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There is a well-established allometric relationship between brain and body
mass in mammals. Deviation of relatively increased brain size from this
pattern appears to coincide with enhanced cognitive abilities. To examine
whether there is a phylogenetic structure to such episodes of changes in
encephalization across mammals, we used phylogenetic techniques to analyse
brain mass, body mass and encephalization quotient (EQ) among 630 extant
mammalian species. Among all mammals, anthropoid primates and odontocete
cetaceans have significantly greater variance in EQ, suggesting that
evolutionary constraints that result in a strict correlation between brain
and body mass have independently become relaxed. Moreover, ancestral state
reconstructions of absolute brain mass, body mass and EQ revealed patterns
of increase and decrease in EQ within anthropoid primates and cetaceans.
We propose both neutral drift and selective factors may have played a role
in the evolution of brain–body allometry.
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