Porpoise Sounds as Sonar Signals

Abstract

Investigations of captive and free-living bottle-nose dolphins, Tursiops truncatus. indicate that these marine mammals rather continuously make two kinds of sound 1) a "whistle," of approximately 0.5 sec. duration, generally beginning at about 7000 cycles per sec. and ending at around 15,000 cycles per sec. (such a continuously changing pitch would reflect a continuously changing echo, and could be used as a kind of frequency-modulated sonar); 2) and series of discrete clicks varying in rate from 5 per sec. to more than 100 per sec. (the dominant frequencies are in the human sonic range, but the clicks also contain many ultrasonic vibrations from 20 kc to 120 kc (rarely as high as 170 kc). Such clicks would be excellent for a pulse-modulation method of echo-ranging, since it has been shown that the frequencies involved are reflected from underwater objects, from the bottom, and from the surface of the water. Furthermore, the higher frequencies are the more useful, since natural sounds in the environment are all under 10 kc, and the higher the frequency the shorter the wavelength and the more definite the information which can be interpreted from the echo. Proof is lacking, however, that dolphins and porpoises hear and utilize the echos.