The Hills Are Alive With the Sound of Bat Song

Close-up of a Brazilian free-tailed bat colony. Photo by Kirsten M. Bohn

When it comes to animal models of human speech, songbirds are the gold standard. Scientists look to birds, and not other mammals, for clues about human speech because most mammals produce simple, monosyllabic, innate vocalizations that are much less flexible than those of birds. Birds sing complex, multisyllabic songs composed of multiple elements with flexible syntax (i.e., the way in which elements are ordered and combined). And both humans and songbirds possess neural circuits that support vocal plasticity that are thought to be absent in other mammals.

Cetaceans (whales and dolphins) and bats may be the exceptions. These animals have evolved a suite of neural adaptations to support echolocation, and they are the only two groups of mammals that demonstrate vocal learning, juvenile babbling, regional dialects, and cultural transmission of vocalizations.

Of the bats, the Brazilian free-tailed bat (Tadarida brasiliensis) is of special interest because it is a mammal that sings like a bird. This bat’s songs are hierarchically structured with specific syllables combined to form phrases, which are in turn combined to form complex songs. Song construction follows rules but specific syntactical arrangements vary from one song rendition to the next.

Florida International University biologist Kirsten Bohn, in collaboration with colleagues from Texas A&M University, has recently discovered that Brazilian free-tailed bats vary the syntax in their songs in response to different social situations. The research was published in June in Animal Behaviour.

Only male bats sing. Songs are especially pronounced during the mating season, when males establish territories with harems of females that they defend against other males. But they also sing year-round and in all-male colonies, so song may play a role in other social situations besides mating.

Bohn noticed that in a captive colony of bats at Texas A&M University, roosting males would start singing whenever another bat flew past. She thought the cue that elicited singing in this social context could be the echolocation calls of the passing bat. To test this, she and her colleagues recorded echolocation passes of bats in the lab and played them back to bats roosting both in the captive colony and in the wild (the wild in this case was the football stadium at Texas A&M, where Brazilian free-tailed bats roost in cracks in the concrete). They also played back other males’ songs.

Brazilian free-tailed bat colony. Photo by Kirsten M. Bohn

“The most incredible result was how strong and rapid male responses were to echolocation passes,” Bohn says. “In every roost where we played echolocation passes, we got responses.” What’s more, the bats responded extremely rapidly — in less than a second.

The second unexpected finding was that bats did not respond to other males’ songs at all. Bohn says this is especially interesting because elements of songs are identical to echolocation passes and both use similar frequencies. Yet somehow bats can distinguish between the two and respond as rapidly as 200 ms.

When Bohn and her colleagues compared songs produced in response to echolocation passes and those produced in other contexts, they found bats used different combinations of phrases. Songs produced during echolocation passes were also shorter than songs produced in other contexts. “These shorter songs are tailored to the context in which they are used because they must rapidly convey sufficient information to capture the attention of a flying bat that would pass beyond hearing range in 1-2 seconds,” writes Bohn.

Bohn thinks that Brazilian free-tailed bats sing in response to echolocation passes to notify the passing bat of the presence and location of an occupied roost. While the experiment doesn’t show if songs are directed at males or females, they could serve both purposes: warning rival males away from an occupied roost and inviting females to join the harem.

Brazilian free-tailed bats roost in the millions and have all-male, all-female, or mixed-sex colonies that fluctuate across seasons. Song flexibility may have evolved in these bats as a response to this highly variable social environment.

Next for Bohn is determining what aspects of bat song are learned and which are innate. To do this, she is looking at whether different vocal dialects exist in different areas of the Brazilian free-tailed bat’s range from Brazil to Texas. She’s also working on the neurological basis of vocal production in these bats. “The vocalizations these bats produce should not be possible with the standard mammalian vocal-motor pathway,” Bohn says. “It’s going to be very exciting to discover how these bats produce these songs, if they use the same neural pathways as birdsong and human speech, and how they evolved.”

 

Reference:

Bohn, K. M., Smarsh, G. C., and Smotherman, M. (2013). Social context evokes rapid changes in bat song syntax. Animal Behaviour 85(6): 1485-1491. http://dx.doi.org/10.1016/j.anbehav.2013.04.002

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