Variations in brain size and proportions can be linked to the cognitive capacities of different animal species, and correlations with ecology may give clues to the evolutionary origins of these specializations. Much recent evidence has implicated the social domain as a major challenge driving increases in problem-solving abilities of mammals. However, the methods of measurement available to researchers are often indirect and sometimes appear to give conflicting answers, and other intellectual challenges may also have been influential in cognitive evolution. While the cause of an evolutionary increase in intelligence may be domain-specific (sociality, for example), and the brain specialization that results may largely implicate a single perceptual system, such as vision, the intelligence shown in consequence can be very 'general-purpose' (as in primates and some avian taxa). Future research needs to get beyond vague ascription of 'greater intelligence' or 'faster learning' towards a precise account of the cognitive mechanisms that underlie particular mental skills in different species; that will allow theory-testing against data from complex, natural situations as well as from the laboratory, on a common metric.