The acoustical function of the lower jaw of the bottlenose dolphin, Tursiops truncatus (Montagu), during echolocation

A dolphin (Tursiops truncatus) was conditioned to perform a discrimination task, by means of echolocation, in a "Go/No-go" paradigm while stationed in an underwater hoop and wearing opaque suction cups over its eyes. To investigate the hypothesis that the fat-filled lower jaw of odontocete cetaceans provides an acoustical pathway to the inner ear, the dolphin was additionally required to perform the task while wearing either of two rubber hoods designed to cover its lower jaw. A control hood, constructed from gasless neoprene, allowed returning acoustic signals to pass. An experimental hood, constructed from closed-cell neoprene, substantially attenuated such signals. The dolphin's performance was significantly hindered while wearing the experimental hood (p {dollar}<{dollar}.001, X{dollar}sp2{dollar}) as opposed to the wearing of the control hood or no hood at all. Target sensitivity dramatically differed across conditions and a relatively unbiased response criterion was maintained. Collateral behaviors observed did not reveal any behavioral strategies used by the dolphin to compensate for signal attenuation at the lower jaw. Acoustical data recorded during the experiment indicated that the use of the hoods did not hinder the emission of outgoing echolocation signals. Spectral parameters of the emitted signals (i.e., amplitude, peak frequency, and bandwidth) did not widely vary across conditions. However, notable variations in temporal parameters were apparent in terms of lower mean click repetition rates in the hooded conditions and shorter mean interclick intervals in the experimental condition. These results provide the first direct evidence of their kind, agree with previous electrophysiological data, and support the hypothesis that the lower jaw is involved in the transmission of high-frequency echolocation signals to the inner ear.