Scaling of Size and Dimorphism in Primates I: Microevolution

Abstract

Previous researchers found positive scaling of body size and sexual size dimorphism (SSD) in primates, known as Rensch's rule. The pattern is present in Haplorhini, but absent in Strepsirhini. I found that positive evolutionary correlations between size and SSD drive positive scaling relationships within Haplorhini as a whole and Platyrrhini, Cercopithecinae, Colobinae, and Hominoidea individually at the generic level and higher, but that evolutionary correlations within genera in these clades are often nonsignificant or negative. I suggest that positive evolutionary correlations result from greater change in male than in female size, usually because of sexual selection acting on polygynous populations. I suggest that negative evolutionary correlations result from greater change in female size, owing to either natural selection or, in Callitrichidae, sexual selection acting on polyandrous populations. The high incidence of negative evolutionary correlations within Haplorhini suggests a relatively large influence of natural selection on SSD, at least with regard to differences in SSD between congeners. I propose two possible explanations for the difference in intrageneric and supergeneric evolutionary patterns: 1) natural selection is a relatively weak force for modifying SSD and has a noticeable effect only when one compares related species experiencing similar levels of sexual selection, and 2) natural selection is a relatively strong force for modifying SSD but is less likely than sexual selection to affect higher level taxonomic comparisons noticeably because of the cumulative effect over time of marginal differences in mortality rates of these two types of selection. I discuss types of data required to test these explanations and implications for reconstructing fossil behavior.