Intrasexual competition and body weight dimorphism in anthropoid primates

Abstract

Body weight dimorphism in anthropoid primates has been thought to be a consequence of sexual selection resulting from male-male competition for access to mates. However, while monogamous anthropoids show low degrees of weight dimorphism, as predicted by the sexual selection hypothesis, polygynous anthropoids show high variation in weight dimorphism that is not associated with measures of mating system or sex ratio. This observation has led many to debate the role of other factors such as dietary constraints, predation pressure, substrate constraints, allometric effects, and phylogeny in the evolution of anthropoid weight dimorphism. Here, we re-evaluate variation in adult body weight dimorphism in anthropoids, testing the sexual selection hypothesis using categorical estimates of the degree of male-male intrasexual competition ("competition levels"). We also test the hypotheses that interspecific variation in body weight dimorphism is associated with female body weight and categorical estimates of diet, substrate use, and phylogeny. Weight dimorphism is strongly associated with competition levels, corroborating the sexual selection hypothesis. Weight dimorphism is positively correlated with increasing female body weight, but evidence suggests that the correlation reflects an interaction between overall size and behavior. Arboreal species are, on average, less dimorphic than terrestrial species, while more frugivorous species tend to be more dimorphic than folivorous or insectivorous species. Several alternative hypotheses can explain these latter results. Weight dimorphism is correlated with taxonomy, but so too are competition levels. We suggest that most taxonomic correlations of weight dimorphism represent "phylogenetic niche conservatism"; however, colobines show consistently low degrees of weight dimorphism for reasons that are not clear.

Sexual dimorphism in primate evolution

“…Most cercopithecoid male primates range from about 30 -80% larger than females in body mass. Body mass differences between males and females are often characterized as species-specific traits, but there is good evidence for biologically meaningful subspecific variation in body mass dimorphism in some species (Turner et al, 1994(Turner et al, , 1997Smith and Jungers, 1997;Plavcan and van Schaik, 1997b;Jones et al, 2000).…”

“…In general, body mass dimorphism reaches its extremes in hominoids and papionines. Platyrrhines on the whole are characterized by lesser degrees of body mass dimorphism, though there are a few species (howler monkeys) that are comparable to cercopithecoids (Ford, 1994;Plavcan and van Schaik, 1997b).…”

“…It is well known that the large sagittal and nuchal crests of male gorillas are a function of the disproportionate increase in the size of the temporalis muscles and nuchal musculature as compared to the brain. Brain size in male and female gorillas is on average quite similar, even though male gorillas can be twice the body mass of females (Smith and Jungers, 1997;Plavcan and van Schaik, 1997b). This means that the brain case in males and females is very similar in size.…”

“…Mate competition has long been thought to be the primary mechanism favoring the development of sexual dimorphism in primates (Darwin, 1871;Crook, 1972;Leutenegger and Kelly, 1977;CluttonBrock, 1985;Kay et al, 1988;Kappeler, 1990Kappeler, , 1991Plavcan and van Schaik, 1992, 1997bFord, 1994;Martin et al, 1994;Lindenfors and Tullberg, 1998). In easily observed species, such as baboons, males are largely intolerant of one another, often engaging in spectacular fights and threat displays.…”

Equality for the sexes in human evolution? Early hominid sexual dimorphism and implications for mating systems and social behavior

“…For most anthropoids, males are bigger than females (4)(5)(6)(7)(8). Humans today display relatively limited sexual dimorphism (Ϸ15%), whereas some of the other hominoids (gorillas and orangutans) are highly dimorphic (Ͼ50%) (5,9).…”