C. atlantica have evolved a completely unique flapping style. Instead of pulling their wings back and clapping them together before peeling them apart to generate lift and propel themselves forward – like L. helicina – C. atlantica sweep their wings widely from front to back, overlapping the tips in an embrace as they meet, forming a cylinder. Cuvierina atlantica then angle the wings upward while peeling them apart and sweeping them wide around to the other side of the body where they meet again in another embrace during the second half of the wingbeat cycle. ‘We also put a lot of algae in the water – much higher than would occur in nature – so that we could track the water motion’, says Murphy, describing how they filmed the water’s movement with a high-speed camera through a microscope lens to track the trajectory of the algae and hence the water flow, with the added advantage that the molluscs could also dine on the morsels. And when the team analysed the swirling motion of the water, they realised that the sea butterflies generated lift when they tilted the wings back when opening out of the first embrace, before forcing a propulsive jet of water down as the wings came together to form the second embrace on the other side of the body. By flexing their wings and bringing them together on both sides of their elongated shell and body, C. atlantica are able to generate thrust on two occasions during each wingbeat cycle, in comparison with sea butterflies that swim like flapping insects, which only generate thrust once. And Murphy is excited about C. atlantica’s flexible approach, which has inspired him to build a pneumatically operated soft-bodied robot to learn more about their exotic propulsion mechanism. 10.1242/jeb.232546 Karakas, F., Maas, A. E. and Murphy, D. W. (2020). A novel cylindrical overlap-and-fling mechanism used by sea butterflies. J. Exp. Biol. 223, jeb221499. doi:10.1242/jeb.221499

38 Puffin hearing unaffected by amphibious lifestyle

My diving is terrible – I can never clear my ears properly, even within 1 m of the surface – which makes the achievements of diving Atlantic puffins even more impressive, plummeting 150 m down with no apparent concern for their ears. ‘It is conceivable that the air-filled sinuses and auditory abilities are modified in auks, perhaps to withstand deep dives or enable underwater hearing’, says Aran Mooney, from the Woods Hole Oceanographic Institution, USA. Yet, it wasn’t clear how the puffins’ amphibious lifestyle might impact their hearing out of water and whether they are distressed by noise. Mooney explains that other species are clearly disturbed by human noise, spending less time on the nest when people are around. Joining Ole Larsen and Kirstin Hansen, from the University of Southern Denmark, and Marianne Rasmussen, from the University of Iceland, Mooney and Adam Smith travelled to northern Iceland to collect puffins as they emerged from their burrows, to test their hearing. ‘The burrows are often on high cliffs, so are a bit perilous to approach and work near, given the sheer drop and potential instability’, says Mooney, who eventually captured nine birds. After transporting them to an improvised lab in a nearby farm shed, the team gently sedated the animals and inserted fine electrodes beneath their skin before recording their responses to clicks ranging from low-pitched 125 Hz tones to 8 kHz beeps at a 20 dB whisper up to 100 dB to find out which frequencies the puffins were most sensitive to. Although the birds could hear sounds up to 6 kHz, their hearing was sharpest between 750 Hz and 3 kHz. And, when the team compared the birds’ hearing with that of other similarly sized birds, it seemed that their aquatic lifestyle has not affected their hearing at all: it’s just as good as that of birds that never plunge beneath the waves. However, the team suspects that the windswept locations of the birds’ nesting sites with the noise of waves crashing below could limit their hearing above ground. The hearing of these charismatic birds is probably fine-tuned to the cries of their own chicks and other puffins. In addition, they are also less likely to be disturbed by their noisy surroundings when secluded in their burrows. However, Mooney believes that the thud of approaching human feet is likely to penetrate their otherwise peaceful homes. ‘Given the influence of human encroachment on bird colonies, the sensitive hearing of these animals and the fact that puffins are a major tourist attraction in many countries, we suggest that human disturbance noise, even low-level sounds from hikers and visitors, has the potential to disturb puffins’, he says. 10.1242/jeb.232314 Mooney, T. A., Smith, A., Larsen, O. N., Hansen, K. A. and Rasmussen, M. (2020). A field study of auditory sensitivity of the Atlantic puffin, Fratercula arctica. J. Exp. Biol. 223, jeb228270. doi:10.1242/jeb.228270

