Fig. 2.

Pulse-echo ambiguity and frequency hopping in FM bat sonar. Examples of spectrograms for successive big brown bat FM biosonar broadcast pulses (P1, P2) recorded by a miniature telemike attached to the flying bat (data replotted from ref. 16). A second echo microphone recorded echoes reflected back to the bat from multiple obstacles while the bat flew toward them. Each broadcast contains two prominent harmonic sweeps (FM1, FM2). (A) IPI of 32 ms is longer than the echo epoch of 27 ms, so all echoes of P1 are received before P2 is transmitted, and no ambiguity occurs. P1 ends at about the same frequency as P2 (Δf = 0; no frequency hopping). (B) IPI of 22 ms is shorter than the echo epoch of 32 ms, too short for all echoes of P1 to be received before P2 is transmitted. There is a region of ambiguity when lingering echoes of P1 are still arriving after P2 is emitted. These echoes could be registered ambiguously as echoes of P2 instead of P1, leading to perception of phantom obstacles at close range, which could disrupt orientation. The bat quantitatively and significantly responds to the occurrence of ambiguity by raising the ending frequency of P1 relative to P2 by the amount Δf, which is about 5 kHz (frequency hopping, ref. 16).