Body Temperature in Poikilothermal Animals

Summary Temperature, especially body temperature, is a very important factor in the lives of animals. Body temperature may be raised or lowered by the transfer of heat directly by conduction, or indirectly through the transformation of radiation into heat; it may also be raised by metabolism and lowered by the evaporation of water. These are the factors which determine the amount by which the body of an animal shall differ in temperature from the surroundings. In aquatic animals, only conduction and metabolic heat are important and the body could not therefore remain cooler than the water. On the other hand, the water acts as a very effective cooling fluid, especially in animals with a good branchial flow of blood, and after a change of water temperature, body temperature becomes steady again in seconds or a few minutes. With one exception all recent measurements show body temperature to be less than 1°C. warmer than the water, even in a large cod. Temperature measurements are open to many errors, and the century‐old belief that the tunny is warm‐blooded requires confirmation. Air, unlike water, is a good heat insulator. For land animals gains of heat through radiation and losses by evaporation are important; an environment at uniform constant temperature probably seldom or never occurs in nature and gradients of temperature in the animal body itself may be considerable. Evaporation of water may cool an insect by several degrees below the air around it, especially if the air is dry and the temperature is as high as 40° C. There is no reason to believe that this cooling is an active regulatory mechanism, even in the special case of the cockroach, though it may save the insect from heat‐stroke during a short exposure. The surface‐volume ratio is larger for smaller animals, heat intake varies with the surface, and water available for evaporation varies with the weight. Consequently, other things being equal, if a small insect keeps cool for long it may desiccate too much. Even for a large insect prolonged exposure to warm dry conditions may be more damaging than a similar exposure to moist conditions, because the animal dies from desiccation. Radiation, especially from the sun, is often an important factor which keeps insects' bodies warmer than the immediate surroundings, and differences of 5–15° C. have been recorded. The colour of an insect's cuticle is not a good indication of the degree to which it absorbs heating radiation. The metabolic heat developed by similar insects at rest is very roughly proportional to the surface area of the body; since direct and indirect heat exchange and evaporation of water are also related to surface area rather than to body weight, mere size of body should have little effect on body temperature under steady state conditions. In most comparisons of different species, of course, the constants to be applied to these factors will be dissimilar. Some insects can increase their metabolic rate enormously, and this power is used in elevating body temperature, especially before starting to fly. It seems unlikely that insects could maintain a high temperature by means of a coat of material of low heat conductivity, for with small objects such a coat actually increases the rate of heat loss because of the enlargement of the surface area. A heat balance‐sheet shows that evaporation is the principal mode of loss of metabolic heat, even in rather moist air. Amphibia appear to have no anatomical protection against losing body water and it is their behaviour which enables them to live on land. In dry air, especially in a current, evaporation is extraordinarily rapid, body temperature falls 5–10 0 C. or more, and death from desiccation occurs in a few hours. In saturated air the body is warmer than the air and some of the metabolic heat is still lost by evaporation. In reptiles the skin is opaque to radiation and so protects the deeper layers of the animal. Like some insects lizards bask in the sun and the body may thus be warmed by as much as 20 0 C. The rate of temperature change is lower than in insects because of the larger size and the consequent smaller surface area per unit weight. Evaporation from reptiles is slow, even slower than from xerophilous insects, if the rate of loss is expressed as a percentage of body weight. Even so a large snake when inactive is cooler than the surrounding air because of such evaporation as does occur. On the whole the heat relations of terrestrial reptiles seem to be similar to those of insects. There is no reason to expect a raised body temperature in large reptiles; the large size of some of the extinct reptiles probably had only the effect of making body temperature lag behind changes in the environmental temperature. It is the high metabolic rate of birds and mammals which enables them to maintain a high constant temperature, not primarily their coats of feathers or hair.