Testing the Role of Frequency-Modulated Bouts in Eptesicus fuscus

Abstract

Social vocalizations serve multiple purposes in animal communication and are particularly common in bats (Order Chiroptera). The big brown bat (Eptesicus fuscus; Family Vespertilionidae) emits downward frequency modulated (FM) sweeps that are used for echolocation. Male E. fuscus have been reported to emit a cluster of 3-4 downward FM signals that are longer in duration than echolocation calls, known as frequency-modulated bout (FMB) signals, and have been hypothesized to play a role in food competition. Other Vespertilionid species emit social calls that share spectro-temporal properties with the FMB. These social calls have a role in food competition and incite positive phonotaxis in females, so it is conceivable that the FMB also has a function in mate attraction. My thesis focused on the role of FMB social calls and if they are attractive to females. I tested female E. fuscus in acoustic playback trials presenting bats with competing signals in a two-alternative forced choice paradigm. Bats were presented with combinations of three types of synthetically-generated sounds—a natural-like FMB stimulus, a time-reversed FMB stimulus, and acoustic system noise generated by a silent signal—and I measured the relative affinity or aversion of bats towards different combinations of these signals. Bats were tested first in the fall mating season and then retested in the spring after winter hibernation season. If FMBs are mainly emitted by males for mate attraction, then I predicted females would show an affinity for them, particularly during mating season. My results somewhat corroborate the hypothesis as the females showed some affinity for these calls. These results provide a foundation for further research into understanding big brown bat mate choice and social behaviour.