P.E. Wheeler

The evolution of bipedality and loss of functional body hair in hominids

The large relative brain size of hominids, and their lack of an effective carotid rete, means they are vulnerable to thermal damage during elevations of body temperature. This would account for the well developed whole-body cooling system of man, provided by a functionally naked skin and associated cutaneous sweat glands. It is argued that the lower direct solar radiation fluxes incident upon a bipedal mammal made possible the reduction of body hair, and explains the absence of this characteristic among savannah quadrupeds. The major thermoregulatory advantages conferred by bipedality, to an animal extremely sensitive to hyperthermia, could also account for the initial evolution of this unusual form of locomotion.

The thermoregulatory advantages of hominid bipedalism in open equatorial environments: the contribution of increased convective heat loss and cutaneous evaporative cooling

The thermoregulatory advantages conferred by bipedalism to a large-brained primate on the African savannah could have been significant factors contributing to the adoption of this unusual mode of terrestrial locomotion. Although the major benefit is a dramatic reduction in direct solar radiation exposure, additional advantages also result from the higher distribution of the body surfaces. Windspeeds are higher and air temperatures lower away from the ground, both factors increasing the rate at which a biped dissipates heat by convection. The greater airflow and low relative humidity above any surface vegetation present will also increase the rate at which sweat can be evaporated from the skin.

The loss of functional body hair in man: the influence of thermal environment, body form and bipedality

An insulating layer of body hair is crucial to the thermoregulatory energetics of most mammals. Only some highly specialized aquatic forms and a few fossorial and terrestrial species which experience special heat stress problems have evolved a naked skin. Mans body size and form suggest that his naked skin is associated with the prevention of hyperthermia in hot climates rather than an aquatic ancestry. The reduced direct solar radiation exposure of hominids in open equatorial environments, resulting from their unique bipedal posture, may explain the absence of this characteristic among other savannah mammals of comparable body mass.

The influence of bipedalism on the energy and water budgets of early hominids

A model is described for estimating the net thermal loads, and associated demands for drinking water, experienced by bipedal and quadrupedal hominids in open equatorial environments. Comparisons of the results obtained with the thermoregulatory capabilities of living primates indicate that the equatorial African savannah would have been a difficult habitat for hominoids to exploit. However, under all temperature regimes examined distinct advantages would have been conferred by bipedalism, allowing hominids to forage at higher temperatures and over greater distances, while consuming less food and water, than quadrupeds.

The thermoregulatory advantages of large body size for hominids foraging in savannah environments

Abstract Estimates are presented of the net thermal loads, and associated drinking water requirements, experienced by naked bipedal hominids weighing between 10–100 kg. The increase in body size observed in the hominid fossil record would have conferred significant advantages to these primates when foraging on dispersed resources in open equatorial environments, where drinking opportunities were limited. Larger hominids dehydrate more slowly and are able to cover a greater distance each day before encountering thermoregulatory constraints. This substantially increases the home range area, and consequently the quantity of dietary resources, to which they would have had access. The potential relevance of these thermoregulatory factors to the high degree of sexual size dimorphism in early hominids is discussed.

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

Abstract 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 bipedalism was the pre-adaptation necessary to make the loss of functional body hair a net thermoregulatory asset in such environments.

The Evolution of Reversed Sexual Dimorphism in Birds of Prey

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Thermal acclimation of metabolism and preferred body temperature in lizards

Abstract 1. 1.|The standard metabolic rates (SMRs) and preferred body temperatures (PBTs) of the tropical cordylid Cordylus jonesi and temperature lacertid Lacerta lilfordi were determined following acclimation to constant environmental temperatures of 20 and 30°C. 2. 2.|Although after 5 weeks the SMRs of Cordylus jonesi and Lacerta lilfordi displayed partial compensations of 20.9 and 10.5%, respectively, their PBTs did not alter over this period. Therefore, acclimation does not maintain complete metabolic homeostasis during either the active or inactive phase of the lizard. 3. 3.| Cordylus jonesi allowed to thermoregulate behaviourally at their PBT during activity possessed similar SMRs to control animals maintained continually at the same background temperatures, indicating that acclimation state in lizards is determined by the body temperatures experienced while at rest. 4. 4.|The particular acclimatory problems of animals exhibiting behavioural homeothermy are discussed.