Bat sonar and the role of frequency diversity

Species of echolocating bats that forage in foliage, in the presence of clutter with attendant multiple, often overlapping echoes, use 3–5 simultaneous harmonic sweeps to cover a broad bandwidth, whereas species of bats that hunt only in the open use a single sweep to cover the same broad bandwidth. It is hypothesized that the frequency diversity evident in the presence of several harmonically related frequencies is vital to success in clutter. It is additionally hypothesized that the bat processes harmonic FM echoes in a manner tantamount to split-beam processing. Simultaneous determination of phase at several related frequencies enables the bat to distinguish between single and multiple targets, as in the theory of coincidence echoes [K. G. Foote, J. Acoust. Soc. Am. 99, 266–271 (1996)]. The plausibility of the second hypothesis is gauged by computations based on empirical knowledge of the signal parameters and timing acuity of the big brown bat (Eptesicus fuscus).