Bats in Costa Rica have evolved a neat trick to help them hear their roost-mates flying above: They use leaves to funnel sound in a natural version of an old-timey ear horn.
The Spix's disc-winged bat (Thyroptera tricolor), named for suction-cuplike discs on its wings and feet, is found in South America. Unlike other cave-dwelling bat species, disc-winged bats roost each day in the unfurling leaves of plants outside of caves. These leaves form a tube shape as they go from folded-up to flat, meaning the bats can roost only for a day before having to find another leaf in the proper shape.
Spix's disc-wing bats are also cliquish. They form groups of five or six and stay together, despite their nightly evictions.
"They can stay together for many, many years, which is quite rare in bats," said Gloriana Chaverri, a postdoctoral researcher at Boston University and the director of research and conservation at the nonprofit Costa Rican ProSur Foundation. "These bats are fascinating." [See Images of the Leaf-Living Bats]
A previous study of the bat's chattering calls revealed that despite the need to recognize roost-mates, roosting bats weren't great at discerning whether they were talking to a close buddy or a stranger. The finding got Chaverri and her colleague Erin Gillam of North Dakota State University wondering whether the roosting leaves were changing the sound of the calls. Because the leaves are shaped like megaphones, the researchers also wondered if they might amplify the roosting bats' cries.
To find out, the researchers recorded calls from bats, including 79 "inquiry" calls, simple one-note cries used by flying bats to try to locate roosting pals. The other 65 recorded calls were complex "response" calls, which can consist of as many as 20 to 25 sounds made in response to an inquiry call, Chaverri said.
The researchers then played the response and inquiry calls from either inside the leaf or outside the leaf, posting a microphone at the opposite end to catch the sound.
Can you hear me now?
They discovered the leaves don't make very good megaphones: The calls of the roosting bats were only amplified by a decibel or two, an inaudible difference for human ears and unlikely to make a big difference in how far a bat can be heard.
But the funneling effect of the leaf inward made a big difference for how well roosting bats could hear their flying friends. Compared with a bat call heard without the leaf, a call heard from within the leaf was up to 10 decibels louder.
Decibels are measured on a logarithmic scale, such that 60 decibels is half as loud as 70 decibels, so as few as 10 decibels can make a big difference. In the case of the bats, the acoustic effect could increase the distance at which a flying bat is heard by 65 feet to 98 feet (20 meters to 30 meters).
However, the boosted cries were distorted, Chaverri said, because not all frequencies of sound amplify equally. The finding explains why roosting bats can hear their friends, but not necessarily recognize them. As a result, bats in the roost cry out in response to any inquiry they hear. It's the job of the flying bat to recognize the complex response call as familiar and join the roost. The findings are surprising, Chaverri said, because while animals are known to use structures such as logs to boost their calls, there are fewer examples of animals detecting calls with the aid of an object.
The researchers report their findings today (Oct. 15) in the journal Proceedings of the Royal Society B.