Taxonomic differences in the mammalian life span-body weight relationship and the problem of brain weight

Despite the highly significant correlation between brain and body weight throughout the entire mammalian class, there are consistent differences between rodents, higher primates, carnivores, and ungulates. Primates have larger brains than carnivores of equal size, while rodents have smaller brains, and ungulates have similar-sized brains as carnivores with the same body weight. Further, life span correlates well with body weight for all mammals together (over 150 species), although there are large and consistent interorder differences. For a given body weight, carnivores have a shorter life span than primates, one as long as rodent, and one longer than ungulates. These differences in life span are not matched by the differences in brain weight. Therefore, the conjecture that brain size is a determinant of life span is not valid.