Implications of Male and Female Contributions to Sexual Size Dimorphism for Inferring Behavior in the Hominin Fossil Record

Abstract

Sexual dimorphism is commonly used to directly infer or support reconstructions of social behavior in early hominins. This is often done by comparing the magnitude of sexual size dimorphism to that seen in extant primates and extrapolating a likely social behavior. Such comparisons are of limited value, though, allowing only the inference of strong male–male competition when dimorphism is strong. Recent studies have begun to focus on the selective factors that impact female body size, and thereby size dimorphism. Considerations of changes in male and female size in the fossil record potentially allow insight into the meaning of changes in sexual dimorphism through time. To illustrate, I compare estimates of body mass dimorphism for four hominin taxa to assess changes in male and female size. Assuming that early Homo represents a single taxon, sexual size dimorphism increased in early Homo through an increase in male size, but was subsequently reduced through an increase in female size in Homo erectus. This would imply a significant increase in sexual selection acting on males in early Homo. An increase in female size with a loss of dimorphism in Homo erectus would imply a simultaneous shift in female optimal body size through selection for increased female fecundity, and/or an increase in female resource abundance, coupled with a shift in selection acting on male size. Although none of these inferences are certain, the exercise illustrates the potential for considering how dimorphism changes through time, rather than simply focusing on the magnitude of size dimorphism in isolation.