by Neil Bauman, Ph.D.
Humans are not the only creatures that use “hearing aids” in order to hear better. You may be surprised to learn that some animals do too. In Costa Rica, the Spix’s disk-winged bats (Thyroptera tricolor), named for suction-cuplike discs on their wings and feet, use leaves to funnel sound in a natural version of old-time “ear trumpets” (1)
The bat “ear trumpets” are made out of naturally-furled leaves. These furled leaves let the bats better hear other bats in their group flying above them from a greater distance than would otherwise be possible. This helps keep the group members from getting separated.
“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.” (1)
Spix’s disc-wing bats are also cliquish. “They form groups of five or six individuals and tend to stay together for many years. They have a complex communication system involving a single-sound inquiry call that they emit when in flight to locate other bats in their roosting group. Members of their group then make response calls consisting of as many as 20 to 25 sounds. The difficulty the bats have is hearing the inquiry calls from large distances.” (2)
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. (1)
This is where the furled leaves come into play. The “ear trumpet” shape of the leaves amplifies the incoming calls up to 10 dB. (1) (We would perceive this as double the volume.) This makes a big difference in how well roosting bats could hear their flying friends.
However, “the boosted cries were distorted because not all frequencies of sound amplify equally. This 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.” (1)
So now you know why these bats deliberately nest in these rounded leaves. It increases their chances of hearing inquiry calls, so that they can send out a recognizable message to their fellow bats at the right time. (2)
The 10 dB increase in sound volume increases the distance at which the flying bats can be heard by their roost-mates by an estimated 65 to 98 feet. (2)
You can see some fascinating pictures of these bats roosting in the bottoms of their “ear trumpet” leaves.
(1) Pappas, Stephanie. 2013. Speak Up! Costa Rican Bats Use Leaves as Hearing Aids.
(2) Bats in Costa Rica Using Leaves for Hearing. 2013. Audiology Worldnews.