Electrophysiological Assessment of Temporal Resolution in Pinnipeds

Studies of auditory temporal processing in marine mammals have traditionally focused on the highly refined temporal resolution capabilities of dolphins and other odontocete cetaceans. However, a recent electrophysiological investigation of manatee (Trichechus manatus) hearing has shown their temporal resolution to be better than expected, leading to speculation that enhanced temporal processing capabilities are adaptive for underwater sound localization. This study measured evoked responses from several California sea lions (Zalophus californianus), a harbor seal (Phoca vitulina), and a northern elephant seal (Mirounga angustirostris) to determine how well the auditory systems of these amphibious mammals resolve rhythmic stimuli. Trains of broadband clicks were presented in air at repetition rates from 125 to 1,500 s, and the averaged evoked responses elicited by these stimuli were recorded from the skin. Rate-following responses were detected in the sea lions at rates up to 1,000 s, with an upper limit of temporal resolution estimated at 875 to 1,000 s. This upper limit is better than previously anticipated and was further substantiated by limited testing with the harbor seal and northern elephant seal. While these findings might support an underwater sound localization hypothesis, measurements comparable to those of the pinnipeds were also obtained in a phylogenetically similar terrestrial mammal: a domestic dog (Canis familiaris). It is therefore possible that increased temporal resolution in pinnipeds and other non-echolocating marine mammals is not a result of the evolutionary pressure of an aquatic environment.