Comparison of Cetacean
and Primate Cerebral Cortexes
The cerebral cortex is the outer layer of advanced brains that evolved
latest in all animals. It is the seat of all higher mental functions. All
mammals and most other animals have one to some degree, but in the
primates and cetaceans it is extremely well-developed.
In cetaceans, the cerebral cortex is very large, especially in relation
to the rest of the brain. Since the cerebral cortex covers the more
primitive areas of the brain in a thin layer, the amount of surface area
available for it is important. The brain manages to obtain more cerebral
cortex through encephalization, or convolutions on the surface of the
brain. The amount of encephalization is considered an important factor in
intelligence, since it can greatly increase the amount of area for the
cerebral cortex (Morgane 1974).
Only in primates and cetaceans does the brain
possess any significant amount of encephalization. As a matter of fact,
cetacean brains have much more encephalization than human brains. For
example a dolphin brain (one of the smaller of the cetaceans) has 40% more
cerebral cortex than a human (Morgane 1974). The human cerebral cortex is divided into
rather distinct layers, and it is believed that this indicates a higher
level of complexity and development over those which lack the layering.
Early studies showed that cetaceans have very little lamination; that most
of the cerebral cortex was muddied together. Later studies proved this
wrong (Bunnel 1974). The cetacean cerebral cortex is
as layered as our own. The individual layers are similar or even identical
between the two.
Another facet is the amount of regional differentiation in the cerebral
cortex (Morgane 1974). A greater amount of separation
in areas would seem to point towards a greater specialization of these
areas. At least for humans it would seem to indicate that. Our brains have
highly specialized sections, but as to whether or not this is necessary to
possess high intelligence remains to be seen. Cetacean brains seem to
possess comparable differentiation, but there has not been enough research
done to be sure.
Cetacean brains have many of the same features as other primate brains.
The frontal lobe is the general region in which abstract thought is
believed to occur. In cetaceans it is as developed, if not moreso in many
species, as it is in humans. The parietal lobe, which is related to
association of senses and generally making sense of the senses is quite
large in cetaceans. In dolphins, it is as large as the human parietal and
frontal lobes put together (Lilly 1975).
The temporal lobe in cetaceans is also very well
developed and extremely large in comparison to the rest of their brain. In
fact, there are portions which are equivalent to Broca's and Wernicke's
areas in the cetacean brain (Bunnel 1974, Jacobs 1974, Lilly 1975). This area
represents all of the senses together, whereas primate brains represent
the senses separately and they are only connected by long bundles of
Lastly, the relative size of the motor and sensory centers are reversed
in cetaceans in comparison to primates. Primates favor the motor cortex,
whereas the cetaceans greatly favor the sensory region (and are not very
balanced at all between the two).
Neural Density and
The cetacean neural density is comparable to that
of humans and some of the higher primates. However, the density is
slightly less in some regions of the cetacean brain (Bunnel 1974). A high ratio is necessary for such things as
emotional control, objectivity, reality orientation, humor, logically
consistent abstract thought, and higher creativity. There seems to be a
clear correspondence between the ratio and these abilities. Cetaceans have
a ratio that surpasses even the healthiest of humans. This is supported by
behavior studies of captured dolphins who show a high amount of
playfulness and enjoyment even when in captivity. Humans under such
circumstances would not fare quite so well.