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Bats use leaves as megaphones to call home

Bats are climbing inside curled leaves and using them to amplify sound, marking the first time an animal has been observed using a tool to increase its vocalization range. A recent study has shown that Spix's disk-winged bat uses the shape of the leaves to boost the sound of both incoming and outgoing calls.

The tiny bats — so named for cute little suction disks on their wings — roost inside curled-up leaves. To identify the correct roosting spot, they call out: when that call is met by a chirped response, they know they've found home. The study, published by the Proceedings of the Royal Society, found that both call and response sounds were amplified by the shape of the leaves. Outgoing replies from roosting bats were faintly boosted by the leaves' trumpetlike effect; incoming calls from flying bats were significantly increased in power as sound waves were funneled down the leaves' lengths.


The study also found that both of these calls were so significantly distorted by their leaf modulation that roosting bats wouldn't be able to understand who was making the sound. As io9 report, National Geographic News' Christine Dell'Amore explains that roosting bats instead "respond indiscriminately" to calls of their species. Response calls, Dell'Amore says, are "more acoustically complex," meaning that despite leaf distortion, enough information gets through for a calling bat to successfully find his friends.

Spix's disc-winged bats roost inside leaves of plants such as Heliconia 
Due to their shape, these leaves can amplify sounds by up 10 decibels
The leaves, however, also cause messages to become muffled which may have caused the bats to develop a more complex response call

Biologists are finding more and more animal species, besides apes and crows, that use tools. Most recently, Gloriana Chaverri of the University of Costa Rica has discovered that a particular South American bat – Spix’s Disc-winged bat (Thyroptera tricolor), so-named because of the circular suckers on the bases of the thumbs and soles of the feet that allow them to hang – use leaves to amplify sound. The bats roost in groups of 5 or 6 inside unfurling Heliconia and Calathea leaves. Because the leaves remain curled up for only about 24 hours, the bats have to find new homes almost every day. Dr. Chaverri's tests in the laboratory confirmed that incoming calls were boosted by as much as 10 decibels and that the responses, although boosted by only 1–2 decibels, were made highly directional by the megaphone-shaped leaf.
Thyroptera tricolor is one of two species in the family Thyropteridae, more commonly known as the Disc-winged Bats or New World Sucker-footed Bats. As the common names imply, these bats are easily recognizable by the presence of a circular sucking disk on the bases of the thumbs and soles of the feet. The suckers are used during roosting, when the bats hang head-up inside the tubular, young leaves of Heliconia trees. The suction created by the disks (and strengthened by licking and sweat gland secretions), is so powerful that only one sucker is needed to support a bat’s full weight (Nowak 1991).

This small species, with a body length of 34-52 mm and an average weight of just 4.2 grams, can be found in tropical forests from southern Mexico to Bolivia and South Brazil and also on the Island of Trinidad (Nowak 1991). While some members of this species elect to follow a solitary lifestyle, it is not uncommon to discover up to ten individuals roosting in the same Heliconia leaf and seeking out new habitat together when that leaf matures and opens. Like many species of bat, T. tricolor is insectivorous and can eat up to one-quarter of its body weight in prey during a single night’s activities.

In appearance, Thyropterids have long, slender snouts and large, funnel shaped ears. The snout lacks a noseleaf, and is instead covered with warts that may serve a sensory function. The nares are circular in shape and are spaced rather far apart from one another. The general head shape of Thyropteridae most closely resembles that of Natalids and Furipterids as all three share a considerably elevated crown and a concave forehead (Hill and Smith 1984).

Specific characteristics of the Thyropterid skull include the presence of a tragus, a relatively small auditory bullae that covers only a portion of the cochlea, and the lack of a postorbital process (Anderson and Jones 1984). Additionally Thyropterids have a complete premaxilla, well-developed premolars, and palatal branches that isolate two foramina (Anderson and Jones 1984). The overall dental formula is 2/3, 1/1, 3/3, 3/3 and gaps can be found separating the incisors and canines and the left and right incisors (Anderson and Jones 1984).

A large number of post-cranial characteristics also diagnose Thyropteridae and according to Anderson and Jones (1984) the list includes: toes with only two phalanges; second finger reduced to an incomplete metacarpal; third phalanx of third finger ossified; a small presternum with a forward facing keel, mesosternum with obsolete keel, and xiphisternum with reduced keel; separate lumbar vertebrae; first and second thoracic vertebrae fused; third and fourth toes fused through to the tips and claws; a reduced fibula; and a fused sacrum, with the last two vertebrae distinct. In males there are both a baculum and pubic symphysis present.

During scanning of the whole skeleton, it was discovered that this female specimen contained a single, well-developed embryo, which can be viewed on its own page here. Thyroptera tricolor breeds twice annually and mating is known to be polygonous. The young are unable to fly for the first month and so after birth the mother will carry her single offspring in flight, with the baby clinging to her abdomen (Anderson and Jones 1984).

Anderson, S. and J. K. Jones, Jr. 1984. Orders and Families of Recent Mammals of the World. John Wiley and Sons, New York. 686pp.

Hill, J. E. and J. D. Smith. 1992. Bats: A Natural History . University of Texas Press, Austin. 243pp.

Nowak, R. 1991. Walker's Mammals of the World, 5th edition. The John's Hopkins University Press, Baltimore and London. 642pp.

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.Disk-winged bats inside leaves on Barro Colorado Island, Panama. Photograph by Christian Ziegler, National Geographic

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.”

Hear, Hear

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.

Say What?

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.

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BatsRule!: Bats use leaves as megaphones to call home
Bats use leaves as megaphones to call home
bats use leaves as megaphones to call home
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