39 Hefty shells help hermit crabs cling on in surf

Sometimes families just outgrow their homes: everyone keeps bumping into each other and there is never enough space. But even if you do upgrade, your new abode may not tick all the boxes, and it turns out that home-hunting hermit crabs are often in the same quandary. ‘Hermit crabs can be forced into “best of a bad job” scenarios in which they must accept whatever relatively suitable shell becomes available’, says Luis Miguel Burciaga Guillermina Alcaraz, from Universidad Nacional Autónoma de México. This compromise might also affect where the nomadic crustaceans choose to dwell. ‘It may be more feasible for crabs to choose a habitat that will be better suited for the shell they have been able to acquire, rather than choosing the shell based on their preferred habitat’, explains Alcaraz. Clibanarius antillensis hermit crabs seem to use loose-fitting, heavy- pointed Stramonita biserialis snail shells when battered by waves, in contrast to more tightly fitting light and rotund Nerita scabricosta snail shells, which they don in sheltered rock pools, so Alcaraz wondered whether the crabs’ choice of shell affects their cost of living and, ultimately, where they choose to settle. Risking the surf on Mexico’s Pacific coast, Alcaraz, Brenda Toledo and Luis Burciaga went in search of Calcinus californiensis hermit crabs in waves crashing at up to 160 cm s–1; ‘we had to avoid being dragged away’, recalls Burciaga. After gathering other crabs from tranquil rock pools nearby, the trio returned to Mexico City, where they encouraged the crustaceans to vacate their homes briefly, and checked how spacious and hefty the shells were. Impressively, the intrepid crabs that were resident in the surf tended to occupy Stramonita shells that were 72% larger than the Nerita shells inhabited by the crabs that opted to shelter in rock pools. After they had returned the animals to their shells, Alcaraz and her students placed a claw hairclip on the crabs’ shells – to distort the weight and to encourage them to move out – while providing a selection of larger and smaller alternative shells (of the same species that they had previously occupied) in either still water or simulated waves to find out how the environment affected the crustaceans’ choice of shell. Allowing the crabs 24 h to relocate, it was evident that the wave-swept crustaceans preferred to inhabit heavier Stramonita shells, even though the shells are more cumbersome and could impede foraging. But why were the crabs selecting heftier Stramonita shells that could hamper them when lighter shells would surely ease their burden? Alcaraz measured the amount of oxygen consumed by the animals as they were pummelled by the simulated waves. Remarkably, the crabs clad in heftier Stramonita shells used 84% less energy when hanging on in the surf than the crustaceans that squeezed themselves into smaller Stramonita shells. ‘The heavier the gastropod shell, the lower the net effects of hydrodynamic forces and, therefore, the less energy required to overcome water flow’, Alcaraz explains. In contrast, the crabs that were given lighter, more rotund Nerita shells used more energy clinging onto the ground; the Nerita-clad crabs are more easily dislodged by the seething water and the globular shells are less streamlined, forcing the crustaceans to use more energy to withstand the force of the waves. Alcaraz suspects that hermit crabs are essentially pragmatic; when it comes to finding a desirable residence, they’re prepared to put up with whatever they can squeeze into until a better opportunity comes along. However, their choice of new abode may require that they retreat to a more tranquil environment if their new home isn’t up to going with the flow. 10.1242/jeb.233577 Alcaraz, G., Toledo, B. and Burciaga, L. M. (2020). The energetic costs of living in the surf and impacts on zonation of shells occupied by hermit crabs. J. Exp. Biol. 223, jeb222703. doi:10.1242/jeb.222703

40 Resilient aquifer stoneflies handle low oxygen well

Sweeping through Montana’s mighty Rocky Mountains, the majestic Flathead River courses through pristine valleys and canyons. But according to Rachel Malison from The University of Montana, USA, the river extends well beyond the babbling torrent visible at the surface, flooding deep into the gravel- and cobblestone-packed aquifer beneath. And these subterranean waterways are bursting with life. In the 1970s, Jack Stanford from the University of Montana, USA, found that stonefly nymphs –

Rachel Malison some of the most vulnerable inhabitants of rivers as they require well-oxygenated water – are remarkably content to reside in aquifers, even though the oxygen levels can plunge as low as 0.14 mg l–1. In addition, Amanda DelVecchia had recently discovered that the stonefly occupants of the Flathead River’s Nyack aquifer dine on microorganisms that only thrive in deoxygenated environments, deepening the paradox. How could insects that usually perish when oxygen levels dwindle thrive in an aquifer where the oxygen supply can be patchy in places? After travelling through the dramatic landscape to the Nyack floodplain, Malison and intern Hailey Jacobsen pumped water from the wells drilled by Stanford in the 1980s to collect the subterranean stoneflies. ‘Sometimes we didn’t get any, sometimes we might get 100’, says Malison, adding that additional creatures, including tiny crustaceans and other microscopic invertebrates, also appeared in the water. Back at the Flathead Lake Biological station, Malison identified three stonefly species (Paraperla frontalis, Isocapnia spp. and Kathroperla perdita) before measuring the insects’ oxygen consumption to calculate their metabolic rate as she steadily reduced their oxygen supply from 12 to 0.5 mg dissolved oxygen l–1. Comparing the metabolic rates of the aquifer stoneflies with stoneflies that had been collected from the river, Malison found that the aquifer residents had a much lower initial metabolic rate than the surface-dwelling insects. In addition, the species in the aquifer coped much better as the oxygen levels fell – their metabolic rate only dropping ~46% when the oxygen was reduced by 83% – in contrast to the river dwellers’ metabolic rates, which dropped by 66%. And when Malison and Jacobsen checked the insects’ recovery after their oxygen ran out, the species that live in streams never regained their full metabolic rate, while the aquifer residents bounced back to almost normal. Knowing that the aquifer-based species could nip in and out of oxygen-depleted water if they are to dine on the specialised microorganisms that thrive there, Malison and Jacobsen then tested how P. frontalis and Isocapnia fared when the oxygen levels dropped from 12 to 0.5 mg l–1 three times over a 9 h period. Impressively, all of the stoneflies survived oxygen levels that would probably have proved fatal for the river-dwelling species; however, the reduction in P. frontalis’ metabolic rate suggests that visiting deoxygenated regions briefly to dine may be stressful. And when Malison, Julia Cotter and Haley Dole embarked on the Herculean task of continually recording the insects’ metabolic rates over a week when the animals repeatedly went without oxygen overnight after the levels had declined gradually during the day, K. Perdita was most robust (91% survival), although Isocapnia seemed to struggle more (38%). Even though most stonefly species are notoriously sensitive to the oxygen levels in their water, the stoneflies that have descended beneath the ground and made their homes in aquifers seem to be more resilient, tolerating oxygen levels that would be fatal for river dwellers. ‘This tolerance suggests that they can enter zones of low oxygen to forage and exploit hot spots in the underground aquifer where food is abundant’, says Malison. 10.1242/jeb.233130 Malison, R. L., DelVecchia, A. G., Woods, H. A., Hand, B. K., Luikart, G. and Stanford, J. A. (2020). Tolerance of aquifer stoneflies to repeated hypoxia exposure and oxygen dynamics in an alluvial aquifer. J. Exp. Biol. 223, jeb225623. doi:10.1242/jeb.225623

