The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Intrasexual competition and size dimorphism among polygynous primates


Anthropology, University of Toronto

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Intrasexual competition exerts a strong influence on the evolution of secondary sexual characteristics. It has long been theorized that the evolution of large male body size in mammals is correlated with the intensity of physically aggressive intrasexual competition that males experience. Despite the predictions of sexual selection theory, when monogamous species are removed from analyses, a number of studies have found no relationship between sexual size dimorphism (SSD; ratio of male-to-female body size) and the intensity of physically aggressive male-male competition, while others have found the expected positive association. This lack of convergence among studies is in part due to difficulty in finding a measure of intrasexual competition that reflects its potential to influence sexual selection. Since the temporal availability of females has a strong influence on the monopolization potential of males, estrous synchrony should be included in any measure of the potential for male-male competition over access to females. Thus, here I present a new formulation of the operational sex ratio (OSR), which incorporates estrous synchrony and the adult sex ratio (ASR). Using phylogenetic generalized least squares regression (PGLS), I find that among polygynous primates, there is no relationship between SSD and the potential for physical intrasexual competition. This indicates that polygynous primates represent an exception to the general rule that intensity of intrasexual competition leads to evolution of large male body size. Possible explanations include use of alternative mating strategies by polygynous males, and reduced reliance on physical competition due to formation of complex social relationships.

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