A Bat With a Listening Horn

Unfurling leaves act as hearing aids for the Spix's disc-winged bat.

What animal uses a leaf for a hearing aid? Answer: The Spix’s disc-winged bat, native to Latin America. Scientists, publishing in Proceedings of the Royal Society B, report that these bats use curled leaves to funnel sound made by their roost-mates. 

The tiny bats, weighing less than a nickel, get their name from the suction-cup discs on their wings and feet that enable them to cling to smooth surfaces. They live in tight-knit groups of five or six and use their small size to hide out in the unfurling Heliconia leaves and the fronds of similar plants. These leaves only stay curled for about 24 hours, which means the bats have to constantly find new resting places. 

When it’s time to sleep, scouts head out to find a suitable leaf. The remaining bats track the scout by making “inquiry” calls, which are simple, one-note cries. 

The scout then employs its new leaf as a horn-like hearing aid. The narrowing sides of the leaf compress the entering sound waves of the “inquiry” call, amplifying the sound by up to 10 decibels and enabling the bats to hear a call from up to 98 feet. (Without the leaf, the cry might only carry 65 feet.)

However, the leaf distorts the sound, preventing those that hear the call from recognizing friend from foe. To solve this problem, the scouts answer back with a “response” call, a more complex cry made up of as many as 20-25 different sounds. 

It’s then up to the flying bats to recognize a friendly “response” and hone in on the new roost. 

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