THE EFFECTS OF ATTENUATING RETURNING ECHOLOCATION SIGNALS AT THE LOWER JAW OF A DOLPHIN (TURSIOPS-TRUNCATUS)

Recordings were made during an echolocation experiment in order to observe any changes in the parameters of outgoing signals as a function of the deliberate attenuation of returning signals at the lower jaw of a dolphin (Tursiops truncatus). A dolphin was conditioned to perform a discrimination task while wearing either of two rubber hoods covering its lower jaw in addition to performing the same task while wearing no hood. A control hood, made of gasless neoprene, allowed returning acoustic signals to pass to the lower jaw. An experimental hood, made of closed-cell neoprene, significantly attenuated such signals. As previously reported (Brill et al., 1988), the dolphin's ability to echolocate was significantly hindered while wearing the experimental hood. The acoustical data we recorded during that experiment suggest that the use of the hoods did not affect the dolphin's ability to emit useful echolocation signals. The dolphin appeared to vary the temporal parameters of its emitted signals in terms of lower click repetition rates in both the control and experimental conditions and interclick intervals (ICIs) of shorter duration in the experimental condition. The lower repetition rates and shorter ICI's in the latter condition are attributed to the attenuation of returning echoes at the dolphin's lower jaw. The results support the theory that returning echolocation signals are best received along the lateral sides of the dolphin's lower jaw.