Sperm whale sound production

largest, yet most elusive, creatures inhabiting deep ocean waters. Adult sperm whales undertake long, deep dives (Watkins et al., 1993) into the darkness and high pressure of the mesoand bathypelagic depths. They do this to locate and catch approximately 1000 kg (Lockyer, 1981) of medium-sized squid and fish (Clarke et al., 1993) each day. The most prominent feature of the sperm whale physique is the large nasal complex (Fig. 1), accounting for up to onethird of the body length of large males (Nishiwaki et al., 1963). The entire forehead is heavily innervated by cranial nerves V and VII (Oelschläger and Kemp, 1998), and the potential level of activity in the muscle complex controlling the forehead is implicated by the highest density of arteries found in any muscle tissue of the sperm whale (Melnikov, 1997). Norris and Harvey (1972) proposed that the sperm whale nose, homologous with the sound-producing nasal complex of smaller odontocetes (Cranford et al., 1996), is a pneumatic sound generator (Fig. 1). Recent investigations have corroborated some of the basic concepts of the Norris and Harvey theory by showing that clicks are produced in the anterior part of the nasal complex (Ridgway and Carder, 2001) and that sound can be transmitted through the spermaceti compartments (Møhl, 2001). The sperm whale sound generator is believed to be driven by air which, when recycled, allows for continuous sound production throughout a dive (Norris and Harvey, 1972). However, air volumes contained in soft structured tissue (Ridgway et al., 1969) are reduced in proportion to increasing ambient pressure (Boyle’s law: PV=C, where P is pressure, V is volume and C is a constant), so the available volume for sound production varies considerably with depth. Sperm whales are vociferous animals and, unlike most odontocete species that have been investigated, their vocal repertoire is made up solely of clicks. It has been suggested that the so-called usual clicks (Weilgart and Whitehead, 1988) are involved in echolocation (Gordon, 1987), whereas stereotyped patterns of clicks, termed codas (Watkins and Schevill, 1977), are allegedly involved in communication to maintain the complex social structure in female groups (Weilgart and Whitehead, 1993). Recent investigations have demonstrated that sperm whale usual clicks are highly directional and have the highest biologically produced source levels ever recorded (Møhl et al., 2000). Clicks of high sound pressure levels and directionality serve biosonar purposes well 1899 The Journal of Experimental Biology 205, 1899–1906 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB4130E