A few years ago, researchers discovered that the babies of at least one species of bat make babbling sounds, much like human infants. Now, it turns out those babbling baby bats aren't just mindlessly cooing--they're imitating the songs of the big guys in their colonies: adult males with territories and harems. Such vocal imitation is rare in the animal kingdom, and it has never been found in nonhuman primates. The discovery should open a new window on the evolution of speech and language, scientists say.
Scientists define complex vocal imitation as the ability to learn a call or song from a tutor--and they regard this talent as a key innovation in the evolution of speech. The rarified list of complex vocal imitators includes birds, elephants, cetaceans, seals, and humans. Researchers had long predicted that bats might also be capable of such imitation because of their extraordinary vocal flexibility; they use echolocation calls to navigate the physical world, for example, and social calls to communicate with their fellow bats.
An S. bilineata male producing territorial songs.
As behavioral ecologist Mirjam Knörnschild of the University of Ulm in Germany listened to sac-winged bats (Saccopteryx bilineata), she thought she heard complicated vocal imitation. These insectivorous Costa Rican bats live in harems of one male and as many as eight females, each of which can have one pup annually. The males defend small territories in their day-roosts with unique multiple-syllabic songs. Adult females don't sing, but their pups (males and females) do plenty of babbling. During such "babbling bouts," the pups often sing nearly complete renditions of the territorial songs, Knörnschild says, albeit shakier renditions. But were the pups simply combining fragments or actually listening and imitating their complete songs?
To find out, Knörnschild recorded the tunes of 17 pups at different ages (2 to 6 weeks old and 7 to 10 weeks old), and those of six harem males, and compared their acoustic properties via sonograms. As the pups grew, their baby songs slowly developed into true territorial songs, identified by what Knörnschild terms "buzz syllables." At every stage of their development, the pups' renditions always closely matched the buzz syllables of their harem males, showing that the youngsters were not simply piecing together random song fragments.
Because males cannot sexually monopolize the females in their harems, pups within a male's territory are not necessarily his offspring. Thus, the vocal similarity between pups and harem males wasn't because of genetic relatedness. Rather, that similarity arose simply because the pups listened to their harem male, as they would "to a tutor," and imitated his songs, which are crucial for acquiring territories, says Knörnschild, whose team reports its findings online today in Biology Letters. In short, the pups must hear the calls to learn how to sing them.
The fact that both females and males learn complex songs makes these bats an excellent model--the first mammal ever--for investigating the link between vocal imitation and the emergence of speech, says W. Tecumseh Fitch, a cognitive ethologist at the University of Vienna, who was not involved in the study.
Why would female pups imitate their harem male's songs? Perhaps because females choose their mates. By singing the songs, the female pups may learn how to evaluate potential mates, says Knörnschild.