CARRIER, D/R; Univ. of Utah, Salt Lake City: Lucy�s diminutive legs.

Australopithecines had short legs. Relatively short hindlimbs characterize most large bodied primates (Jungers, 1984). The short legs of australopithecines are generally thought to represent arboreal specialization. There are reasons, however, to suspect that the evolution of relatively short legs may also be due to specialization for physical aggression. Fighting is a behavior in which short legs can be expected to improve performance, by lowering the center of mass and reducing the length of the ground reaction force moment arm. Fighting, associated with male-male competition, is prevalent in all extant great ape species. To address whether the evolution of short legs is associated with aggression, I examined the relationship between limb length and body size sexual dimorphism (SSD) in Hominoidea (i.e., Hylobatidae and Pongidae). Among anthropoid primates, SSD is strongly correlated with levels of male-male competition (Plavcan and Van Scaik, 1997), which allows SSD to be used as an index of male-male aggression. Relative leg length is strongly correlated with SSD (R² = 0.825, P-value = 0.0018). Ape species with the shortest legs for their body mass tend to be the most dimorphic. Independent contrast analysis also indicates a negative correlation (R² = 0.940, P-value = 0.0003). This negative correlation remains significant after the effect of body size is removed (R² = 0.665, P-value = 0.025). In contrast, the evolution of relative hindlimb length is not significantly correlated with the evolution of relative forelimb length (R² = 0.274, P-value = 0.228). Although australopithecines possessed a striding bipedal gait, they retained relatively short legs for over 2 million years. The observed negative correlation between evolution of leg length and evolution of sexual dimorphism in apes is consistent with specialization for high levels of physical aggression in australopithecines.