The influence of the loss of functional body hair on the water budgets of early hominids

Estimates are presented of the net daytime thermal loads, and associated drinking water requirements, experienced by fully haired and naked skinned hominids adopting quadrupedal and bipedal postures in open equatorial environments. Naked hominids could probably have tolerated higher environmental temperatures and levels of metabolic heat production, but this would have required the expenditure of large amounts of water. At normal levels of metabolic activity, a naked skin actually reduces the water requirements of bipedal hominids exposed to temperature regimes typical of the African Savannah. A naked skin confers no such advantage on a quadruped. The findings support the contention that biped&m was the pre-adaptation necessary to make the loss of functional body hair a net thermoreguIato~ asset in such environments.