41 Intrepid lice survive extreme pressure when hitching rides on elephant seals

Lice usually have a pretty cushy existence. Snuggled up warm in the pelt of their host with a plentiful supply of fresh bodily fluids on tap, the worse many have to deal with is an irritated swat or drenching from a shower. But not the lice that choose to reside on walruses, seals and sea lions. These flightless parasitic insects are taken on rollercoaster rides beneath the waves whenever their host plumbs the depths; and, in the case of the lice (Lepidophthirus macrorhini) that reside on southern elephant seals, this can mean Martín Brogger and Mauricio Luquet (ALUAR SAIC) submersions down to 2000 m. It wasn’t even clear whether the irksome pests could survive. Soledad Leonardi from the Instituto de Biología de Organismos Marinos (IBIOMAR), Argentina, and Claudio Lazzari from the University of Tours, France, explain that elephant seals pick up their first lodgers as pups, but no one was sure whether the insects that infest the adults are descendants of that initial population, or new arrivals that hitched a ride after earlier settlers had perished during deep sea excursions. Wondering just how much pressure these mini-beasts can take, Leonardi, Ricardo Vera, Julio Rua and Florencia Soto travelled to the Peninsula de Valdés, Argentina, to collect adult lice and nymphs from elephant seal pups. ‘Soledad’s colleagues restrained the 150 kg pups with their bare hands, while she collected the lice with tweezers from the pups’ flippers’, says Lazzari, explaining that the insects were then immersed in sea water before being transported to IBIOMAR. ‘They can cope with hypoxia [low oxygen], but they can’t handle water loss when exposed to air’, he says. Then Vera, Rua and Leonardi, rigged up a scuba air tank attached to a pressured chamber to simulate the experience of plunging to 300, 800, 1500 and 2000 m for 10 min. After retrieving the lice from the tank, the team found that the adults and nymphs began moving around and recovered instantly from simulated submersion to 300 and 800 m. However, the nymphs seemed to struggle more under higher pressure, taking 30 min to recover from the 1500 m simulated descent, although the nymphs that experienced the pressure exerted at 2000 m began crawling within 10 min of returning to the surface. In addition, when the researchers simulated successive dives 40 min apart, the adult lice again recovered straight away, although the nymphs took 5 mins to recover after the equivalent of a 300 m dive preceded by a 2000 m dive 40 min earlier. Even when the pressure was inadvertently turned up to 450 kg cm−2, effectively submerging an adult louse to 4500 m, the insect recovered fine. ‘Seal lice exhibit extraordinary pressure tolerance; in addition, they tolerate rapid changes in pressure corresponding to the rapid descent and return to the surface of a diving host’, says Lazzari. But how do they pull off this remarkable feat? Leonardi and Lazzari suspect that scales on the insect’s thorax and abdomen could reinforce the body, although they also point out that they carry less air in their tracheolar (air tube) system than mammals do in their lungs, making them less compressible. The researchers also suggest that the insects might carry a thin layer of air with them, which could act like a gill extracting oxygen from the water near the surface, and they may be able to reduce their metabolism to conserve oxygen. Whatever the reason behind the itchy insects’ impressive ability to withstand pressure, it seems that they have made a success of surviving where no other insects have ventured before, and understanding how they do so could hold the key to fathoming why insects usually fail to thrive in the sea. 10.1242/jeb.234260 Leonardi, M. S., Crespo, J. E., Soto, F. A., Vera, R. B., Rua, J. C. and Lazzari, C. R. (2020). Under pressure: the extraordinary survival of seal lice in the deep sea. J. Exp. Biol. 223, jeb226811. doi:10.1242/jeb.226811

42 Catfish keep head flat when gulping

Compared with the monotonous chomping of mammal jaws, the expansive gape used by fish as they slurp in food is quite miraculous. ‘It’s a very cool and intricate behaviour’, says Ariel Camp from the University of Liverpool, UK, describing how the animals protrude their lips as they fling their jaws wide. Many fish use the forceful muscles packed along the trunk of their bodies to power the expansive mouth movement, Aaron Olsen pulling the top of the skull upward with the muscles along the back while rotating the lower jaw down with muscles on the belly side. However, catfish don’t seem to lift the top of their heads when gulping down lunch, which made Patricia Hernandez, at George Washington University, USA, Aaron Olsen and Elizabeth Brainerd, from Brown University, USA, and Camp wonder how much of a role the powerful trunk muscles might play in the process. ‘We wanted to know if catfish only use one of their two big body muscles to powerfully suck up food and if this meant they were limited to less powerful food sucking than other fish’, says Camp. But getting to grips with the fine detail of how three channel catfish, Ictalurus punctatus, manoeuvre their jaws in front of an X-ray camera required enormous patience. ‘We tried to train the catfish by always feeding them in the part of the tank where they needed to be for filming’, recalls Camp, adding with a chuckle, ‘but the fish all had their own little personalities; we just had to wait until they were in the mood for food’. In addition, the team had to position minute metal markers on various jaw bones to track their movements, and within the upper and lower body muscles to monitor muscle shortening as they contracted. ‘We hadn’t done that on this species before, so it was tricky’, says Camp, who then filmed the fish dining, with the help of Hernandez. However, when Olsen painstakingly tracked the movements of each jaw bone and Camp reassembled them in a 3D animation of the manoeuvre, the team was astonished to see that instead of moving upward, the fish’s head usually stayed in place and on one occasion it even moved down, ‘which is not how we normally expect fish to move their heads and backbones during feeding’, exclaims Camp. Most surprisingly, the fish’s back muscles did not shorten at all. However, the muscles along the belly side of the fish did shorten by up to 8%, rotating the fish’s shoulder bones to fling open the bottom jaw. And when the team compared the suction inside the catfish’s mouths as they threw their jaws wide, it was every bit as strong as the suction produced by similarly sized bass. So what are the back muscles doing if they aren’t actively contracting to contribute to the catfish’s powerful slurp? Camp suspects that they hold the fish’s head in place while the lower body muscles yank the jaw open. ‘We think the epaxial [back] muscles may be generating force to prevent the head from moving relative to the body, the same way you’d use your arm muscles to hold a heavy bag of groceries. The muscles aren’t shortening, but they’re still exerting force’, says Camp. The team also suspects that holding the skull in place could help the bottom-feeding fish to position their mouths above a tasty morsel. ‘Fish have different ways of using their big body muscles to help them eat’, says Camp, and she and her colleagues are eager to find out whether other fish with similar body shapes use the catfish’s alternative guzzling strategy. 10.1242/jeb.234823 Camp, A. L., Olsen, A. M., Hernandez, L. P. and Brainerd, E. L. (2020). Fishes can use axial muscles as anchors or motors for powerful suction feeding. J. Exp. Biol. 223, jeb225649. doi:10.1242/jeb.225649

