Why males are larger than females in most mammals?

Examining the potential drivers of these differences.

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Females are often larger than males in most species. Contrary to popular belief, most mammalian species exhibit a bias in favor of male sexual size dimorphism. The theory that intramale sexual selection led to the evolution of sexual dimorphism in mammals is widely accepted.

A new study examined the potential drivers of these differences- casting doubt on the theory that only sexual selection is at play in mammals–that males compete to mate with females, and bigger males are more likely to win.

Marcelo H. Cassini from IBYME tested this hypothesis in primates by relating sexual size dimorphism to seven proxies of sexual selection intensity: operational sex ratio, mating system, intermale competition, group sex ratio, group size, maximum mating percentage, and total paternity.

He fitted phylogenetic generalised least squares models using sexual size dimorphism as the dependent variable and each of the seven measures of intensity of sexual selection as independent variables. The comparative analysis was then conducted with data from 50 extant species of primates, including Homo sapiens, Pan troglodytes, and Gorilla spp.

The study found that alongside sexual selection, natural selection may be an evolutionary driver of sexual size differences in mammals.

He noted, “Sexual dimorphism was positively related to the four measures of female monopolisation and in some cases to group size, but was not associated with maximum mating percentage or total paternity.”

Additional regression analyses indicated that maximum mating percentage and total paternity were negatively associated with group size.

The findings of this comparative study of sexual dimorphism and sexual selection intensity in primates indicate that natural selection should be taken into account in addition to sexual selection as an evolutionary driver of polygyny and sexual size dimorphism in mammals. Additionally, the use of intramale sexual selection theory to explain the evolution of sexual dimorphism and polygyny in mammals should be reviewed.

Journal Reference:

  1. Sexual size dimorphism and sexual selection in primates. DOI: 10.1111/mam.12191

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