Variability in the echolocation of Tadarida brasiliensis: effects of geography and local acoustic environment

We examined variation in the echolocation calls of Brazilian free-tailed bats, Tadarida brasiliensis, on a broad geographical scale and in response to local environmental variables. Significant differences in call structure were observed among populations throughout the species range in the United States, but this variation was not associated with geographical distance or local weather conditions. Observed variability between sites was primarily due to differences between bats, and the flexibility in call structure that can be achieved by individuals. During this study, we observed that bats recorded in the presence of high-frequency sounds from chorusing insects used higher call frequencies than bats recorded in silence. This led us to test the hypothesis that bats adjust echolocation call structure in response to local ambient noise. We broadcast experimentally manipulated ultrasonic insect sounds to free-flying Brazilian free-tailed bats and found a positive correlation between the frequency of the insect sound stimulus and the call frequencies used by bats. These results document that bats adjust echolocation call structure to avoid acoustic interference from ambient noise in their local environment.