That birds sing—to attract mates, communicate with young, or defend territory—is a given. But among mammals, breaking into song is a relatively rare occurance. If any mammal were to meet their match through melody, though, it would probably be a bat, an animal that already relies heavily on sound and keen hearing for survival. And in fact, according to a study published last week in Behavioral Ecology and Sociobiology, male lesser short-tailed bats produce intricate songs that disclose their size, enabling females to assess potential mates through song alone.
“Birds and bats may have evolved song because flight is energetically expensive,” says Cory Toth, behavioral ecologist and lead author on the study. “Song broadcasts your intentions without having to move around as much.”
The lesser short-tailed bat is a teeny thing—only about 3 inches from head to claw—and is considered vulnerable in its native home in New Zealand, where deforesting is on the rise. These bats comprise one of only two known species of bats to engage in "lek mating," wherein males put on elaborate displays for visiting females who assess their breeding potential, and one of only a handful of bats that sing. (Others include heart-nosed bats and black flying foxes.)
Now bats join an elite club of mammals—including mice, whales and humans—that serenade potential mates with intricate, information-rich melodies. “This is a really interesting and exciting [finding],” says Michael Smotherman, a biologist at Texas A&M University who was not involved in the research. “[This] unique bat does something uncommon even for bats.”
At night, males will nestle into small cracks in trees called singing roosts, where they engage in marathon arias that can last up to eight hours. Like sirens, they call out into the dark in the hopes of enticing a female to the roost.
Singing roosts can be solitary, occupied by only one male. Or they can be what the researchers designate as “timeshares,” which are sequentially inhabited by up to five males. Males also personalize their roosts: they collect their urine with their hind feet and rub it under their chins, which they use to mark the roost entrance like a pungent welcome mat. This behavior may help them return to the same spot night after night, which in turn makes it easier for females to track their vocal performance over the breeding season.
But while it was known that males vocalize throughout their springtime mating season, no one had looked closely at the role they played in attracting females, says Toth. He decided to go behind the scenes of these bats’ operettas, recording over 25,000 vocalizations from 16 male lesser short-tailed bats over two months of their breeding season and analyzing their complexity and composition.
The bats’ songs turned out to be remarkably intricate, showcasing dozens of distinct syllables (which are roughly similar to the ones in human speech). Four syllable types were especially common: trills, which rapidly fluctuate in frequency; trill-downsweeps, or trills immediately followed by a sharp, sustained drop in frequency; upsweep-trill-downsweeps, which are comprised of a rapid increase in frequency, a trill, then a drop in frequency; and upsweep-trills, where the frequency rises into a trill.
The bat vocalizations were sophisticated enough, Toth found, that they could even be attributed to individual singers. That made sense, because females would need to distinguish between individual males in the nightly vocal parade. The presentation of song is a very high-stakes display for males: because they don’t offer females protection, present gifts, or parent offspring, males must advertise their fitness solely through sexual displays—in this case, outsinging your neighbor.
But could bat songs actually give a female more information about the quality of her potential mate? The answer turned out to be yes: smaller males produced longer trill-downsweeps, the second-most common syllable type. And smaller males, in these bats, seem to be preferred. Toth had previously observed that smaller males spent more time in their roosts, which tended to be solitary, and sang more frequently. And these smaller, more industrious singers tended to father more offspring. (Larger males do get an occasional chance to mate, perhaps thanks to their ability to keep a roost surfeit with song through the multi-male relay strategy.)
For bats as well as humans, trills are a difficult vocal exercise; male birds will often produce them to show off their vocal acumen. So next, Toth’s colleagues, including zoologist and senior author Stuart Parsons, hope to disentangle the connection between song syllable and bat size. “One possibility is that trill-downsweeps require more energy,” says Kathleen Collier, a biologist in Parson’s lab at Queensland University of Technology who is following up on Toth’s work. Perhaps more compact males may not have to work as hard to fly or sing, and can afford these costly syllables.
In her next experiments, Collier plans to plant manipulated audio recordings of bat vocalizations in roosts, to test for variations in bat song that can lure females into visiting certain trees. Tracking the success of these bat remixes will help confirm that, by listening for more vocally advanced songs, females can literally size up males through their serenades.