Colour and Motion Affect a Dune Wasp's Ability to Detect Its Cryptic Spider Predators
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www.nature.com/scientificreports OPEN Colour and motion afect a dune wasp’s ability to detect its cryptic spider predators Dulce Rodríguez‑Morales1, Horacio Tapia‑McClung2, Luis E. Robledo‑Ospina3 & Dinesh Rao1* Ambush predators depend on cryptic body colouration, stillness and a suitable hunting location to optimise the probability of prey capture. Detection of cryptic predators, such as crab spiders, by fower seeking wasps may also be hindered by wind induced movement of the fowers themselves. In a beach dune habitat, Microbembex nigrifrons wasps approaching fowerheads of the Palafoxia lindenii plant need to evaluate the fowers to avoid spider attack. Wasps may detect spiders through colour and movement cues. We tracked the fight trajectories of dune wasps as they approached occupied and unoccupied fowers under two movement conditions; when the fowers were still or moving. We simulated the appearance of the spider and the fower using psychophysical visual modelling techniques and related it to the decisions made by the wasp to land or avoid the fower. Wasps could discriminate spiders only at a very close range, and this was refected in the shape of their trajectories. Wasps were more prone to making errors in threat assessment when the fowers are moving. Our results suggest that dune wasp predation risk is augmented by abiotic conditions such as wind and compromises their early detection capabilities. Prey strategies to avoid attack by ambush predators are more efective in the early part of the predation sequence 1. Ambush predators are ofen cryptic, with their body colouration matching their environment2. Tey are usu- ally still since movement can break their crypsis 3; they ofen have venom to debilitate their prey 4 and most importantly they hunt at a moment when the prey is vulnerable, i.e., during foraging or mating—when prey awareness is compromised1. Terefore, for a prey to overcome an ambush predator’s strategy, evaluation of a risky site is crucial. A prey’s ability to detect an ambush predator is constrained by its perceptual capabilities through chemical, vibratory or visual mechanisms 3. Visual detection of a predator depends on the spectral sensitivity of the prey’s eye (the ability of the eye to respond to specifc wavelengths of the light spectrum 5), spatial acuity (the capacity to discriminate shape and pattern details6 and temporal resolution (time taken to process visual information5). Furthermore, abiotic factors such as wind or obstacles can add to the visual clutter in a habitat 7,8 and consequently hinder predator detection. Te problem of detecting ambush predators is commonly faced by pollinating insects that approach fowers harbouring crab spiders (Araneae: Tomisidae)13. Crab spiders are famously cryptic—their body colouration blends into the background of the fowers9. Some species can change their colour to match the fower10 and oth- ers are mottled in various shades11. Tis crypsis may serve to avoid detection by potential prey12,13, perhaps by interfering with search image formation. However, there is still controversy whether the intended receivers of the crypsis are the spider’s prey or predators. Crypsis was found to be inefective when considering the entire community of fower visiting potential prey 14 and a recent study argued that crypsis in crab spiders reduce their risk of predation by birds15. Tough some crab spiders can increase the number of potential pollinators approach- ing the fower using deceptive signalling that exploit an insect’s ability to perceive UV colouration 16–18, several studies have shown that the presence of a spider on a fower deters pollinators19,20. Clearly, some pollinators are capable of detecting these spiders21,22 and minimise their risk by evaluating the fower before landing23. What is not known, especially in natural conditions, is if insects can respond to a predation risk by altering their fight trajectories before landing and whether motion of the fowers afects their evaluation. In this study, we evaluated the predator detection strategies of dune wasps (Microbembex nigrifrons; Hyme- noptera: Crabronidae) as they approached a spider occupied Palafoxia lindenii fowerhead under two conditions 1Instituto de Biotecnología y Ecología Aplicada (INBIOTECA), Universidad Veracruzana, Xalapa, Veracruz, Mexico. 2Instituto de Investigación en Inteligencia Artifcial (IIIA), Universidad Veracruzana, Xalapa, Veracruz, Mexico. 