Bats don’t sound like birds to the naked ear; most singing species broadcast predominately in the ultrasonic range, undetectable by humans. And in contrast to the often lengthy songs of avian species, the flying mammals sing in repeated bursts of only a few hundred milliseconds. Researchers must first slow down the bat songs—so that their frequencies drop into the audible range—to hear the similarities. Kirsten Bohn, a behavioral biologist at Johns Hopkins University, first heard Brazilian free-tailed bats (Tadarida brasiliensis) sing more than 10 years ago, when she was a postdoc in the lab of Mike Smotherman at Texas A&M University. “I started hearing a couple of these songs slowed down,” she recalls. “And it really was like, ‘Holy moly—that’s a song! That sounds like a bird.’”
In an exception to supersonic inaudibility, at least one bat species does sing loud and proud: the greater sac-winged bat (Saccopteryx bilineata), which broadcasts audible territorial defense songs from roosts tucked away in the canopies of tropical rainforests. “You can hear these songs from over a hundred meters away,” says Mirjam Knörnschild, an animal behaviorist at the Free University of Berlin who studies the species in Costa Rica and Panama.
Greater sac-winged bats may even learn their songs in ways that mirror songbirds—and their pups “babble” the species’ vocal repertoire as do songbird chicks and human babies, Knörnschild has shown (Naturwissenschaften, 93:451-54, 2006). “The species I work with happens to learn its territorial songs by imitating tutors, which is very close to what a bird is doing,” she says. “There are fairly few species of bats well-described that sing, and even fewer that are capable of vocal learning. In that regard, Saccopteryx is the only one at present that is singing a song that is learned by vocal imitation.”
The emerging similarities between bats’ and birds’ songs may be tied to the fact that both types of animals engage in one of the most resource-intensive modes of locomotion: flying. “You can use singing to control your territory without flying,” which is energetically expensive, says Smotherman. “And singing works well to control fairly large territories for fairly small animals that fly. . . . There’s an important tie between singing and flight.”