The Locomotor Energetics of Extinct Hominids (original) (raw)

Bipedality is the defining characteristic of Hominidae and, consequently, the energetics of human bipedal locomotion has been of interest to anthropological researchers for many years. Scenarios of hominid evolution have been created to explain the rise of bipedality as predicated on such diverse factors as selection for energy efficiency (Foley, 1992), maximal thermal energy transfer (Wheeler, 1991a,b) and the use of display to maintain group cohesion (Jablonski and Chaplin, 1993). Without regard for which selective forces were at work, bipedality has been seen as a unitary adaptation, rather than as a general one with a variety of possible styles. Just as there are myriad forms of obligate and facultative quadrupedality, however, so too can there be different forms of bipedality. The style of bipedality that served the early hominids of the late Miocene was almost surely different from that of late Pliocene Homo. The fossil record has begun to reveal that hominids can possess at least two body configurations (Jungers, 1991): relatively short-legged and long-legged versions. The former is characteristic of the early hominids, the australopithecines, while the latter is limited to Homo. Australopithecines are frequently seen as a transitional group, not fully modern in their form of bipedality and, consequently, energetically inefficient. Despite warnings from a few biomechanists (Witte et al., 1991), anthropologists have often equated short legs with inefficiency [an inherent bias of Steudel (1994), Webb (1996), Jungers (1982, 1991) and McHenry (1991b), to name a few]. The implicit logic is that short legs imply short stride lengths and that short strides require a higher cadence to maintain a particular velocity (Jungers, 1991). Unfortunately for anthropological analyses that rely on this logic, energetic expenditure is not solely a matter of the number of strides taken, but also of the energy required to take a single stride. This latter variable is dependent on the configuration of the locomotor anatomy, and shorter limb length implies, among other things, a smaller mass moment of inertia, which decreases energy expenditure. Consequently, it is reasonable to question the assumption that the short-legged