High Frequency Auditory Response of a Bottlenosed Porpoise, Tursiops Truncatus (Montagu)

Abstract

The high frequency auditory response of a bottlenosed porpoise, Tursiops truncatus (Montagu), was measured in a simple conditioned response test. The porpoise is a mammal that necessarily hears by bone conduction. Therefore, its frequency range of auditory response may be expected greatly to exceed the frequency range over which it can discriminate pitch, on the basis of what is known of hearing in man, whose auditory response by bone conduction has been reported as passing 100 kc [e.g., Pumphrey, Nature, 166, 571 (1950)]. The porpoise gave a persistent indication of an upper limit for even this presumably colorless type of response. It was exposed to tones between 0.15 and 153 kc at sound levels ranging between −20 and +28 db from 1 dyne/cm2, mostly 0 to +10 db. Above 120 kc there was a marked falling off in positive responses, and at 126 kc the porpoise responded to only 50 percent of the signals; above this the falling off continued, but never as low as 10 percent. The extreme upper frequency limit of sound production (phonation) in these animals was not found; there was still appreciable energy at 196 kc.