Social context evokes rapid changes in bat song syntax

The capacity to modify vocal syntax to changes in social context is an important component of vocal plasticity and complexity in adult vertebrates, especially in human speech. The ecological significance of this behaviour has been well established in some avian species but not in mammals where complex, multisyllabic vocalizations are rare. The Brazilian free-tailed bat, Tadarida brasiliensis, is a mammal that sings like a bird, producing hierarchically structured songs that vary in the order and number of phrases (i.e. syntax) from one rendition to the next while simultaneously following specific organizational rules. Here, we used playback experiments to examine the function of songs and tested whether song syntax is correlated with social context. Free-tailed bats responded rapidly and robustly to echolocation calls that mimicked a bat flying past the roost but did not respond to conspecific song playbacks. We compared songs that were directed at a passing bat with songs that were produced spontaneously and found that bats produced longer songs with different phrase content and order when singing spontaneously than when singing to bats approaching their roost. Thus, free-tailed bats quickly varied song composition to meet the specific demands of different social functions. These distinct singing behaviours are similar to those of some songbirds, suggesting that bats and birds have converged upon a similar set of production modes that may reflect common neural mechanisms and ecological factors.