Sexual Dimorphism in Fossil Hominids

Body weight sexual dimorphism in A. afarensis is above that seen in modern humans and chimps, but well below that characteristic of gorillas and orangs. As in other animals, sexual selection probably accounts for this dimorphism and is most likely related to a polygynous mating system. The level of body weight dimorphism in A afarensis corresponds to what might be expected from moderate intensity male/male competition. Kin-related multi-male groups are most likely based on the composition of the A. L. 333 site at Hadar and on theoretical considerations.

Canine dimorphism in A. afarensis is well below that seen in the great apes (except for the lower canines in Pan paniscus), but forelimb size dimorphism is apparent. This may be related to the fact that forelimbs were freed from any role in terrestrial locomotion and thereby took over the role of threat and aggression previously played by the canines.

Body weight dimorphism declines throughout time from A. afarensis to A. africanus to A. robustus with a dramatic reduction in H. erectus. Taxonomic attributions of specimens formerly assigned to H. habilis obscure the analysis of dimorphism in pre-erectus Homo, unfortunately.

The dramatic reduction in body weight sexual dimorphism at 1.7 Myr with the appearance of H. erectus is due to increase in both male and female body size. The cause of this change is probably related to the release of dietary constraints on body size, selection for larger brains and biomechanical constraints on maximum male body size. It may also be due to reduced male/male competition.

Sexual dimorphism is very high in some traits such as mandibular size in later H. erectus, but there are too few postcranial remains of later H. erectus and early H. sapiens to assess body weight dimorphism. Weight dimorphism in Neanderthals and early anatomically modern H. sapiens is as low as that seen in modern humans.

from: McHenry, H.M. (1996) Sexual dimorphism in fossil hominids and its socioecological implications. IN: The Archaeology of Human Ancestry edited by James Steele and Stephen Shennan. Routledge, London. pp. 91-109.