43 Lizards pant to keep cool

Caleb Loughran It’s a common misnomer that lizards are cold blooded. They are anything but and also run the risk of overheating when temperatures rocket, just like many other creatures. ‘The most common way lizards control their body temperature is by moving back and forth from sunny to shaded sites’, says Caleb Loughran from the University of New Mexico, USA, explaining that many species seek shade or strike cooling poses to keep their temperatures down. But Loughran and Blair Wolf, also from the University of New Mexico, wondered whether some lizards may also resort to a strategy used by over-heating dogs and birds to keep cool: panting. ‘Cooling down by evaporating water by panting might prove to be an important alternative to moving from a hotter to cooler environment when overheating is a risk’, says Wolf. Setting out in search of lizards ranging from sizeable chuckwallas (Sauromalus ater) and medium-sized eastern collared lizards (Crotaphytus collaris) to tiny side-blotched lizards (Uta stansburiana), Loughran and Wolf collected more than 260 lizards from 17 species residing in locations ranging from baking deserts to cooler locations high in the Manzano mountains. ‘Aspidoscelis are often difficult to capture because they are almost always moving and slide right through the lasso that we use to catch them’, laughs Loughran. Then, the duo settled each lizard into a plastic chamber to film the reptiles and capture whether they began panting, as they gradually increased the air temperature from 35°C to near 50°C over a 4 h period. In addition, the scientists measured the reptile’s body temperature to track whether panting gave the animals any relief from the spiralling temperatures. Impressively, all of the lizards began panting at body temperatures above 36.8°C, although 9 species only resorted to this technique for cooling at temperatures above 40°C. And when Loughran and Wolf tracked the lizard’s body temperature after they began panting, the duo could see that 14 species were able to reduce their body temperature by up to 4.8°C, with the average lizard cooling by 2.7°C. Also, when the team compared the temperature of the reptiles’ natural habitats with the temperature at which they began panting, the species from cooler locations resorted to panting at lower temperatures than the animals from hotter climes. However, three species – Yarrow’s spiny lizard (Sceloporus jarrovii), crevice spiny lizard (Sceloporus poinsettii) and the Chihuahuan spotted whiptail (Aspidoscelis exsanguis) – gained little or no benefit from panting. The scientists also noticed that the smaller members of a species tended to hold off panting until higher temperatures, which they suspect could be due to the smaller reserves of water carried by dinkier animals. In addition, the duo checked to see whether panting could be the first warning that a lizard is in peril from the heat, but when they analysed the pattern it was clear some lizards were still in a good state, while others were near to collapse. In short, panting is not a reliable symptom that an overheated lizard may be getting into trouble. Having confirmed that lizards can resort to panting when the temperatures rocket and are, in many cases, capable of slowing their body temperature rise, Loughran and Wolf point out that the cooling strategy is only practical when there is sufficient water on hand. ‘A hotter environment means using more water to stay cool or being less active, both of which may increase extinction risk in a rapidly warming climate’, says Wolf. Keeping their body temperatures down by panting could give some species the edge over others residing in the same locations when climate change really begins to take its toll. 10.1242/jeb.235036 Loughran, C. L. and Wolf, B. O. (2020). The functional significance of panting as a mechanism of thermoregulation and its relationship to the critical thermal maxima in lizards. J. Exp. Biol. 223, jeb224139. doi:10.1242/jeb.224139

