New research shows that tiny bats in Central America use rolled-up leaves as acoustic horns to amplify incoming bat signals. Excluding human activities, it's the first time this technique has ever been observed in nature.
This species, called Spix's disc-winged bat (Thyroptera tricolor), likes to roost inside tapered tubes that form when new leaves of plants such as Heliconia or Calathea start to unfurl. But while studying these bats, biologists Gloriana Chaverri and Erin Gillam noticed that individuals inside the leaves were often unable to recognize calls made from their comrades on the outside. This got them thinking that something acoustic might be going on.
And indeed, as any recording student knows, amplification increases as sound waves get compressed inside a narrow space. The opposite happens as an acoustic space gets larger or wider, the result of increased sound directionality. At the same time, however, fidelity can get compromised, resulting in a muted, vague sound.
Which explains the scientists' initial findings — that roosting bats don't often respond to the "inquiry" calls made by bats flying on the outside, including those from members of their own group.
So why go to all this evolutionary trouble just to ignore these amplified incoming calls? It may have something to do with the "response" calls of the roosting bats. Christine Dell'Amore of National Geographic News explains:
While the calls might have sounded louder, they may have been so distorted that the identity of the caller was no longer discernible to the listening bats.
As a result, scientists think roosting bats are probably unable to differentiate between calls made by group members and non-group members, and that they likely just respond indiscriminately.
However, roosting bats also make their own unique call, called a “response” call. Scientists had previously observed that flying bats typically have no trouble identifying which leaves house members of their group.
This could be because response calls are more acoustically complex, Chaverri and Gillam speculate, so that even though the quality of the calls are degraded by the leaves, enough usable information still gets through that the listening bats know who’s calling.
The next step for the researchers is to see if the bats are actually selecting leaves that are better suited for sound amplification, and if its shape affects the sounds the animals make.
Read the entire study at Proceedings of the Royal Society B: "Sound amplification by means of a horn-like roosting structure in Spix's disc-winged bat."