Why man is such a sweaty and thirsty naked animal: a speculative review.

The argument that this shift from forest to grassland resulted in certain physiological adaptations to environmental heat loads that still characterize man's species is presented.

Abstract

OUE present view of the earliest differentiation of man's ancestors from the primate stock assumes a gradual shift in habitat from forest to open grasslands in the tropics and sub-tropics of the Old World during the Pliocene epoch. This period encompassed some critical innovations in: morphology (complete upright posture and gait, increased cranial capacity, etc.), economy (increasingly carnivorous), behavior (tool making, speech), and probably others not integrated into the central theory of evolution. This paper presents the argument that this shift from forest to grassland resulted in certain physiological adaptations to environmental heat loads that still characterize our species; Barnicot (1959) suggested just such a relationship a decade ago. Specifically, the physical avenues of heat exchange between an animal and its environment will be reiterated to establish a common vocabulary; the interplay between these avenues and selected tropical environments will then be examined with reference to human responses to point up the specializations which characterize our species. Water requirements and the associated patterns of thirst and drinking habits are reviewed since they may have influenced the daily regimen. Finally, the experimental data now available on non-human primates under heat stress will be presented to see whether the human response is characteristic of the