6
The Auditory Central Nervous
System of Dolphins
SAM H. RIDGWAY
1. Introduction
1.1 Audition Before Birth
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.
1.2 Early Brain Development
Oelschlager and coworkers have stated, "The auditory pathway is
extremely well developed and seems to have been the dominant functional
system in odontocete evolution. Moreover, it is largely responsible for the
273
The Auditory Central Nervous
System of Dolphins
SAM H. RIDGWAY
1. Introduction
1.1 Audition Before Birth
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.
1.2 Early Brain Development
Oelschlager and coworkers have stated, "The auditory pathway is
extremely well developed and seems to have been the dominant functional
system in odontocete evolution. Moreover, it is largely responsible for the
273