The evolution of large brain size in birds is related to social, not genetic, monogamy

There are several hypotheses suggesting that social complexity, including pair bonding, is important in the evolution of increased brain size. I examined whether genetic or social monogamy was related to large brain size in birds. Recent work has indicated that the length and strength of pair bonds are associated with large brain size. I tested several hypotheses for the evolution of large brain size in 42 species of bird by including life history variables in a regression model. A test on 100 phylogenetic trees revealed no phylogenetic signal in brain size. Controlling for body size, a principal components analysis was run on the life history variables and degrees of extra-pair paternity. The main principal component (PC1) was regressed on brain size revealing a strong, positive association. Social, but not genetic, monogamy was positively related to brain size. Large brain size is related to the selective pressures of procuring extra-pair copulations whilst maintaining a social partnership. However, other life history variables also loaded positively and significantly on brain size. These results indicate that the evolution of large brain size in birds was driven by several important selective pressures. (c) 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 668-678.