Sexual dimorphism in non-human primates

Sexual dimorphism in non-human primates has long been observed in the primate order, with numerous studies performed to document and explain the phenomenon. Recent studies have mainly used the technique of comparative analysis to examine both the variation in the expression of the dimorphism among primates and the fundamental causes of sexual dimorphism.^[1] Primates usually have dimorphism in body mass^[1][2][3] and canine tooth size^[4][5][6] along with pelage and skin colour.^[7] The dimorphism in primates has been attributed to many factors:

• Mating system – Polygynous species are more sexually dimorphic than monogamous species, such as the New World monkeys.^[8] It is also associated with greater male gonadal investment than what is found in closely related monogamous species.^[9][10]
• Size – Larger species are more sexually dimorphic than smaller species.^[8]
• Habitat – Terrestrial species tend to be more sexually dimorphic than arboreal species. It is possible that male-male competition in terrestrial species is more dependent on body size.
• Diet – Frugivores, for reasons that are not clear, are slightly more sexually dimorphic than folivores.^[10] Energy may be less of a constraint. or maybe females may be more clumped around an area of fruit trees.
• Copulatory plugs - Copulatory plugs are negatively associated with the degree of sexual dimorphism in primates suggesting they may be used as an alternative form of mate guarding, thus reducing selection pressure on large male size in some species.^[11]

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Comparative analyses have substantiated the sexual selection hypotheses, and have generated a more complete understanding of the relationship between sexual selection, natural selection, and mating systems in primates. Studies are helping to find the relative contribution of the various selective and non-selective mechanisms in sexual dimorphism evolution and expression.^[12][13] These studies have shown that dimorphism is the product of changes in both male and female traits.^[1][14] Ontogenic scaling, where relative extension of a common growth trajectory occurs, may show some insight into the relationship between sexual dimorphism and growth patterns.^[15]

Sexual dimorphism in the primate face

Sex-linked skeletal and dental variation in primates has been the focus of considerable anthropological and anatomical research since 2002.^{[15][16][17][18][19]} This area is currently being investigated as sexual dimorphism is one of the major potential sources of variation that must be taken into account when assessing the number of species represented in a fossil sample and the second is that there have been some strong links between sexual dimorphism and ecological variables in extant species. Thus, by establishing the nature of sexual dimorphism in fossil species it has been possible to make conclusions about their social systems and habitat preferences.

Sexual dimorphism in primate teeth

Primates, such as baboons, are well known for their highly sexually dimorphic canine teeth, with males possessing canines that are up to 4 times taller than those of females.^[20] Primate canine dimorphism has been extensively documented, with the consensus that large male primate canines serve as weapons for intrasexual competition, and some evidence that large female canines also have this function. Primate canine teeth are as large or larger as similarly sized carnivores; they also seem to have been selected for their strength and length does not necessarily impact on canine function. In deciduous teeth it has been shown that Cercopithecidae, with the exception of Papio cynocephalus, showed no significant sexual size dimorphism and that it would be problematic to use deciduous teeth in species determination.^[21] Phylogenetic comparative methods have been used to analyze the consequences of sexual selection on canine size and canine size dimorphism in primates.^[6] The analyses indicated that in Haplorrhini the degree of sexual selection is correlated with canine size dimorphism, as well as with canine size in both sexes. Matched-pairs analyses on canine size after correcting for the effects of body size showed significant effects of sexual selection on relative canine size, indicating that canine size is more important in haplorrhine male-male competition than body size.

See also

• Sex differences in humans

References