Several comparative research programs have focused on the cognitive, life history and ecological traits that account for variation in brain size. We review one of these programs, a program that uses the reported frequency of behavioral innovation as an operational measure of cognition. In both birds and primates, innovation rate is positively correlated with the relative size of association areas in the brain, the hyperstriatum ventrale and neostriatum in birds and the isocortex and striatum in primates. Innovation rate is also positively correlated with the taxonomic distribution of tool use, as well as interspecific differences in learning. Some features of cognition have thus evolved in a remarkably similar way in primates and at least six phyletically-independent avian lineages. In birds, innovation rate is associated with the ability of species to deal with seasonal changes in the environment and to establish themselves in new regions, and it also appears to be related to the rate at which lineages diversify. Innovation rate provides a useful tool to quantify inter-taxon differences in cognition and to test classic hypotheses regarding the evolution of the brain.
2004 S. Karger AG, Basel