Abstract
Mammals possess relatively larger brains compared to body size, and there are many studies focusing on the evolutionary changes of the relative brain size in mammals. A recent study showed that increased resolution in olfaction drove the enlargement of mammalian brains. However, the olfactory bulbs are degenerated among highly encephalized mammals, primates and whales. Several species of whales possess functional olfactory bulbs, but their olfactory bulbs lack a specific area known to induce innate avoidance behavior against odors of predators and spoiled foods. In this chapter, evolutionary changes of the encephalization quotient among mammals and the degeneration processes of olfactory bulbs among whales are discussed from paleontological, anatomical, and genomic points of view.
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