44 Cuvier’s beaked whales take record-breaking dives in their stride

The blue whale might be one of the most enigmatic creatures on the planet, but the true megastars of the diving world are Cuvier’s beaked whales (Ziphius cavirostris). They are capable of reaching depths of almost 3000 m, and calculations suggested that these relatively diminutive whales should only remain submerged for about 33 min before their oxygen runs out and they resort to anaerobic respiration. Yet experience told Nicola Quick Danielle Waples, taken under NOAA/NMFS permit and colleagues from Duke University, USA, 14809-03 and NOAA General Authorization 16185 that the shy mammals were capable of diving for far longer. Wondering how often the animals embark on these epic dives and how long it takes them to recover after returning to the surface, William Cioffi, Jeanne Shearer, Andrew Read, Daniel Webster (from the Cascadia Research Collective) and Quick went in search of the elusive animals in the abundant waters off Cape Hatteras, USA. ‘Because the animals spend so little time at the surface, we needed calm seas and experienced observers to look for them’, says Quick, adding, ‘the average period they spend at the surface is about 2 min, so getting a tag on takes a dedicated crew and a manoeuvrable vessel’. The brief surfacing periods also limited the amount of time available to transfer the precious information to a satellite each time the animals returned from a dive. Deploying 23 tags over a 5-year period, the team recorded more than 3600 foraging dives, ranging from 33 min to 2 h 13 min, all of which were well in excess of the point when diving Cuvier’s beaked whales were thought to run out of oxygen. Knowing that approximately 95% of the dives performed by other mammals are complete before their oxygen supplies dwindle, the team rechecked their plot and realised that if the same proportion of Cuvier’s beaked whale dives are completed before their oxygen stores expire, then they could remain submerged for an incredible 77.7 min before resorting to anaerobic respiration. ‘It really did surprise us that these animals are able to go so far beyond what predictions suggest their diving limits should be’, says Quick. In addition, the team picked up two extraordinary dives in 2017, which exceeded even their wildest dreams. One was almost 3 h long, while the other lasted 3 h 42 min. ‘We didn’t believe it at first; these are mammals after all, and any mammal spending that long under water just seemed incredible’, says Quick. But, how long did it take for the whales to recover from dives of up to 2 h 13 min? After analysing the length of time between foraging dives – which exceeded 33 min – Quick was astonished that there was no clear pattern. Although one whale resumed diving for food within 20 min of a 2 h dive, another that had completed a 78 min dive spent almost 4 h making shorter dips and returning to the surface before initiating the next foraging dive. ‘Going into the study, we thought that we would see a pattern of increased recovery time after a long dive. The fact that we didn’t opens up many other questions’, says Quick. Puzzled by the extraordinary resilience of the Cuvier’s beaked whale, Quick and Andreas Fahlman, from Fundación Oceanogràfic de la Comunitat Valencia, Spain, suspect that the animals may have an exceptionally low metabolism, coupled with larger than usual oxygen stores and the ability to withstand stinging lactic acid building up in their muscles when they switch to anaerobic metabolism when dives exceed 77.7 min. Quick is also intrigued by the reasons behind the two record-breaking dives; ‘it may be that there was a particularly productive food patch,…there was some perceived threat…[or] some noise disturbance influenced these dives’, she says. 10.1242/jeb.234187 Quick, N. J., Cioffi, W. R., Shearer, J. M., Fahlman, A. and Read, A. J. (2020). Extreme diving in mammals: first estimates of behavioural aerobic dive limits in Cuvier’s beaked whales. J. Exp. Biol. 223, jeb222109. doi:10.1242/jeb.222109

45 Lesser long-nosed bats have finely tuned sweet tooth

Marco Tschapka Flight is the most metabolically demanding life process, with some creatures running their powerful engines on a high energy liquid fuel: nectar. Although some nectars are packed with up to 50 g of sugar for every 100 g of fluid (50% sugar), the nectars produced by bat pollinated flowers are more dilute; the bats never encounter nectars containing more than 33% sugar. ‘Foragers should prefer foods that yield more energy’, explains Michael Walter from the University of Tübingen, Germany, so it would make sense that bats could taste the difference between concentrated and dilute nectars, to ensure that they select the most calorific blooms when refuelling. But no one had checked. Knowing that the ability to discriminate between the sweetness of different flowers is even more essential for nectar feeders when the nectar is watery, Hans-Ulrich Schnitzler and colleagues from the University of Tübingen and the University of Ulm, Germany, tested the lesser long-nosed bat’s (Leptonycteris yerbabuenae) sweet tooth. Offering eight bats a choice between two syrup solutions – some differing by as little as 0.5% (4.75% sugar versus 5.25%), while other differences were larger (21% versus 29%) – Walter, Aaron Verdong, Vanessa Olmos and Christina Weiss filmed the bats each night as they foraged from the artificial feeders in the lab to determine whether the mammals could differentiate between the different syrup concentrations. ‘It took us over 7 months to run the whole experiment, as we offered each bat the choice between at least 13 different sugar solution pairs’, says Walter. After filming almost 31,000 feeder visits, the team counted how many times each bat sipped from the most concentrated syrup, reasoning that if the animal could distinguish between the two, it would prefer to frequent the feeder that provided it with the most sugar and energy. Incredibly, when the results were in, seven of the animals were capable of discerning a difference of just 0.5% when offered the choice between the 4.75% and 5.25% syrups. ‘No one had tried to offer them such small concentration differences before and they could easily distinguish between the two’, says Walter, adding, ‘I’m always amazed by the capabilities of these small animals’. However, as the sugar solutions became stronger (around 20% sugar), only one bat was able to differentiate between syrups differing by 1.5% (19.25% versus 20.75%), while the remaining animals were only capable of tasting sweetness differences of 4% at an average concentration of 20% sugar (18% versus 22%). And when the team calculated how much syrup each bat consumed while foraging, it was clear that the bats dining from flowers stocked with stronger nectars (greater than 10% sugar) could easily sip sufficient nectar to meet their energy needs over the course of a night. However, the bats that were offered syrups averaging 5% sugar only consumed 2/3 of the energy imbibed by the better fed bats. There simply wasn’t enough energy in the dilute diet for the bats to satisfy their energy demands; ‘There is only so much water that a small bat belly can absorb’, chuckles Walter. Lesser long-nosed bats seem to have a finely tuned palate, which allows them to discriminate between dilute nectars to identify the most bountiful blooms that will satisfy their appetite. And when the team compared the bats’ abilities with those of other creatures that thrive on nectar, it was apparent that lesser long-nosed bats have an even more discerning sense of taste than hummingbirds, bumblebees and even other nectar sipping bats, which might help them to get to the front of the line when filling up at the best stocked flowers. 10.1242/jeb.236984 Walter, M. H., Verdong, A., Olmos, V., Weiss, C. C., Vial, L.-R., Putra, A., Müller, J., Tschapka, M. and Schnitzler, H.-U. (2020). Discrimination of small sugar concentration differences helps the nectar-feeding bat Leptonycteris yerbabuenae cover energetic demands. J. Exp. Biol. 223, jeb215053. doi:10.1242/jeb.215053

