Similarities in the echolocation beam pattern of vespertilionid bats.

Abstract

Echolocating bats can navigate and fourage by sound, emitting short high-frequency sound pulses and listening to echoes reflected from obstacles and prey. Small insectivorous bats typically emit higher frequencies than larger bats. This correlation has been explained as an acoustic constraint by prey size since small bats feed on small prey and small objects only reflect echoes efficiently at short wavelengths, i.e., high frequencies. However, very small bats use higher frequencies than what is needed for effective reflection from their prey. Thus, we propose an alternative or supplementary hypothesis: Small bats emit high frequencies to obtain highly directional biosonar sound beams. Directionality increases with the size of the sound emitter relative to the wavelength. Hence small bats can counteract the decrease in directionality caused by their small size by increasing their emitted frequency. Echolocation calls were recorded from five species of echolocating bats of different sizes flying in the laboratory using a multimicrophone array and their beam pattern was calculated. The results show high similarity in beam patterns across species, indicating that directionality is indeed a constraint on echolocation frequency. [Funded by the Oticon Foundation.]