Sexual Size Dimorphism in Australopithecus: Current Understanding and New Directions

Abstract

Sexual size dimorphism in extant and extinct species has often been viewed as a proxy for sexual selection, and by extension, mating system. As a result, various measures of relative size variation have been calculated for australopiths (particularly Australopithecus afarensis) as a means to infer mating system and social structure in these extinct hominins. Such analyses are confounded by several factors, including (1) different levels of sexual dimorphism may be present within one species when comparing different systems such as canine size, postcranial size, and body mass; (2) evidence suggests that sexual size dimorphism responds not only to sexual selection acting on both male and female size, but also to natural selection acting differentially on the sexes; and (3) measures of relative size variation within skeletal and/or dental samples of unknown sex are not direct measures of size dimorphism, but rather estimates which are subject to both known and unknown sources of error. This chapter addresses concerns and possibilities for future analyses that relate to these confounding effects, and goes on to present the current understanding of size dimorphism within Australopithecus. Specific methodologies for measuring relative size variation within fossil taxa are discussed, particularly newer techniques which incorporate information from across multiple skeletal elements. Finally, the biological and phylogenetic significance of different possible levels of dimorphism within Ardipithecus ramidus and various species of Australopithecus is discussed in the context of extracting information on selection pressures beyond simple reconstruction of mating systems and social structure.