46 Champion annual killifish embryos survive more than 16 months out of water

Fish are usually synonymous with water. Take away the water and frankly the future is bleak. But not for annual killifish (Austrofundulus limnaeus). Although the adults don’t fare well when their temporary homes dry up, they leave behind eggs in a form of suspended animation – known as diapause – as the seeds of the next generation, ready to resume developing as soon as the rains return. And these eggs are extraordinarily tough. ‘Annual killifish Daniel Zajic can survive for months to years in dry mud’, says Daniel Zajic, now at Linfield University, USA. The dormant eggs seem to seal themselves shut about a week after their ponds have vanished, to prevent themselves from losing further water, but this could also affect basic life support by preventing them from absorbing oxygen. Zajic, Jonathon Nicholson and Jason Podrabsky at Portland State University, USA, wondered how the resilient fish eggs deal with dehydration. But first the trio wanted to know just how long the embryos – some in suspended animation and others that had resumed developing – could survive in dry air (85% relative humidity). Zajic collected eggs at different life stages and placed them in a hermetically sealed sterile box for more than 18 months, patiently checking their survival every day and then 2–3 times a week after a few months. Amazingly, over 10% of the fish embryos in suspended animation survived more than 16.5 months out of water, with one particularly hardy individual surviving 19.4 months (587 days). However, the embryos that had emerged from suspended animation were less resilient than their dormant cousins, although they were still able to endure the dry conditions better than other fish, with around half surviving a whole month and some even making it over 3 months. Nicholson then sealed some of the dormant embryos and some developing embryos in sterilised individual vials with normal air to measure their oxygen consumption, in order to find out how much energy they were using in the dry conditions. ‘Keeping everything clean and sterile is very important when measuring oxygen consumption … microbes can quickly deplete available oxygen and affect the measurements’, says Zajic. However, the team was surprised to see that the dormant embryos – which are meant to switch off oxygen consumption and use anaerobic respiration instead – were still consuming oxygen after more than a month. Their ability to prevent further water loss didn’t seem to affect their ability to consume oxygen, even though they must have somehow sealed their eggs to prevent themselves from drying out completely. The team suspects that the dormant embryos may have increased their oxygen consumption to run metabolic processes that protect them from the effects of dehydration. However, when the researchers analysed the oxygen consumption of embryos that had kick started development in dry air, their oxygen consumption plummeted by 60% after 7 days, probably due to the general stress. Most intriguingly, almost 20% of the reactivated embryos seemed to stall their development 7 days after resuming and the team suspects that this pause could help the fish population to re-establish itself quickly when their ponds eventually refill. ‘Having embryos at different stages of development in the soil that respond differently guarantees survival of at least some embryos, even if others are lost if early rainfall turns out to be a false start’, says Zajic. 10.1242/jeb.234997 Zajic, D. E., Nicholson, J. P. and Podrabsky, J. E. (2020). No water, no problem: stage-specific metabolic responses to dehydration stress in annual killifish embryos. J. Exp. Biol. 223, jeb231985. doi:10.1242/jeb.231985

47 Serotonin key for trap-jaw ant aggression

Hitoshi Aonuma Any creature with the term ‘trap-jaw’ in its name is bound to be fairly ferocious, and trap-jaw ants make the most of their ballistic mandibles, turning them on foes and prey, in addition to using the structures to catapult themselves to safety when confronted by larger adversaries. While some trap-jaw ant nest residents are particularly aggressive, other inhabitants of the same nest simply turn and walk away when attacked. Wondering what lay at the heart of the insect’s decision to withdraw or confront an opponent, Hitoshi Aonuma, from Hokkaido University, Japan, started prodding Odontomachus kuroiwae ants with a paintbrush to find out which members of the nest were most aggressive and which preferred simply to wander off. Surprisingly, only 10% of the 580 goaded insects took any interest in the intrusion: 14 of the ants turned around to confront the brush with their jaws disarmed (closed), while 29 wheeled about with mandibles cocked aggressively, ready to take on the intruder. The remaining nest-mates (90%) simply scurried away from the encounter. Curious to discover the difference between the aggressive ants and their docile companions, Aonuma compared the insects’ brains and found that the antagonistic ants had higher levels of octopamine, dopamine and serotonin, vital chemical messengers – neurotransmitters – that transmit signals between nerves. But which of these essential nervous system chemicals held the key to the insects’ hostility? This time, Aonuma collected meek nest-mates and fed one of the three neurotransmitters to the ants, to see whether any of the compounds could transform a placid ant into a belligerent aggressor. Impressively, the ants that had been fed serotonin became more confrontational, taking on the paintbrush instead of sauntering away, while the ants that consumed dopamine became slightly more combative. Aonuma was also able to amplify the ants’ aggressive tendencies by feeding them compounds that their bodies could convert into serotonin or dopamine. And when he fed drugs that counteract the effects of the essential neurotransmitters to naturally aggressive ants, the insects became more accommodating, preferring to walk away from an irritating tap on the rear instead of turning to confront the intrusion. ‘This study demonstrates that the serotonergic system contributes to the initiation of defensive responses to unexpected tactile stimuli and that dopamine can weakly contribute to the initiation of defensive responses in the trap-jaw ant’, says Aonuma, who is keen to learn more about the mechanisms that make some ants docile and others more easily provoked. 10.1242/jeb.237271 Aonuma, H. (2020). Serotonergic control in initiating defensive responses to unexpected tactile stimuli in the trap-jaw ant Odontomachus kuroiwae. J. Exp. Biol. 223, jeb228874. doi:10.1242/jeb.228874

