Bipedal Form and Locomotor Function: Understanding the Effects of Size and Shape on Velocity and Energetics.

Abstract

Volumes have been written to address the locomotor anatomy of hominids, yet despite this intense scrutiny, we still do not fully understand the relationship between bipedal form and locomotor function. Energetic expendi-ture (as a measure of locomotor function) can be modeled using techniques that exploit empirical or theoretical methods, but none of these approaches fully captures how energetic expenditure varies within or among species at a particular, or through evolutionary, time. Form, too, is imperfectly understood. That form in the most general sense strongly affects energetic expenditure seems clear, but exactly how size should be represented (which com-bination of lengths and masses) and what effect shape has is not understood. Further complicating these issues is the pragmatic problem that extinct animals are often represented by incomplete fossils that only provide a hint of potential form differences. Despite these problems, we believe some trends can be discerned. Size, especially leg length, is associated with velocity and, therefore, correlated with daily range. In the absence of confirmed differences in shape, size does not drive energetic efficiency. Consequently, differences in size among extinct bipeds should be interpreted as indicat-ing differences in environmental context, not in levels of efficiency. We believe that small non-modern hominid species, i.e., Homo floresiensis and Australopithecus(Paranthropus) sp., are small because they did not need to be big in order to walk fast and go far. The “Energetic Studies in Hominin Evolution” Symposium, Paleoanthropology Meetings, Philadelphia, PA, 27—28 March, 2007; symposium papers guest edited by Karen Steudel-Numbers (University of Wisconsin) and Cara Wall-Scheffler (Seattle Pacific University).