Echolocation of moving targets by the fish-catching bat,Noctilio leporinus

Abstract

1. Behavioral measures of target velocity discrimination, Doppler shift discrimination, and Doppler shift compensation are described for the fish-catching bat,Noctilio leporinus. These bats echolocate using a short constant frequency/frequency modulated (short-CF/FM) sonar pulse. 2. Two bats, trained by operant conditioning, performed an in-flight discrimination between sequentially-presented targets which differed in the velocity of movement in the line of the bat's flight path. Both bats discriminate target velocity differences as low as 40 cm/s. This discrimination ability is independent of the absolute speed or direction of target movement. 3. These same bats were subsequently trained by operant conditioning to discriminate between the presence or absence of an upward frequency (simulated Doppler) shift in artificial echoes of their sonar pulses. The smallest frequency shift that can be discriminated is 570 Hz, corresponding to the Doppler shift produced by a relative velocity of 170cm/s. 4. Since these bats cannot detect the Doppler shift associated with moving targets at the threshold of target velocity discrimination, we conclude that Doppler information is not used to make velocity measurements. Target velocity discrimination must instead be based upon range-rate information obtained from the wide band FM component of the sonar pulse. 5. Three of four bats partially compensate for Doppler shifts of their echoes produced by their flight velocity. During flight, the frequency of their sonar emissions is lowered enough to compensate for about 50% of the flight-induced Doppler shift. None of the bats compensate for target-induced Doppler shifts. 6. The functional significance of short CF signals and the echolocation of moving targets byNoctilio and other bats are discussed.