Relationship Between Brain Volume and Brain Width in Mammals and Birds

In this study, we explored the relationship between brain volume and brain width in a wide range of extant mammals and birds in an effort to determine whether brain width could be used as an appropriate variable for estimating the brain volume of extinct species. The relationship between brain volume and brain width in extant species was assessed using computed tomography images of mammalian skulls from 55 species representing 13 orders, and avian skulls from 64 species representing 21 orders. Brain volume and brain width showed a strong linear correlation in both mammals and birds. We also discovered that brain volume of extant as well as extinct mammals and birds can be estimated on the basis of brain width. The brain widths of Cynodontia, Triconodonta, and non-avian Theropoda were relatively narrower than those of extant mammals and birds. This data indicates that compared to their early ancestors, the brain width of both mammals and birds has increased with respect to brain endocast volume. Thus, on the basis of our results we have concluded that the relationship between brain volume and brain width is useful for estimating the brain volume of extinct mammals and birds. In addition, it was found that relative brain width of both mammals and birds has increased throughout their evolutionary history from early ancestors to extant species.