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 tropicallocations some aerial-feeding bats used echolocation calls with most energy <20 or >60 kHz. The impact of frequency-specificattenuation, perhaps combined with frequency-specific, hearing-based defenses of some insects, suggests that by usingecholocation calls <20 kHz, bats could both extend the effective range of echolocation and make their calls less conspicuous toinsect ears. Bats using calls >60 kHz would be less conspicuous to the insects. We found two patterns of echolocation-callbehaviour. Most adjacent echolocation calls, and all that were dominated by sounds >20 kHz, showed large (80%) overlap inbandwidth. The other pattern involved much less overlap in bandwidth (0–30%) between adjacent calls and was evident in thecalls of the molossid Tadarida midas, which used echolocation calls dominated by sounds <20 kHz. This behaviour wouldallow the echolocating bat to extend its effective range of perception by separating in frequency the echoes returning fromadjacent calls.