Australopithecus afarensis was NOT Very Sexually Dimorphic

Lucy and other members of the early hominid species Australopithecus afarensis probably were similar to humans in the size difference between males and females, according to researchers from Penn State and Kent State University.

"Previous convention in the field was that there were high levels of dimorphism in the Australopithecus afarensis population," said Philip Reno, assistant professor of anthropology, Penn State. "Males were thought to be much larger than females." Sexual dimorphism refers to differences between males and females of a species. These can show up, for example, in body size and weight or in the size of the canine teeth. For Australopithecines, canines of males and females are about the same size, but it was assumed their body sizes differed. Other primates have varying degrees of sexual dimorphism. Gorillas are highly dimorphic, with males weighing as much as 200 pounds more than females. Chimpanzees are only moderately sexually dimorphic with males weighing about 18 pounds more than females on average. Humans are moderately sexually dimorphic. Previously, researchers assumed that A. afarensis was similar to or even more dimorphic than gorillas in sexual size differences.

Lucy is probably the most famous example of A. afarensis, a supposed female who measures 3.5 feet in height. Also often used as an example of this species is A.L. 128/129, another small specimen assumed to be female. However, A. afarensis existed long before brains in the human line became large enough to require the alteration in the pelvic structure that both allows for large-headed baby births and easy identification of female specimens. "There is no reason why Lucy, if female, would have the wide notched pelvic bone of a human female," said Reno. "We can't really sex Australopithecines."

While Lucy may not be female, she is the earliest discovered and most well preserved example of A. afarensis and so has been used as a model for the study of other specimens. Recently, another reasonably intact A. afarensis, Kadanuumuu, was uncovered and he stood 5 to 5.5 feet tall. Reno and C. Owen Lovejoy, distinguished professor of human evolutionary studies, Kent State, developed the Template Method to compare different skeletons and determine the range and dimorphism of A. afarensis. They report their results in today's (Apr. 28) issue of PeerJ.

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