Compromises: sound frequencies used in echolocation by aerial-feeding bats

Hunting aerial-feeding bats, species that take airborne prey (usually flying insects), use echolocation to detect, track, and assess targets. The echolocation calls of aerial-feeding bats at sites in Canada (British Columbia and Ontario), Mexico, Brazil, and Zimbabwe were significantly dominated by frequencies between 20 and 60 kHz, although at the more tropical locations some aerial-feeding bats used echolocation calls with most energy <20 or >60 kHz. The impact of frequency-specific attenuation, perhaps combined with frequency-specific, hearing-based defenses of some insects, suggests that by using echolocation calls <20 kHz, bats could both extend the effective range of echolocation and make their calls less conspicuous to insect ears. Bats using calls >60 kHz would be less conspicuous to the insects. We found two patterns of echolocation-call behaviour Most adjacent echolocation calls, and all that were dominated by sounds >20 kHz, showed large (80%) overlap in bandwidth. The other pattern involved much less overlap in bandwidth (0-30%) between adjacent calls and was evident in the calls of the molossid Tadarida midas, which used echolocation calls dominated by sounds <20 kHz. This behaviour would allow the echolocating bat to extend its effective range of perception by separating in frequency the echoes returning from adjacent calls.