Behavioral and Ecological Influences on the Echolocation of Brazilian Free-Tailed Bats, Tadarida brasiliensis

This dissertation investigates variability in the echolocation calls of Brazilian free-tailed bats, Tadarida brasiliensis (Chiroptera: Molossidae), and explores how bats adjust echolocation call structure in response to different behavioral and ecological conditions. Substantial geographic variation exists in the echolocation call structure of T. brasiliensis throughout the species range in the US, but this variation does not correlate with geographic or climatic patterns. Most variation in call structure is due to differences between and within the calls of individuals. When exposed to broadcasts of high frequency insect sounds, free-flying bats consistently responded by shifting call frequencies away from the broadcast frequencies. This response suggests that bats are sensitive to local acoustic interference that decreases the efficiency of echo reception. In another investigation of reactions to interfering sounds, bats responded to echolocation playbacks by rapidly shifting their call frequencies away from playback frequencies, indicating that a jamming avoidance response was occurring. Bats more frequently shifted their calls upwards to higher frequencies, which may be due to maximal jamming power of the lower frequency portion of echolocation calls. Flexibility in their echolocation calls also was evident in calls produced by T. brasiliensis while emerging from roosts in a tight column formation. Bats emitted two distinct call types during emergence, sweep and hook calls, which were substantially different from foraging calls. Call structure differed between roosts, which may be related to differences in the spacing of bats within emergence columns. In a final experiment, it was found that bat activity was substantially greater in response to echolocation playbacks that contained feeding buzz calls compared to broadcasts that did not contain these signals, indicating that bats eavesdrop on the echolocation calls of conspecifics. Overall, this study documents the highly flexible nature of echolocation in Brazilian free-tailed bats and demonstrates that bats respond acoustically to behavioral and ecological influences.