3Red de Ecoetología, Instituto de Ecología A.C., Carretera antigua a Coatepec No. 351, Xalapa, Veracruz, Mexico. *email: [email protected] Scientifc Reports | (2021) 11:15442 | https://doi.org/10.1038/s41598-021-94926-7 1 Vol.:(0123456789) www.nature.com/scientificreports/ Figure 1. Colour modelling analysis summary of the Hymenopteran visual perception of Mecaphesa dubia spiders when located in the side or in the top part of the Palafoxia lindenii fowerhead at diferent observation distances (2, 5, 10, 15 cm). (A) False colour image simulating the perception of the wasp visual system. Te image of the spider in the diferent parts of the fower were created for visualization purposes by assigning the colour blue, green and red for the UV, SW, and MW photoreceptor, respectively. (B) Results of a Receptor Noise Limited flter method, which performs pixel noise reduction afer the acuity control based on Gaussian flters while preserving chromatic and luminance edges, simulating spectral sensitivity and visual acuity of the wasp visual system at diferent distances of the spider located in the diferent parts of the fowerhead. (C) Te proportion of perceptual overlap between the spider and the fowerhead in the Hymenopteran colour space (higher overlap implies that it is more difcult for the viewer to perceive diferences). of wind induced movement, i.e., when the fower was still and when it was moving. We measured the appearance of the fower and spider using psychophysical visual modelling from the perspective of a hymenopteran visual system and related the appearance to changes in wasp fight trajectories and landing decisions. If a wasp can detect the presence of the spider, we expected that their fight characteristics would refect it and that the wasp would be able to detect spiders more easily when the fowers were still. Results Visual modelling. We generated pseudo-colour images of the spiders on the fowerhead (Fig. 1A) that took into account both spectral sensitivity as well as visual acuity. Te log Receptor Noise Limited (RNL) cluster modelling of the chromatic distances (∆S), perceptual thresholds (1 Just-Noticeable Diference), and visual acu- Scientifc Reports | (2021) 11:15442 | https://doi.org/10.1038/s41598-021-94926-7 2 Vol:.(1234567890) www.nature.com/scientificreports/ Figure 2. Sample trajectory of a Microbembex nigrifrons wasp approaching an unoccupied Palafoxia lindenii fowerhead as seen from the top. Wasp positions are subsampled for clarity. Te point when the wasp made a decision (in this case to avoid the fower) is highlighted in red. Circles around the fowerhead represent the diferent distances used in the visual modelling analysis. Te insets (A) and (B) show the Mecaphesa dubia spiders tethered above and to the side of the fowerhead respectively. ity of the wasp visual system showed that spiders may be detected only at a distance of around 5 cm from the fowerhead (Fig. 1B). When comparing the overlap of the colour maps (Fig. S1) that represent the perception of the spider on the fowerhead by the wasp visual system with respect to the spider position, we found that the perceptual overlap is higher when the spider is on the top of the fowerhead (F = 19.7, df = 1; p < 0.001) and at a larger distance away from the fowerhead (F = 5.23, df = 3; p = 0.004; Fig. 1C). When the overlap is higher, the wasp would fnd it harder to visually separate the spider from the fower background. Te interaction between position and distance was not signifcant. Tus, the perceptual overlap was signifcantly higher between 2 cm and 15 (p = 0.014) or 10 cm (p = 0.004), while there is no diference between the other pairwise comparisons. Wasp behaviour. Dune wasps approached fowers in a typical zig-zag fight (Figs. 2, 3; see S2 for a video and S3 for a plot of all trajectories). Wasps were signifcantly more likely to land on fowers that were moving, irrespective of the presence of a spider (GLM, Logit Link, χ2 = 9.1, df = 1,37, p = 0.003) whereas spider location (χ2 = 3.2, df = 2,37, p = 0.193) and the interaction (χ2 = 1.3, df = 2,37, p = 0.512) were not signifcant. Tere was no signifcant efect of spider or fower treatment on the distance at which they made a decision to avoid or land on the fower (GLM, Identity link function, Gamma distribution, Spider: χ2 = 0.0006, df = 1,38, p = 0.97; Movement: χ2 = 1.39, df = 1,38, p = 0.24; Interaction: χ2 = 0.17, df = 1,38, p = 0.67; Moving: Mean ± S.D. = 5.14 ± 2.91 cm, Still: 6.45 ± 3.16 cm). However, there seems to be a trend showing that wasps make their decisions earlier when approaching moving fowers in comparison with still fowers. Tere was no efect of spider location (ANOVA: χ2 = 2.77, df = 2,37, p = 0.25), movement (χ2 = 0.54, df = 1,37, p = 0.463) or the interaction (χ2 = 1.99, df = 2,37, p = 0.36) on inspection time. Distance profles. Spider location, fower movement and their interaction signifcantly infuenced wasp dis- tance profle sinuosity (GLM, Identity link function, Spider: χ2 = 7.75, df = 2,37, p = 0.021; Movement: χ2 = 10.87, df = 1,37, p < 0.001; Interaction: χ2 = 7, df = 2,37, p = 0.030). Sinuosity was higher when the spider was on the side of the fowerhead in the still treatment, suggesting that wasps could respond to the presence of the spider in this condition.