Hunting the Silent Cricket

Hunting the Silent Cricket: Convergent Evolution in Acoustic Insects A TALE FROM THE AMAZON Santiago and I were walking back from a night of collecting specimens in the forest. We had left the research station at sunset, had paddled our low dugout through flooded woodland using the short, pointed-tipped paddles which are good for pushing through mud and shallow swamp, out onto the Amazon river and to the indigenous village of Mocagua, Colombia, where we had met up with our guide and walked through town to a stagnant pond (which had to be crossed in a second canoe), and to the narrow trail which led into the rain forest, and nobody knew how far. For five hours we had wrestled with branches and vines, hiking and searching for crickets, filling the small plastic vials one by one with our quarries, many of which would turn out to be precious representatives of previously unknown species. We had turned over leaves, pried away bark, peered into holes, and sifted through debris, always keeping a wary eye out for those enormous, hand-sized bird spiders which, like crickets, are excellent jumpers. Crickets had been caught and crickets had escaped, until it became clear that the batteries in our headlamps would not last much longer. Our guide had led us back out of the forest, alive and unharmed. By the time we returned to Mocagua, the generator had long since quit, revealing the stars of the Southern Hemisphere, and casting the platform huts into darkness. An army of dogs announced our return to the village. Some candles were lit in the one building which was used as the general store, the meeting-place, the dance hall, and the saloon: an open-air porch with a thatched roof, without windows or doors. For the third night in a row, we were met by a small group of Tikuna men who offered us rum and beer at the end of the night. Santiago and I always felt a bit awkward joining them like that, me a conspicuous six feet tall and him still taller, both of us in long sleeves and trousers and smeared with mud from the night’s work -- as the men sat together, freshly washed, shirtless and barefoot and comfortably cool with bottles of warm beer in the Amazon night. Our guide took a seat next to them. The Tikunas of that region are accustomed to field biologists, but seem incredulous and entertained at the lengths we will go to, to study the details of nature. Conversing in Spanish, they like to recite our own story back to us, in the form of a question, to make sure they’ve got it right: “You came to Colombia -- on an airplane -- and you traveled down the Big River all the way here, just to catch some crickets?” “That’s right,” I say, foreseeing their next question: “Aren’t there any crickets where you come from?” “Sure, but not like these.” I lay a few vials out on the table, between the beer bottles and the shot glasses. Chestnut hands reach for the vials and hold them to the candlelight, as our finds are examined and passed around to the others. “Ah, yes...” says one, “special crickets. Everyone comes here to find special things: special birds, special flowers, special snakes...” The men bob their heads in agreement. “I don’t care if they are special,” says our guide, “I use them for fishing bait.” 2 “That’s why you find over half of what we catch. You’ve got the eye for ‘em!” We sit in silence for a while, until one of the men says, “I know a story about a cricket.” A cricket had been up late one night drinking rum, and there was a full moon, and he went out to sit on a bridge over the River. He saw his reflection in the water and began admiring himself out loud, so that all the other animals could hear every word. “Just look at me! Why, I’m the finest creature in the forest,” he said. “I’m more beautiful than Hummingbird, I sing sweeter than Tree Frog, and what’s more, for my size, I am ten times stronger than Jaguar. They ought to make me the king around here, since there’s no one better suited to it.” He went on like this for some time on the bridge, drinking rum and admiring his reflection in the water and congratulating himself on his good qualities, until a particular monkey who liked to cause trouble ran off to tell Jaguar, who was the true king of the forest. Jaguar, being a social fellow, sought out the cricket and sat beside him on the bridge in the moonlight. “Hello, Jaguar,” said the cricket. “Good evening, Cricket,” said Jaguar. “Say, Cricket, I hear you’re up here drinking rum, gazing at your reflection in the river, enjoying how great and wonderful you are, and announcing to everyone that you are stronger and prettier than me and should be elected King of the Forest. Is that so?” “Yes, sir” said the cricket, who quickly added, “but drinking always blurs my vision.” ACOUSTIC COMMUNICATION IN CRICKETS Crickets in nature are not known to drink rum and converse with predatory mammals, but the one in this joke is an apt caricature of a real cricket anywhere in the world. In nature, male crickets of most species stay up all night advertising themselves to females through audible chirps produced by rubbing the front pair of wings together – a behavior known as stridulation -- and like the cricket in the story, they also fall silent 3 when danger is near. Although female crickets lack the structures for stridulation, adults of both sexes have two auditory organs, one on each front leg, consisting of a perforation and a tympanal membrane, mechanically similar to the ears of mammals. Acoustically, a cricket’s song is characterized by a particular pulse rate, frequency, and amplitude, voiced in a specific combination which is unique to each species, helping to ensure that males and females pair up with the right kind of mate. Listening closely on a summer night will reveal not one type of cricket song, but a multitude of variants, representing the different calling species in the area at the time. The system, however, is much more complicated than a simple game of call and response. Through several decades of behavioral experimentation, biologists have plainly shown that female crickets are extremely picky, and tend to go for conspecific males with the ‘best’ songs1. The parameters which constitute the best songs seem to be arbitrary and vary from species to species, but females have their criteria and there is little that males can do about it. Successive generations of competitive males and choosy females are believed to drive the evolution of males to make their songs as alluring as possible – a process Darwin called sexual selection2. Males with the most attractive songs will be more likely to mate and will therefore leave the greatest number of progeny. Since there is a partial genetic basis to calling song3, many of the offspring will inherit the ability to sing like their fathers. A number of researchers argue that females don’t care as much about the ‘best’ songs as they do about the information songs contain which can 1 Reviewed in Otte, 1992 2 Sexual selection differs from natural selection in that the affected traits do not necessarily enhance the health, strength, or lifespan of the organism, but rather enhance its reproductive output due to the increased frequency of mating. 3 Webb & Roff 1992, Shaw 1996 4 help them select the best mates. Such cases are known in several species of frogs, where females prefer calls which are indicative of males with large bodies. Evolutionary theory suggests that if a male can produce the song which most closely matches the criteria of conspecific females, his talent is interpreted as an index of his biological fitness, or capacity to produce large numbers of strong and fertile offspring. In anthropomorphic terms, each male courts the females of his species through a broadcast of braggadocio -- singing his sweetest song in an attempt to persuade them that he is the finest mate, worthy of being the father of their children and beyond that, the king of the forest (or at least of all conspecific males). In today’s noisy world of frogs, birds, mammals, and other animals which produce acoustic signals, the chirping of crickets is usually heard as a subtle undertone within nature’s soundscape, when in fact it is one of the most ancient systems of communication by sound in the history of the Earth. Orthopteran insects – the lineage to which crickets belong, together with locusts, other grasshoppers, and their sisters, the katydids – are first known as fossils from the late Paleozoic, almost 100 million years before the appearance of the dinosaurs. The first Orthopterans were more like modern katydids than modern crickets, but their wing morphology shows that they stridulated using the same mechanism as crickets and katydids today4. Time travelers to the steamy Carboniferous5 swamps so often depicted in museum dioramas would find their surroundings to be eerily quiet -- with no birds singing, frogs croaking, dogs howling, monkeys screaming, or any other animal sounds at all, save for those made by the ancestors of crickets and other 4 This evidence comes from the examination of fossilized forewings, which clearly show the specialized vein which is the point of friction during song production. 5 The penultimate period in the Paleozoic era, lasting from approximately 360 to 290 million years ago.

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