Evolution of body size and sexual size dimorphism in the order primates: Rensch's rule, quantitative genetics, and phylogenetic effects

Rensch’s rule, positive scaling of sexual size dimorphism (SSD) with body size, has been shown to be present in the evolutionary history of Primates. However, in some radiations there is negative or absent scaling of size and dimorphism. Lande’s (1980) quantitative genetics model predicts (1) positive scaling (Rensch’s rule) when selection acts more intensely on male body size than female body size, (2) no scaling when selection acts equally on both sexes, and (3) negative scaling when selection acts more intensely on female body size. Presented here is a combined response model which describes how sex-specific means and relative variabilities of continuous traits respond to sex-specific selection pressures. Model predictions are compared to data from extant primate populations to identify likely sources of unequal sex-specific selection pressures. Evolution of size and dimorphism within primate radiations is discussed in the context of a new conceptual model for the evolution of various scaling patterns of size and SSD. Under sexual selection by male competition or female mate choice, male body size is expected to be the primary target of selection. Under sexual selection by female competition or male mate choice, or natural selection by resource limitation, female body size is expected to be the primary target of selection. Because any of these forces may be in operation at any particular point in time, surveys of recent evolutionary changes in dimorphism capture positive and negative scaling patterns of SSD and size, and thus Rensch’s rule does not emerge. However, the phylogenetic history of living taxa is not representative of all taxa that ever lived, but only of those taxa that have living descendants. Species subject to intense resource scarcity are more likely to go extinct than species that have abundant resources, so phylogenetic history will be biased against the inclusion of resource limited taxa. In the absence of polyandry, positive scaling of size and SSD (Rensch’s rule) is the most likely pattern to emerge in radiations characterized by sexual selection on male size. This conceptual framework is used to study sex-specific body size evolution in Pan and Australopithecus afarensis.Anthropolog