Echolocation and hearing in the mouse-tailed bat, Rhinopoma hardwickei : acoustic evolution of echolocation in bats

1. Mouse-tailed bats (Rhinopoma hardwickei) use short-duration, multiple-harmonic, constant-frequency (CF) or slightly frequency-modulated (FM) signals for echolocation of flying insect prey. The frequency components are 18–20 kHz (first harmonic), 36–40 kHz (second harmonic), 56–60 kHz (third harmonic), and 75–80 kHz (fourth harmonic). The second harmonic is the strongest component, with the third harmonic 2 to 10 dB weaker, and the first and fourth harmonics about 10 to 20 dB weaker than the second. 2. The bat's hearing, as indicated by N4 auditory evoked potentials, is moderately sharply tuned to the second harmonic, broadly sensitive to the first harmonic and to lower frequencies, and moderately sensitive to the third and fourth harmonics. The degree of tuning is sufficient to indicate some specialized function for the second harmonic, perhaps in the task of detecting targets at maximum range, since this is the lowest of the strong harmonics and least affected by atmospheric attenuation. These bats probably do not perform the Doppler compensation response even though their hearing is tuned. 3. The pattern of emission of sonar sounds (repetition rate, duration) during interception of prey is similar to that observed in most other species of bats. The signals are emitted at repetition rates of 10–20/s during the search stage, 20–40/s during the approach stage, and about 100/s during the terminal stage of the pursuit process. The sonar sounds emitted during the search stage are 6–10 ms long, during the approach stage they are 2–4 ms long, and during the terminal stage they are less than 1 ms long. 4. The principal acoustic dimension of echolocation sounds that relates to perception of targets is signal bandwidth, whichRhinopoma hardwickei manipulates throughout the pursuit process by shortening the duration of its sonar sounds and also by slightly broadening the FM sweeps in its terminal-stage sounds. The onset-time of the long, search-stage sounds and the longer, early approach-stage sounds is more abrupt than for the shorter, late approach-stage and the short terminal-stage sounds. This apparently deliberate transient beginning broadens the signal's bandwidth at the onset relative to the narrower bandwidth prevailing for the rest of the CF harmonic structure which follows. In the shorter sounds, which are emitted during the late approach stage and terminal stage, the onset is more gradual, but the duration (the envelope) and the offset-time are now short enough that the bandwidth of the signal as a whole is increased. The bandwidth is manipulated primarily by these changes in signal envelope and secondarily by the increased FM sweep-width in the terminal stage. 5. Except for duration these signals are relatively inflexible and suggestive of a primitive kind of echolocation in which only one dimension is changed to achieve qualities which most other species of bats obtain by changing a variety of signal dimensions simultaneously. The abrupt signal onsets may be an indication that multiple-harmonic CF and FM echolocation sounds evolved from click-like echolocation sounds of the type emitted byRousettus and some terrestrial mammals.