Sexual Size Dimorphism in Australopithecus: Postcranial Dimorphism Differs Significantly Among Australopithecus afarensis, A. africanus, and Modern Humans Despite Low-Power Resampling Analyses

Abstract

Objectives

Dimorphism estimates are used to infer competition levels, social structure, and mating system in fossil hominins. However, previous studies have reached conflicting conclusions about the degree of postcranial dimorphism present in Australopithecus afarensis, and statistical comparisons of postcranial size dimorphism between A. afarensis and other early hominins are lacking. This study addresses reasons for differences in published studies and directly compares dimorphism in A. afarensis, A. africanus, and extant hominids.

Materials and Methods

Eight postcranial variables represent size for three extant hominids (gorillas, humans, and chimpanzees) and two extinct hominins (Australopithecus afarensis and A. africanus). A modified version of Gordon et al.'s (2008) geometric mean method is used to perform significance tests for direct comparisons of estimated sexual size dimorphism in two fossil samples with different patterns of missing data.

Results

Both Australopithecus species are highly dimorphic—significantly more dimorphic than chimpanzees and modern humans. A. afarensis is also significantly more dimorphic than A. africanus.

Discussion

Previous studies (and this analysis) are typically too low-powered to find significant differences between humans and extant African apes when sampled in the same manner as fossils, rendering negative results for fossil comparisons noninformative. In this study, effect sizes for differences in dimorphism between fossils and other species are large enough to be significant, even at low power. Results suggest intense sexual selection maintained high dimorphism in both fossil species, but also that different species-specific suites of selection pressure produced diversity in the degree of dimorphism present across Australopithecus species.