6. Auditory eNS of Dolphins
279
3. Pathways and Nuclei Between Ear and
Higher Centers of the Brain
3.1 Size ofAuditory Structures
Morgane et al. (1980) comment that "In all the toothed whales or odontocetes (Tursiops truncatus, Delphinapterus leucas, Phocaena phocaena,
and Inia goeffrensis) the massive eighth nerve roots and the absence of
olfactory bulbs and tracts ... contrast with the relatively small roots of the
eighth nerve and the presence of long olfactory tracts in Balaenoptera
physalus." The dolphin auditory nerve has several times as many fibers
as the human eighth nerve. Also, Soviet investigators (see Bullock and
Gurevich 1979) have reported fiber diameters in the eighth nerve of T
truncatus about twice as large as in humans. (A doubling of fiber diameter
approximately doubles nerve impulse conduction speed.) De Graaf (1967)
measured diameters up to 12 11m in tracing fibers from the cochlear nucleus
to the lateral lemniscus of the bottlenosed whale (Hyperoodon ampullatus). Studies of fiber spectra have revealed larger myelinated fibers in odontocete than in mysticete whales. In turn, mysticete whales have larger eighth
nerve fibers than do humans. Mysticete cetaceans (baleen whales) are
apparently not specialized for very high frequency hearing, but audiograms
have not been done on any mysticetes and almost nothing is known about
their auditory central nervous system. In one mysticete, the fin whale,
Balaenoptera physalus, eighth nerve fiber diameters of about 51lm were
measured using light microscopy (Jacobs and Jensen 1964). In contrast,
plots of fiber diameters have shown that the largest numbers of fibers in
the eighth nerve of the sperm whale, Physeter macrocephalus, are 91lm in
diameter, in T. truncatus about 711m. In these toothed whales, auditory tracts
reaching the cerebral cortex are extensive. Some observers of the dolphin
brain suggest that the cerebral cortex may have reached its great development on the basis of acoustic input (Langworthy 1932; Wood and Evans
1980; Ridgway 1990).
Auditory structures are generally larger in odontocete whale brains than
in mysticete brains (Breathnach 1960). Hypertrophy of the auditory system
may be the primary reason for the dolphin's large brain. The medial geniculate body is about 7 times, the inferior colliculus (Fig. 6.5) is 12 times, and
the nucleus of the lateral lemniscus of the dolphin is over 250 times as large
as the equivalent structures in humans (see Bullock and Gurevich 1979).
The ventral cochlear nucleus and some other brain stem nuclei also appear
massive when compared with the human equivalents. De Graaf (1967)
noted that auditory brainstem nuclei were much larger, and fiber diameters
larger, in the odontocetes he studied (H. ampullatus and the killer whale,
Orcinus orca) than in the mysticetes (two different species of B. physalus
and Balaenoptera acutorostrata, the minke whale). He observed, however,
that in comparison with most other mammals the mysticete auditory brain
279
3. Pathways and Nuclei Between Ear and
Higher Centers of the Brain
3.1 Size ofAuditory Structures
Morgane et al. (1980) comment that "In all the toothed whales or odontocetes (Tursiops truncatus, Delphinapterus leucas, Phocaena phocaena,
and Inia goeffrensis) the massive eighth nerve roots and the absence of
olfactory bulbs and tracts ... contrast with the relatively small roots of the
eighth nerve and the presence of long olfactory tracts in Balaenoptera
physalus." The dolphin auditory nerve has several times as many fibers
as the human eighth nerve. Also, Soviet investigators (see Bullock and
Gurevich 1979) have reported fiber diameters in the eighth nerve of T
truncatus about twice as large as in humans. (A doubling of fiber diameter
approximately doubles nerve impulse conduction speed.) De Graaf (1967)
measured diameters up to 12 11m in tracing fibers from the cochlear nucleus
to the lateral lemniscus of the bottlenosed whale (Hyperoodon ampullatus). Studies of fiber spectra have revealed larger myelinated fibers in odontocete than in mysticete whales. In turn, mysticete whales have larger eighth
nerve fibers than do humans. Mysticete cetaceans (baleen whales) are
apparently not specialized for very high frequency hearing, but audiograms
have not been done on any mysticetes and almost nothing is known about
their auditory central nervous system. In one mysticete, the fin whale,
Balaenoptera physalus, eighth nerve fiber diameters of about 51lm were
measured using light microscopy (Jacobs and Jensen 1964). In contrast,
plots of fiber diameters have shown that the largest numbers of fibers in
the eighth nerve of the sperm whale, Physeter macrocephalus, are 91lm in
diameter, in T. truncatus about 711m. In these toothed whales, auditory tracts
reaching the cerebral cortex are extensive. Some observers of the dolphin
brain suggest that the cerebral cortex may have reached its great development on the basis of acoustic input (Langworthy 1932; Wood and Evans
1980; Ridgway 1990).
Auditory structures are generally larger in odontocete whale brains than
in mysticete brains (Breathnach 1960). Hypertrophy of the auditory system
may be the primary reason for the dolphin's large brain. The medial geniculate body is about 7 times, the inferior colliculus (Fig. 6.5) is 12 times, and
the nucleus of the lateral lemniscus of the dolphin is over 250 times as large
as the equivalent structures in humans (see Bullock and Gurevich 1979).
The ventral cochlear nucleus and some other brain stem nuclei also appear
massive when compared with the human equivalents. De Graaf (1967)
noted that auditory brainstem nuclei were much larger, and fiber diameters
larger, in the odontocetes he studied (H. ampullatus and the killer whale,
Orcinus orca) than in the mysticetes (two different species of B. physalus
and Balaenoptera acutorostrata, the minke whale). He observed, however,
that in comparison with most other mammals the mysticete auditory brain