| Abstract: |
Provides explanation for how humans adapted to tropical grasslands. Hair loss and seating are discussed. There are four channels of energy exchange between the animal and its environment: conduction, convection, radiation, and evaporation. Conduction is the flow of heat without displacement of the material and can occur in gases, liquids, or solids. Example is touching an object which is either hot or cold. In man, it is not a major source of heat exchange. Convection is a flow of heat by the physical movement of the gas or liquid in contact. Example is the increase in cooling (or heating) produced by wind. This exchange occurs at the outer boundary of a thin layer of still air whihc surrounds the body. Radiation is an exchange of heat from a warmer to a cooler object whihc is independent of the intervening medium. It includes the direct acquisition of heat form the sea as ultra-violet, visible, and infra-red energy and the more subtle exchanges by whihc we radiate heat to, or receive it from our surrounding. This is an exceedly important change for heat exchange in man. Evaporation results in the lost of "latent" heat required to vaporize water from the skin and from the membranes of the respiratory tract. pg 15 larger body size becomes advantageous since it minimizes the surface area (cm\+2\+) per unit of body volume (cm\+3\+) and this spreads the radiant heat load throughout a greater mass, sparing the critical centers. pg 17 Man has been observed to sweat at the rate of three liters per hour for short periods of heavy work in high heat. One liter per hour is a reasonable figure for many hot conditions. pg 18 At rest or light work the clothed man gained about two-thirds as much heat as the mude man and sweated commensurately less, and this is roughly the same savings in heat gain that one gets from going out of the sunshine into shade. pg 19 Every liter of water that an average 70kg man loses represents nearly a 1.5% loss in body weight. A man who has accumulated a 2% weight loss from sweating is thirsty, by 10% he is helpless, and death occurs at 18-20%. pg 20 Man reaches satiety after rapidly consuming 1 liter of water and cannot imbibe over 2 liters in a minute period. pg 23 Man's specialized dependence on thermal sweating appears to be an evolutionary adaptation to the tropics.
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