48 Colour is key when female chameleons choose Mr Right

Alexis Dollion Flickering from green to blue, red and yellow in the blink of an eye, panther chameleons (Furcifer pardalis) turn up the colour for a range of reasons, from warning off intruders to evading predators. Some chameleons even increase the thermostat by shifting to a darker shade when they are chilly. But it wasn’t clear whether certain aspects of colour are also the key to female chameleon hearts. ‘To date, the role of physiological colour change in mate choice is poorly understood’, says Alexis Dollion from the Université de Paris, France. Together, Sandrine Meylan and Anthony Herrel, with colleagues from the Sorbonne Université and Muséum national d’Histoire naturelle and Olivier Marquis from the Parc Zoologique de Paris, France, set up chameleon speed dates while monitoring the males’ appearance to find out if any particular skin hue, colour intensity and brilliance (which combine to produce colour saturation) and brightness changes might make a male irresistible. But the team also needed to make sure that the mood lighting was just right. ‘Chameleon vision extends into the ultraviolet, which some animals, like humans, cannot see’, he says; females could miss out on essential aspects of the males’ vivid serenades if the lighting wasn’t sufficiently natural and the UV shades were lost. Setting up a bank of lamps, ranging from incandescent bulbs and LED lights to a UV fluorescent tube, the team photographed male panther chameleons every two minutes during the throes of courtship to capture their appearance in the moments before the female audience succumbed to their charms. Comparing pixels in images recorded from several locations on each male’s body as he wooed his would-be sweetheart, the team realised that the females were impressed by males that changed the brightness least. In addition, the males that pushed the boat out and went for the most dramatic colour changes seemed to have better success with the females than more conservative suitors, whose shades remained similar to their initial palette. However, when the team compared the effects of colour changes in different regions of the males’ skins, the females preferred males that kept the brightness of the band along their side fairly steady. Also, the males that played it safe and retained similar hues in their stripes and background skin, and those that changed the saturation, seemed to attract more female attention. Turning their attention to the UV ‘before’ and ‘after’ shots, the scientists could see that the attractive male chameleons also alter their appearance by increasing the brightness of their UV shades and switching their UV colours more. In fact, the males that did not splash out on ostentatious UV displays seemed to be actively avoiding catching the eye of nearby ladies, probably because they prefer the quiet life by evading predators and other unwanted attention. In short, the male panther chameleon’s cutaneous pyrotechnic displays seem to fan the flames of female ardour and the team is curious to find out whether different portions of the reptiles’ skins convey different information that the females use when selecting Mr Right. 10.1242/jeb.237230 Dollion, A. Y., Herrel, A., Marquis, O., Leroux-Coyau, M. and Meylan, S. (2020). The colour of success: does female mate choice rely on male colour change in the chameleon Furcifer pardalis? J. Exp. Biol. 223, jeb224550. doi:10.1242/jeb.224550

49 How whale-surfing remoras stay in touch with their steeds

When it comes to hitching a ride, whale- surfing remoras are perfectly at home latching onto the largest creatures on the planet. ‘From static photos, it’s easy to assume that the remora is anchored in a single spot on their host for lengthy periods’, says Brooke Flammang from the New Jersey Institute of Technology (NJIT), USA. But when she saw Jeremy Goldbogen’s movies of blue Stanford University & Cascadia Research Collective. whale-riding remoras at the 2015 Society Image collected under NMFS permit #16111 for Integrative and Comparative Biology conference, the images blew her mind. ‘Jeremy joked that he had, “inadvertently gotten hundreds of hours of remora footage”’, she laughs. But the movies, captured by tags attached to the colossal mammals, revealed the inverted fish skating across the whales’ skin to different locations. Transfixed, Flammang was frantically trying to figure out how the fish remained in contact with the whales, without being torn free by the fast-flowing water as the massive mammals dived. A few days later, at a lunch with Goldbogen, from Stanford University, USA, and her long-time collaborator Jason Nadler, from Georgia Tech Research Institute, USA, they were also gripped by the mystery. ‘We wanted to know how the remoras moved along the whale and why they attached where they did’, says Flammang. But first, David Cade, also from Stanford University, spent hours trawling through the movies for photobombing remoras to figure out where the fish preferred to grip onto their whale steeds. Eventually, it was apparent that the fish favoured three zones on whale bodies: behind the blowhole; behind and next to the mini-fin on the whale’s back; and above and behind the pectoral fin. The fish even remained attached when the whales surfaced, clinging on out of the water. In addition, when the remoras lifted off from one location, skimming a few centimetres above the whale’s skin to another, they were never torn free by the water rushing past. Curious to learn how water slides over the whale’s body, Michael Beckert and Nadler initially built a computer program. However, to reveal the fine detail of the water movement on the remora’s scale, Flammang needed a truly formidable supercomputer. Flammang teamed up with Simone Marras (also from NJIT), in collaboration with Oriol Lehmkuhl, Guillame Houzeaux and Mariano Vázquez at the Barcelona Supercomputer Centre, Spain, to perform the immense 48 h long calculation over more than 9 million points dotted around the 18 m body of a whale swimming at 1.5 m s−1. Analysing the calculations, the team discovered that the drag experienced by the fish sheltering behind the dorsal fin, blow hole and pectoral fins was ~80% less than the drag if they were swimming freely at 1.5 m s−1. They also revealed drag reductions ranging from 50% to 75% as the remoras lifted off 1 cm from the whale’s skin and skimmed across various locations on the mammal’s body. The fish manoeuvre with ease in the slow-moving cushion of water carried along by whales as they scythe through the sea. Erik Anderson and Flammang also investigated how remoras attach to a moving surface in the lab, to find out whether the water sandwiched between the fish and its whale-ride when skimming across the surface might help to hold the fish in place. Impressively, a region of fast-flowing water in the space between the fish and whale effectively sucks the remora toward the whale’s body. So, whale-surfing remoras stay in touch with their steeds by riding in the cushion of water carried by the colossal beasts and Flammang and her colleagues are hoping to use the remora’s know-how to keep the cameras used by scientists in place for longer. 10.1242/jeb.237883 Flammang, B. E., Marras, S., Anderson, E. J., Lehmkuhl, O., Mukherjee, A., Cade, D. E., Beckert, M., Nadler, J. H., Houzeaux, G., Vázquez, M. et al. (2020). Remoras pick where they stick on blue whales. J. Exp. Biol. 223, jeb226654. doi:10.1242/jeb.226654

