Brain shape and encephalization in birds

marugan-lobon-et-al-2016Encephalization quotients (EQ) have been extensively used to characterize brain evolution, but this univariate metric only includes information on relative size. Marugán-Lobón and his colleagues recently analysed the association between endocranial shape changes and EQ by applying geometric morphometrics to a sample of modern bird endocasts. A Principal Component Analysis accounting for phylogenetic history showed that the bird endocasts varied essentially in the relative expansion of the forebrain and in the degree of flexion of the braincase. The distribution of the specimens in the morphospace has a phylogenetic structure, with morphological affinity between close evolutionary clades, particularly the landbirds, which display larger forebrains. Size explains 10% of the shape variation. EQ accounts for changes in relative forebrain expansion, with larger EQs associated with larger forebrains. A second study was computed correcting for phylogeny, i.e. computing regression analyses on the phylogenetic independent contrasts of shape and size against EQ. When allometric and phylogenetic signals were removed, shape variation was mostly associated with the degree of flexion of the endocasts, and EQ was not significantly correlated with these morphological changes. The authors conclude that, excluding the general effect of size, EQ does not explain shape differences among birds’ endocasts. Therefore, other factors are probably responsible for brain variation in birds.

Sofia Pedro


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