The evolution of body size and sexual size dimorphism in primates

Abstract

Primates vary widely in respect to body mass, as well as sexual size dimorphism. Despite largely being considered the result of sexual selection, the processes that give rise to sexual size dimorphism are still widely debated, with a number of alternative theories having been and still being proposed. Relatively recent studies have found that allometric relationships among primates follow two prominent and widely cited “rules” of evolutionary biology, Rensch’s rule and Cope’s rule. Using phylogenetic comparative methods that enable the detection of long-term trends from extant data, and by looking at male and female evolutionary history
independently, I test the idea that sexual selection for increased male size is not only the primary mechanism behind sexual size dimorphism in primates, but also the observed trends of Cope’s and Rensch’s rule. I find that although multiple processes may lead to sexual size dimorphism in primates, the most extreme cases, those observed in the catarrhines, are most likely the result of selection for
larger males. The most notable example of this occurred early on in catarrhine evolution, with several lineages subsequently undergoing further selection on male size. I also find that selection for increased male size in catarrhines as the most likely cause behind the pattern of Rensch’s rule and Cope’s rule observed in primates, suggesting that these “rules” should not in fact be considered allometric rules, but are instead trends that result directly from sexual selection for larger male
size. I also find that species adopting polygynous and polygynandrous mating systems are significantly more sexually dimorphic in size than monogamous and polyandrous species. These results open up intriguing new avenues of future study in which the relative roles of natural selection and sexual selection in the evolution of morphological traits can be teased apart, and further light shed on questions that have pervaded evolutionary biology for centuries.