By Ker Than-
A species of tiny bat seems to be using rolled-up leaves like trumpets to amplify calls, a new study says.
A few years ago, biologists Gloriana Chaverri and Erin Gillam were in Costa Rica studying Spix’s disk-winged bat, a species that is known to escape predators and harsh weather by roosting inside the folded leaves of plants such as the lobster-claws plant and calatheas.
The creatures are so small—each one weighs only about four grams—that multiple bats can fit inside a single leaf.
While studying the bats, the scientists noticed that individuals roosting inside leaves often could not recognize calls made by members of their own group flying outside. (See “‘Whispering’ Bat Evolved to Trick Prey.”)
“We started to wonder if the leaf was somehow affecting call fidelity,” or how accurately the call was perceived by the bats, said Chaverri, who is at the University of Costa Rica and whose research was funded in part by the National Geographic Committee for Research and Exploration.
“Then, given the shape of the leaf, we began to wonder if the leaves could increase sound intensity while affecting fidelity, much like acoustic horns do.”
So the scientists conducted an experiment in which they recorded social calls made by the bats and played them back through a speaker that was positioned at different ends of a leaf—either the narrow bottom end, which had been cut to create a hole, or the wider rim of the leaf.
A microphone was placed at the opposite end of the speaker to record how the leaf structure affected the bat calls.
The researchers’ findings, detailed in this week’s issue of the journal Proceedings of the Royal Society B, showed that furled leaves significantly amplified incoming bat calls because the narrowing sides of the leaves compressed entering sound waves. Outgoing calls were found to be only faintly amplified. (Interactive: Hear tropical bat calls.)
Interestingly, the experiment also showed that both types of calls were significantly distorted by the leaves themselves. The end result was that certain frequencies of the bats’ social calls were amplified, while others were muted.
These findings help explain Chaverri and Gillam’s original observation: Roosting bats often didn’t respond to the “inquiry” calls made by bats flying outside of their leaves, even if they were members of their own group.
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. (Also see: “Bats Use Magnetic ‘Compasses’ to Navigate, Study Says.”)
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.
Next, the team plans to study whether the bats somehow select leaves that are better suited for sound amplification, and if a leaf’s shape affects the sounds the animals make.