6. Auditory eNS of Dolphins
281
great deal remains to be done to understand the auditory organization of
the large dolphin brain. Good developmental studies are needed. The
arrangement of fiber diameters relative to the structure of the cochlea, especially the basilar membrane width, the arrangement of eighth nerve fiber
diameters as these fibers enter the cochlear nucleus, and many other such
arrangements throughout the auditory brain stem and pathways to higher
centers should be investigated. Such detailed investigations could give
significant information about how the dolphin brain processes acoustic
information. Such processing is at the basis of the sophisticated echolocation system of dolphins (Au 1993).
4. The Cerebellum
Recent findings have shown that the cerebellum (Figs. 6.3, 6.4, and 6.5) may
be intimately involved in acoustic processing of certain types. In the embryonic stage, the dolphin cerebrum and cerebellum are about equal in width.
According to paleontologists, the early forebears of modern cetaceans were
the first giant mammals adapted to life-long swimming. Fossil studies have
shown that these ancient cetaceans already had a very large cerebellum that
was much wider than the cerebrum (Edinger 1955). In modern odontocetes,
the cerebrum has so enlarged as to cover the anterior portion of the
cerebellum.
In comparison with the anatomy of the cerebellum of terrestrial
mammals, the cetacean lobus simplex and paramedian lobules are much
larger (Jansen and Jansen 1969). The paraftocculus and nucleus interpositus, in these comparative terms, are enormous. The hemispheral parts of the
anterior lobe, the ftocculonodular lobe, the lateral nucleus, and the ansiform
lobule are relatively small. In the same terms, the medial nucleus is
moderately developed.
Lange (1975) has studied cell numbers and cell densities in the cerebellar cortex. He made comparisons between humans and various mammals,
attaching particular significance to the ratio of Purkinje cells to granule
cells. He found ratios of 1:2,991 in humans; 1: 1,898 in rhesus monkeys,
Macaque mulatta; 1: 1,812 in T truncatus; 1: 1,790 in pilot whales,
Globicephala melas; 1: 1,508 in cats, and 1: 609 in hedgehogs.
The size of the cerebellum relative to brain weight is largest in mysticetes.
Without free digits and presumably without fine movements, these huge
animals may have a cerebellum as large as 1,500 g. In these baleen whale
species, the cerebellum may be 20% to 25% of total brain mass. In humans,
the cerebellum is typically about 10% or 11 % of total brain mass. In delphinoids (toothed whales of the superfamily Delphinoidea, which includes
all of the dolphins plus white whales and narwhals) such as T truncatus,
with a brain of similar size to the human, the cerebellum is more typically
about 15% of total brain mass. We do not understand why these animals
281
great deal remains to be done to understand the auditory organization of
the large dolphin brain. Good developmental studies are needed. The
arrangement of fiber diameters relative to the structure of the cochlea, especially the basilar membrane width, the arrangement of eighth nerve fiber
diameters as these fibers enter the cochlear nucleus, and many other such
arrangements throughout the auditory brain stem and pathways to higher
centers should be investigated. Such detailed investigations could give
significant information about how the dolphin brain processes acoustic
information. Such processing is at the basis of the sophisticated echolocation system of dolphins (Au 1993).
4. The Cerebellum
Recent findings have shown that the cerebellum (Figs. 6.3, 6.4, and 6.5) may
be intimately involved in acoustic processing of certain types. In the embryonic stage, the dolphin cerebrum and cerebellum are about equal in width.
According to paleontologists, the early forebears of modern cetaceans were
the first giant mammals adapted to life-long swimming. Fossil studies have
shown that these ancient cetaceans already had a very large cerebellum that
was much wider than the cerebrum (Edinger 1955). In modern odontocetes,
the cerebrum has so enlarged as to cover the anterior portion of the
cerebellum.
In comparison with the anatomy of the cerebellum of terrestrial
mammals, the cetacean lobus simplex and paramedian lobules are much
larger (Jansen and Jansen 1969). The paraftocculus and nucleus interpositus, in these comparative terms, are enormous. The hemispheral parts of the
anterior lobe, the ftocculonodular lobe, the lateral nucleus, and the ansiform
lobule are relatively small. In the same terms, the medial nucleus is
moderately developed.
Lange (1975) has studied cell numbers and cell densities in the cerebellar cortex. He made comparisons between humans and various mammals,
attaching particular significance to the ratio of Purkinje cells to granule
cells. He found ratios of 1:2,991 in humans; 1: 1,898 in rhesus monkeys,
Macaque mulatta; 1: 1,812 in T truncatus; 1: 1,790 in pilot whales,
Globicephala melas; 1: 1,508 in cats, and 1: 609 in hedgehogs.
The size of the cerebellum relative to brain weight is largest in mysticetes.
Without free digits and presumably without fine movements, these huge
animals may have a cerebellum as large as 1,500 g. In these baleen whale
species, the cerebellum may be 20% to 25% of total brain mass. In humans,
the cerebellum is typically about 10% or 11 % of total brain mass. In delphinoids (toothed whales of the superfamily Delphinoidea, which includes
all of the dolphins plus white whales and narwhals) such as T truncatus,
with a brain of similar size to the human, the cerebellum is more typically
about 15% of total brain mass. We do not understand why these animals