Body size and intelligence in hominoid evolution

Great apes and humans are the largest-brained primates. Aside from a few extinct subfossil lemurs, they are also the largest in body mass. Body size is a key aspect of a species’ biology, a large organism having different energetic, ecological, and physical constraints than a small one. Brain size, in so far as it determines abilities to acquire, process, and act on information, is also a key aspect of a species’ biology and is linked to body size. Large animals have different informational problems to solve than do small ones, hence their respective sensory organs and nervous systems are sized and organized differently. Mammalian body and brain size scale consistently with each other (Figure 18.1). This relation is generally described by allometric exponents that vary between 2/3 and 3/4 (e.g., Bauchot & Stephan 1966, 1969; Hofman 1982; Jerison 1973; Lande 1979; Martin 1981; Martin & Harvey 1985; Stephan 1972). From a paleontological perspective, the body–brain size relation offers an appealing way to evaluate the cognitive abilities of fossil taxa, a problem of particular importance for understanding hominid evolution. A convincing theoretical basis for this general allometric statistical pattern, however, remains elusive (Deacon 1997; Harvey & Krebs 1990). Body mass is not a strict determinant of brain size, as species of similar size can have different brain sizes and cognitive abilities (Pagel & Harvey 1989). In addition, comparative analyses indicate considerable variation among taxa from general mammalian patterns (Pagel & Harvey 1989). Hominid brains, for example, are double or more their expected size as mammals. There are also phylogenetic differences in typical brain–body size relations within primates that reflect grade shifts in encephalization across taxa (Armstrong 1985a,b; Martin & Harvey 1985, Pagel & Harvey 1989). One reason for the lack of a universal brain–body size correlation among mammalian species is that factors other than body mass or metabolism, such as locomotion, diet, predation risk, social structure, and life history, affect relations between body and brain size (see recent reviews in de Waal & Tyack 2003; other chapters in this volume). All of these factors and others may contribute to selective pressures for cognitive abilities. As such, allometric scaling models developed from analyses of relations between physical variables such as metabolic rate and body mass may not be appropriate models for relations between body and brain size. Evolving a large brain depends upon a complex balance of costs and benefits, which vary from species to species. There are not likely to be simple explanations based upon simple physical principles. Observed correlations between brain size and body size, the variability in these relations, and the reasons underlying phylogenetic differences, require consideration of both direct and indirect influences. Direct influences include structural and metabolic constraints on encephalization and size-related needs: large-bodied animals are better able to support large crania and energetically expensive neural tissue than are small-bodied animals, and larger bodies may require more neurons to control. If this were all there was to the relation between body size and brain size, then we would expect simple and consistent statistical associations. However, additional indirect influences can independently affect both body size and intelligence, including the effects of selective pressures shaping other aspects of a species’ biology, such as locomotion, diet, predation risk, social interactions, and life history. If these indirect influences are important for determining brain–body size relations, then we expect more