50 Goby fins have fingertip sensitivity

Adam Hardy Groping around in my bag for my keys is a daily ordeal. I’m not going to list the catalogue of junk, but I can distinguish every article by touch until I eventually locate the elusive keys. Our fingertips are exquisitely engineered, deftly detecting the differences between surfaces and shapes, but we are not the only animals that touch objects. ‘A whole host of fishes contact the bottom of bodies of water, plants or other animals using their fins’, says Adam Hardy from The University of Chicago, USA, leading Hardy and his graduate advisor, Melina Hale, to wonder whether fish may also be able to feel surface differences with their fins. However, before they could begin unravelling the question, Hardy and Hale had to find a fish that seems to spend a lot of time in touch with riverbeds and the bottom of lakes. ‘Round gobies (Neogobius melanostomus) were a great choice for these experiments given that they are a bottom-dwelling fish that love to perch on rocks and other materials’, says Hardy, who biked from the university campus to Lake Michigan during the summer to catch the fish. ‘It’s always a good day when you can go fishing for work’, he chuckles. After collecting a few gobies, Hardy filmed the fish as they manoeuvred over a piece of slate or a wavy piece of plastic on the tank bottom, and also when they wedged themselves against the side of the tank. Sure enough, the fish’s fins splayed out over each of the surfaces, contacting the structures like a hand laid upon them. Yet, to find out whether the fins were providing the fish with different touch sensations, Hardy knew he had to record nerve signals from individual fin rays. Gently brushing a short horizontal bar moving along a fin ray toward the tip at speeds ranging from 5 mm s–1 to 20 mm s–1, Hardy recorded the electrical signals in nerves as the bar moved over the fin and it was clear that the fins sensed when they were being touched. In addition, each nerve only sensed contact along a tiny portion of each fin ray, possibly allowing the fish to feel fine surface details. But, were the fins sensitive enough to detect the difference between different grades of gravel? This time, Hardy designed a rotating wheel with 2 mm wide ridges along the edge – separated by gaps of 3, 5 or 7 mm – to mimic sediments ranging from coarse sand to granules and pebbles. Then he rolled each wheel along the fish’s fin rays at speeds ranging from 20 to 80 mm s–1. ‘It took numerous design iterations to create the wheels’, says Hardy, but as he painstakingly recorded the nerve signals produced when the ridges contacted the fin rays, the nerve signals synchronised with each ridge contacting the ray. ‘They matched the pattern of the ridges moving across the skin even as the speed of the wheel increased’, he adds. Most impressively, the gobies’ fins seemed to be as sensitive to the coarse surfaces as monkey finger pads. ‘Primates are often held up as the gold standard in tactile sensitivity, so it was really exciting to see that fish fins exhibit a similar tactile response’, says Hardy. He and Hale also suspect that the goby’s tactical sensitivity may have originated far back in evolution. ‘This primate hand-like touch also suggests that the ability to detect surface differences via touch has been around a lot longer than we previously thought’, he says. 10.1242/jeb.238089 Hardy, A. R. and Hale, M. E. (2020). Sensing the structural characteristics of surfaces: Texture encoding by a bottom-dwelling fish. J. Exp. Biol. 223, jeb227280. doi:10.1242/jeb.227280

51 Fiddler crabs ignore near misses when threatened from all sides

Everyone suffers with attention overload from time to time. Sometimes there are simply too many meetings and deadlines for one brain to keep track of, so you have to prioritise. Yet, compared with the life and death attention grabbers that stalk many creatures’ lives, we’ve got it easy. ‘Prey animals are often exposed to multiple simultaneous threats, which significantly complicates the decision-making process’, says Zahra Bagheri from the University of Western Australia. So how do animals that are facing several threats simultaneously prioritise which one, or ones, to take seriously? To find out how fiddler crabs respond when confronted with two looming threats, Bagheri, Callum Donohue and Jan Hemmi, also from the University of Western Australia, rigged up a pair of black spheres suspended on fishing line travelling threateningly toward the crustaceans: one on a direct collision course and the second coming in at an angle to produce an alarming near miss. Intriguingly, the crabs that were on a near-miss collision course began scuttling for safety when the alarming ball was as much as 300 cm away. However, the crabs that were in direct line with the incoming sphere stood their ground until it was within 100 cm; only then did they beat a retreat. In contrast, when confronted with both inbound balls simultaneously, instead of turning tail when the near-miss ball was within 300 cm, the crabs held on until the ball heading straight for them came within a threatening 100 cm. The crabs appeared to be completely disregarding the less dangerous threat, focusing exclusively on the inbound ball that could genuinely bowl them over. ‘Crabs that face multiple predators simultaneously behave as if they only face the single, directly approaching predator’, says Bagheri, adding, ‘this suggests that the crabs do not perceive, or do not respond to, the increased risk of predation posed by two simultaneous predators’. The team suspects that the crabs weigh up the relative dangers of each impending threat and focus on the one that poses the greatest risk, ignoring others that would simply result in a narrow escape. After all, there’s only so much attention to go around, so the key to survival is to use it wisely. 10.1242/jeb.238188 Bagheri, Z. M., Donohue, C. G. and Hemmi, J. M. (2020). Evidence of predictive selective attention in fiddler crabs during escape in the natural environment. J. Exp. Biol. 223, jeb234963. doi:10.1242/jeb.234963 52

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