Tree cricket (Oecanthus spp.) baffle singing

Semantic satiation is when a repeated word starts to lose its meaning, becoming instead just a collection of sounds. For me, ‘baffle’ is one of those words that rapidly reaches satiation. Baffle. Baffle. Baffle. See?

Perhaps not. But it’s a phenomena I’ve had to confront when dealing with the subject of today’s post. Because we’re looking into the way that tree crickets use leaves to intensify and direct their song, and the term for that arrangement is a baffle. Actually, sound baffles can also be used to dampen sound, for example in concert halls or along the edges of motorways. Here, though, we’re interested in how a tiny insect uses them to change its acoustic appearance.

Harping on

Male tree crickets create a distinctive, high-pitched song in their efforts to attract females. They are widespread around the world—anyone who’s sat outside on a warm summer evening has likely heard their chirping serenade. The way that the males produce this sound (and it is only males, not females) is by vibrating their tougher front wings. A stiff section on the left wing serves as a scraper, which is rapidly drawn across a series of microscopic teeth on the right wing that are collectively called the file.

The result is stridulation, from the Latin for a creaking sound. The full cricket song is actually a nested set of pulses and tones making up a single chirp, with a series of chirps contributing to the complete song. Each tone (or burst) is the sound from a single tooth being ‘plucked’, and the full chirp is therefore the sound of one run of the scraper along the file. For the tree cricket Oecanthus burmeisteri, each chirp takes around 300ms to complete (less than half a second).

Here’s a schematic of the kinds of sound groupings we’re talking about:

With those basics covered, let’s spend a little more time with O. burmeisteri, a native cricket of southern Africa. That’s just what Transvaal Museum entomologist Lieselotte Prozesky-Schulze and her ornithologist husband were doing in late December, 1973.

They noticed that one of the male crickets they encountered in Pretoria had chewed out a hole in a small sunflower leaf. It had then positioned itself partway through the hole, with its wings tightly pressed against the edges of the gap. Searching around, they noticed that the same stance was also used by some nearby male crickets:

Being astute nature-watchers, they reasoned that this might have something to do with sound production, since burrowing mole crickets are known to create specially-shaped burrows to increase the projection of their calls. Prozesky-Schulze made her own calls to some acousticians at South Africa’s government research organisation, the CSIR (or Council for Scientific and Industrial Research). And together they found that, yes, there was a 2.5 to 3.5 times increase in the amplitude—or loudness—of the holed-up cricket chirps.

From their 1975 report in Nature, at the top is an unaided chirp, and below is the same cricket, in its self-made leaf-hole. The amplitude is the up-down size of the sound wave—note that making it louder doesn’t change the spacing, or wavelength, of the chirp:

Why is this happening? It’s because the leaf is acting as (you guessed it) a baffle. The word is used here in its sense of an obstruction, which directs or controls motion. When the cricket’s wings are pressed against the sides of the hole, the leaf’s size makes it much more efficient at spreading the sound than the tiny resonating part of its own wings. It also stops the pressure waves from the front and back of the wings from interfering destructively. Crickets are so small that they can efficiently produce only very high-pitched sounds (as opposed to big animals like elephants or whales), but those high frequencies don’t travel very far at all. The baffle changes the acoustic equation.

Fast forward about 40 years, and the cause of baffled crickets has been taken up in earnest by Natasha Mhatre, formerly of the University of Bristol and now based in Canada. In a series of elegant studies over the past decade, focused on the South Indian tree cricket (Oecanthus henryi), Mhatre has shown that the size and shape of both the cricket’s wings and the chosen leaf affect the loudness of its song.

She found that her tree crickets were using much more of their wing’s surface to resonate out their call than was seen in field crickets, for example. The latter use a special harp-shaped part of the wing—naturally called the harp—to amplify their song. But the smaller tree crickets had expanded on that strategy. The cluttered bush and tall grass environments favoured by tree crickets also favour transmission of lower frequency, higher powered sounds. The expanded wing resonator helps with that.

The leaf baffle ramps things up even further. Mhatre found that although not all the crickets made baffles, those that did so preferred to make their hole near the centre of the leaf, which is close to the ideal acoustic position. They didn’t use the direct centre of the leaf, because that would involve chewing through the main structural rib that allows the leaf to hold its shape. They also chewed out a hole that was just the right size to rest their wings against, maximising the value of the baffle. And they only used leaves that were large enough to give a sound boost—on smaller leaves they would just call without modifying the leaf. Essentially, they were manipulating their environment to optimise their song.

