The Auditory Central Nervous System of Dolphins

Dolphins, as with all cetaceans, are conceived, gestated, and born and live their entire lives in the water. During the long gestation period in the uterus, the developing dolphin is exposed to abundant sound from the environment because its mother’s body tissues, including the amniotic fluid that bathes the developing fetus, are well matched to seawater in acoustic impedance. Othello Langworthy, a neurobiologist who made many early observations on the brains of dolphins and whales, suggested that the cetacean auditory system, and indeed the great expanse of cetacean neocortex, reached its high state of expansion on the basis of this early auditory input during brain development (Langworthy 1932; Ries and Langworthy 1937). Although there have been no specific studies on hearing by the cetacean fetus, clearly other mammals have demonstrated a capability to hear in the womb well before birth. For example, the human fetus responds to acoustic stimulation. In a group of 31 women, roughly 50% of fetuses responded to acoustic stimulation by acceleration of the heart beat at 24 weeks after conception and 100% responded to sound by week 28 (Johansson et al. 1992). Bottlenose dolphins (Tursiops truncatus), with a gestation period of roughly 52 weeks compared to 38 weeks in the human, are born at an advanced stage of development (Fig. 6.1). At birth, the neonate will be 100 to 125 cm in length and may weigh 15 to 20 kg. Most likely, the ear is functional and the dolphin auditory system is receiving acoustic input for a long period in the womb.