- Male tree crickets use leaves as baffles to appear louder to females.
- They cut a pear-shaped hole in a leaf, stick their forelegs through the hole and sing, amplifying their chirps by as much as four times.
- Tree crickets can choose the most suitable leaf, and craft the most acoustically optimal baffle, in the first try.
When a male tree cricket decides to serenade, he sometimes searches for a large leaf. He makes his way deliberately to the underside of the leaf, cuts out a pear-shaped hole that can just about fit his tiny body, sticks his front legs through the hole, and starts singing. He can press his wings closer to the leaf to make the call louder. He uses the leaf as a baffle (or a megaphone) that amplifies his chirps by as much as four times, broadcasting his presence to far-off females.
When a cricket makes a hole in a leaf and sticks the front part of its body through it, it is converting the leaf to a baffle, and preventing acoustic short-circuiting. A baffle prevents the sound from the front and back of the speaker from colliding and causing noise interference.
In a series of experiments, Natasha Mhatre from the University of Toronto and collaborators from the University of Bristol and Centre for Ecological Sciences, Indian Institute of Science, showed that tree crickets choose the ‘best’ leaf, and cut out the most acoustically optimal baffle — all in the first try. No editing, no trials.
The cricket broadcasting studio
Males in all cricket species use sound to attract mates. Louder calls travel further, render the male more attractive, and make it easier for females to locate the singing male. Cricket species use many strategies to make their singing more powerful. A spider-like bushcricket from Columbia chirps as loudly as a power saw, using its specially modified wings. The mole cricket has modified front legs, which it uses to dig loudspeaker-shaped burrows in the soil to sing from. Tree crickets make their own amplifiers from leaves.
Tree crickets sing by ‘stridulation’ — they rub modified front wings together to produce a series of chirps and trills. Forewings are modified to form the elytra, a tough leathery sheath that protects the soft body beneath. Each wing bears a stridulatory file, a row of regularly spaced teeth. “It is a kind of violin, of which the ribs [teeth] represent the strings,” describes John Stark eloquently in his 1828 book. The hardened edge of the second wing serves as a scraper, or if we were to use Stark’s violin analogy, the bow. The cricket rubs the scraper and file together to sing.
The problem with this arrangement is that the sound produced on either side of the wings cancels out, a process called as acoustic shortcircuiting. Sound waves emanating from either side of the source – the wings in this case – cancel each other out. Tree crickets are very prone to this problem because their wings are small (~9 mm in length) compared to the wavelength of their call (~110 mm at 3.1 kHz).
They use baffles to tackle the problem.
A leaf to help… but not always
Using simulations, the researchers modelled the acoustics of crickets (Oecanthus henryi) calling in free space, and those calling with baffles. Using a baffle had a definite advantage, providing a 1.8 fold increase in sound pressure levels and a four-fold increase in sound.
Baffles made in nature by crickets vary a lot, because crickets encounter leaves of varying sizes, and produce holes of different sizes. Further simulations that tested the effectiveness of baffles made of different sized leaves, and different types of holes, showed that larger leaves with cricket-sized holes make the best baffles.
In lab experiments and in the field, the researchers constantly noted that crickets seem to have the rather uncanny knack of carving out a perfect hole for their bodies. “They seem to know, somehow, they cut a big piece very close to the size and then adjust a little,” said Mhatre.
You would expect that crickets in the wild would be going all out to make baffles and that you’d find larger leaves in a cricket dominated area riddled with pear-shaped incisions, but that’s not the case. In natural populations observed over three field seasons, the probability of finding a male with a baffle was as low as 26%, and there was also wide variation in leaf sizes used to make baffles. Even under controlled laboratory conditions, about a third of the males did not make baffles, though they were provided with large leaves suitable for baffle making.
“Thus baffled calling is not a stereotyped behaviour, tree cricket males must decide whether to call with or without a baffle,” write the authors in the paper. The authors feel that the variation in baffle making in a cricket population indicates that there are multiple evolutionary forces driving the behaviour.
Rittik Deb from the Centre for Ecological Sciences, one of the authors, lists out these pressures. “The advantages are obvious — louder sound, larger sound field, hence more females attracted,” he said. On the other hand, the costs are numerous. “Firstly, there might be a cost associated with finding an ideal leaf. The cricket has to invest time that it could’ve spent signalling. The more it travels, the higher are the chances of getting eaten. Once the leaf wilts in a few days, the cricket is back to square one.”
A louder signal also makes it easier for predators to locate the cricket. Also, there are costs because of the plant the cricket uses in the wild, Hyptis suaveolens. “Making the baffle can be costly with respect to the effort and mouthparts used for the process, as these are thick leaves, filled with hair like structures and likely to be high in chemical content, they have a strong smell,” said Deb.
Lars Chittka, a professor at the Queen Mary University of London, agrees that there could be multiple evolutionary pressures on baffle making. “It might be that non-baffle making individuals persist in the population… [because] the time costs with generating them are sometimes not worth it. Instead, you can start calling and attracting females sooner without such an ‘amplification tool’,” he said. He also pointed out that there could be a component of individual flexibility: each male could have a different propensity to make baffles, “but some only start making them after repeated calling without baffles doesn’t produce the desired success.”
Bull’s eye at the first try
Most males choose the ‘best’ leaf and make a near-optimal baffle, in a single attempt. Once a baffle is made, it is difficult to edit. Two holes on a leaf can fuse together. But how does the cricket know what to do? The authors hypothesise that tree crickets optimise acoustics by using three hard-wired rules of thumb: find the largest available leaf, choose a place as close to the centre of the leaf as possible, and cut a hole that can just accommodate wings. These simple rules, or ‘heuristics’, have enough information for the cricket to produce an optimal baffle, write the authors.
“[An] inherited/hard-wired heuristic is the most parsimonious explanation, given the information we have about other, similar traits in insects, given the generality of this behaviour in tree crickets, and given the accuracy with which baffles are constructed, seemingly without the opportunity to learn,” said Thomas Blankers, a postdoctoral fellow at Cornell University, who was not involved in the study. He agrees with the authors that trials with naive males, which have never built a baffle before, are necessary to make conclusions.
How would such a strategy evolve? Baffles are effective in two ways: a ‘good’ male can use a baffle to further enhance his attractiveness by adding variation to his calls. A smaller male, or one in poorer condition, can use a baffle as a compensatory mechanism to sound bigger, louder and more attractive. If we take a look at cricket evolution, the tendency to make baffles could have arisen as a coping mechanism for small wings, said Mhatre.
Baffle making is next on a growing list of sophisticated tool use in invertebrates, like bumblebees learning to roll a ball to get a reward, and octopuses using coconuts for self-defence. Baffles are an “external structure” that “the animal has fashioned to fulfil a certain function,” said Chittka. “These animal-made structures are absolutely fantastic evolutionary innovations that lack any parallels in the world of vertebrates. But on the other hand, these structures aren’t quite tools in the same way as a man-made tool that has been fashioned by insight and individual trial-and-error processes.”
“That said only a tiny, tiny minority of humans actually innovate tools—the vast majority of us simply use the structures that have been invented by others, often generations ago, without any form of deeper understanding—e.g. everyone can use a megaphone as a tool to amplify sound, yet very few people understand how a megaphone works, and even fewer could build one,” said Chittka.
Citation:
Mhatre, N., Malkin, R., Deb, R., Balakrishnan, R., & Robert, D. (2017). Tree crickets optimize the acoustics of baffles to exaggerate their mate-attraction signal. eLife, 6, e32763. DOI: 10.7554/eLife.32763.