In fact, the one cricket in Mhatre’s experiments that initially cut a hole off-centre on a leaf then changed tack and made a second hole close to the centre. This gave it a clear signal boost. Mhatre reasoned that there are costs to this kind of destructive trial and error though, because the more holes you put in a leaf, the less effective it becomes overall as a baffle. Many species of Oecanthus make leaf baffles, and they typically put the hole in the right place the first time, which led Mhatre and her team to suggest that they are using specific ‘rules of thumb’ (rules of claw?) that reliably work:

If a male decides to make a baffle, three simple rules can produce an optimal baffle in a single attempt: (i) find the largest available leaf, (ii) place the hole as close to the centre of the leaf as possible and (iii) cut a hole that can just accommodate wings.

An important note here is that the cricket cannot actually evaluate how far its song is going out into the world. The heuristics work despite the cricket’s knowledge of a baffle’s effects, not because of it.

A biological loop-hole

Ok, but is there any proof that all this hard work actually pays off for the crickets? At this point Mhatre’s Indian colleague Rittik Deb and his team in Bangalore step in. Deb showed that baffles were mainly made by smaller crickets with softer natural calls—males that were more often overlooked by females in the mating game. Instead of being a tool used by everyone to boost their song, baffling therefore becomes what’s known as a condition-dependent strategy. Basically, big loud crickets do their thing, and the rest go through the expense of finding the right leaves and making holes just to stay in the game.

Deb went out into the field (literally) and found that only 25 out of 463 calling Oecanthus males, or about 5%, were using a baffle. Those aren’t exactly the results you’d see for a hard-wired or innate behaviour—sometimes called fixed action patterns—that some have posited for insect tool use. Critically, those 5% were significantly smaller than their comrades, and without their leafy sound systems, those smaller crickets sang significantly softer songs.

And the tactic worked, as this figure from Deb et al. (2020) demonstrates. On the left are results from a single male tree cricket, without fail attracting more females with the baffle (BF) than without (NBF). On the right are pooled data that consistently place the baffled crickets higher in the female attraction ranking:

So there we have it. Baffling works, and it’s easy enough to implement using simple rules, but it comes with a construction cost that makes it most beneficial to the little guys.

The only question left is one we keep returning to: but is this tool use? Mhatre actually tackled this head-on in a 2018 publication. She concluded that it is, using the widespread definition of tool-use given by Shumaker et al. in their comprehensive book from 2011 (we covered that definition and its implications in an earlier post).

The key is that manipulated objects don’t need to be entirely detached from their surroundings in order to be considered a tool. For example, you could hit your sibling with the end of a hose and that would be tool-use, albeit of an unkind sort, even if the other end of the hose is securely attached to your house and its plumbing. The cricket’s holey leaf is still attached to a host plant, but the insect does hold and orient the tool, draping it over its body, as well as manufacturing it in the first place.

You may agree, you may not. Tool-use is looking less and less like a coherent category of behaviour the more we learn, and ironically, tree crickets aren’t about to make it any less baffling. Baffling. Baffling. No, sorry, it’s just turned back into sounds again.

Further viewing. Two quick videos for you. First up is a short and a jauntily-scored look at cricket baffling, from GeoBeats Science. And following that, a slowed down version of unbaffled tree cricket song from Alyce Santoro that reveals the hidden rhythms within the chirps—with the catchy title ‘DJ Oecanthus’. Enjoy!

Sources: Turner, J. (2000) The Extended Organism: The Physiology of Animal-Built Structures. Harvard University Press. || Prozesky-Schulze, L. et al. (1975) Use of a self-made sound baffle by a tree cricket. Nature 255: 142-143. || Mhatre, N. et al. (2012) Changing resonator geometry to boost sound power decouples size and song frequency in a small insect. PNAS 109: E1444–E1452. || Mhatre, N. et al. (2017) Tree crickets optimize the acoustics of baffles to exaggerate their mate-attraction signal. eLife 6: e32763. || Mhatre, N. & D. Robert (2018) The Drivers of Heuristic Optimization in Insect Object Manufacture and Use. Frontiers in Psychiatry 9: 1015. || Mhatre, N. (2018) Tree cricket baffles are manufactured tools. Ethology 124: 691-693. || Deb, R. et al. (2020) Baffling: a condition-dependent alternative mate attraction strategy using self-made tools in tree crickets. Proceedings of the Royal Society B 287: 20202229.

Main and third image credit: Natasha Mhatre || Second image credit: Turner (2000) Fig.10.1 || Fourth image credit: Prozesky-Schulze et al. (1975) Fig.2 || Fifth image credit: Deb et al. (2020) Fig.2