PHYSIOLOGY, THERMOREGULATION AND BIPEDALISM

It has long been recognized that the bipedal posture reduces the surfa ce area of the body exposed to the sun. In recent years, a theory has been developed by Wheeler that bipedalism evolved in the ancestor of t he Hominidae in order to help relieve thermal stress on the animals in open equatorial environments. Bipedalism was said to afford a distinc t adaptive advantage over quadrupedalism by permitting hominids to rem ain active in the open throughout the day. The heat load of the hypoth etical hominid comprises the external environment as modelled by Wheel er and the animal's internal environment (i.e., the internal heat gene rated by its metabolic and locomotor activities, and its evaporative a nd respirative cooling capacities). When these factors are integrated in the calculation of the animal's thermal budget, the putative advant age of the bipedal over the quadrupedal posture is considerably reduce d. The simulations conducted in this study suggest that the increased time afforded to early hominids in the open by bipedalism was relative ly short and, therefore, of little or no adaptive significance. These results suggest that thermoregulatory considerations cannot be implica ted as a first cause in the evolution of bipedalism in the hominid anc estor.