Relative Brain Size in Marine Mammals

It has previously been suggested that small ratios of brain mass to body mass (relative stupidity; Robin 1973) may be more important for species survival than relative intelligence in diving mammals. The basis of this argument is that a large brain, consuming large amounts of limited oxygen, would be more of a detriment than an asset to diving mammals. This paper explores the relationship between brain mass and body mass in marine mammals and subsequently examines the possible determinants of brain size. An allometric relationship describing brain mass and body mass of 648 mammalian species encompassing 16 orders was derived. When compared with an average mammal of similar body size, pinnipeds (seals, sea lions, fur seals, and walruses) have encephalization quotients similar to those of terrestrial carnivores. Odontocete cetaceans (toothed whales), with the exception of the sperm whale (Physeter catodon), have relative brain sizes larger than expected, similar to the anthropoid primates. Sirenians (dugongs and manatees), the sperm whale, and the mysticete cetaceans (baleen whales) all have relative brain sizes smaller than the mammalian average and similar to the large ungulates. The large brains of the odontocete cetaceans may be due in part to their use of echolocation in a three-dimensional medium, which presents a number of difficulties that the less encephalized, microchiropteran bats do not have to cope with in the aerial environment. The auditory system of small, toothed whales may also have facilitated sociality and therefore have led indirectly to further increases in encephalization. The small brains of the sperm whale and the mysticete cetacea are discussed in terms of the gigantism possible in the aquatic environment, which allows specialization and increased size of the head region without a requirement of a similar increase in neural tissue.