Temperature regulation is extremely uneconomical if it depends only on variations in metabolism. Therefore, mechanisms for losing excess heat have been developed by both birds and mammals. In an aquatic environment, the entire heat transfer between the animal and the media is through conduction. However, in terrestrial habitat, only small amounts of heat are exchanged this way. Most of the Homeotherms are terrestrial.
In man, heat loss due to radiation accounts for about 55% and heat loss due to evaporation is about 44%. The amount of heat loss depends on the ambient temperature and the humidity. Loss of heat by radiation and conduction is usually effective in a cool environment. Whereas, at high temperature, the animals will actually receive heat by these routes. Evaporation however, is always negative factor and it requires considerable energy expenditure. For instance, to vaporise one gram of water from the moist surfaces of the skin or respiratory epithelia, 0.6 kcal is required. This technique of cooling has been exploited in quite different ways by birds and mammals.