The Energetic Paradox of Human Running and Hominid Evolution [and Comments and Reply]

Abstract

The energetic cost of running is relatively high in man. In spite of this, humans are adept endurance runners, capable of running down, for example, zebra and kangaroo. Distance running is made possible for man in part by an exceptional ability to dissipate exercise heat loads. Most mammals lose heat by panting, which is coupled to breathing and locomotor cycles during running. This interdependence may limit the effectiveness of panting as a means of heat dissipation. Because sweating is not dependent on respiration, it may be more compatible with running as a thermoregulatory mechanism. Furthermore, man's lack of body hair improves thermal conductance while running, as it facilitates convection at the skin surface. While horses, for example, have been shown to possess energetically optimal speeds in each gait, the energetic cost for a man to run a given distance does not change with speed. It is hypothesized that this is because bipedality allows breathing frequency to vary relative to stride frequency. Man's constant cost of transport may enable human hunters to pursue the prey animal at speeds that force it to run inefficiently, thereby expediting its eventual fatigue. Given what is known of heat dissipation in Old World Anthropoidea, the bipedality of early hominids, and human exercise physiology, one factor important in the origin of the Hominidae may have been the occupation of a new niche as a diurnal endurance predator.