Brain Size

Brain Size

Mammalian and bird brains are larger in proportion to their body size than nonavian reptile brains. An average-sized mammalian brain weighs 9.9 g for a 1 kg animal, a bird brain weighs 6.7 g, and a nonavian reptile brain weighs only 0.7 g. Thus, when compared to nonavian reptiles, both birds and mammals are "brainy," but their cerebrums are not strictly homologous. Differences exist in how visual information is processed as well. Birds and other sauropsids continue to rely on the optic lobes of the midbrain for visual processing, as did the first vertebrates. The optic lobes are small in mammals and are primarily used for optical reflexes such as tracking motion, while a new portion of the cerebrum (the visual cortex) actually interprets the information.

Evaluating an animal's intelligence can be difficult. For starters, brain size scales with negative allometry; that is, larger animals have proportionally smaller brains than smaller animals. The exact cause of this negative scaling is unknown, but it may be related to the fact that larger bodies do not require significantly more brain tissue to function. If 200 nerve cells in the brain are required to control a mouse's right hind leg, there is no reason to believe that more cells are required to control an elephant's right hind leg. Of course, larger animals have greater brains than smaller ones; they are just not as enormous as one would expect for their size, all else being equal. Brain size comparisons sometimes overlook changes in bodily size. For example, television shows frequently assert that dolphin brains are the same size as human brains to stress how intelligent dolphins are, however most dolphins are much larger than humans and would be expected to have larger brains.

Although we consider the tendency to evolve a large brain to be a natural result of mammalian evolution, the situation is not so straightforward. Pack-hunting carnivores, for example, have larger brains than solitary ones, although many small-brained animals exhibit sophisticated behaviour and a significant ability for learning. Although the average brain size of ungulates has increased over time, and the largest known brains belong to contemporary species, some live ungulates have brains as small as some early Cenozoic forms. In other words, rather than observing an average rise in brain size, we see an increase in the range of brain sizes, with small-brained animals coexisting alongside much bigger brained ones.

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