Temperature Relations

Body temperature has major significance for an animal's physiology. Temperature determines the state of matter and influences the rate of chemical reactions in biotic as well as abiotic systems. The body temperature of active animals generally ranges from —2 °C (freezing point of seawater) to +50 °C (where protein structure becomes unstable). This body temperature range can be even greater for animals in an inactive or dormant state; some can survive temperatures as low as —200 °C or as high as 120 °C!

All animals exchange heat with their environment. The vast majority of animals passively thermoconform to the temperature of their surroundings. However, some manipulate their thermal exchange to thermoregulate their body temperature within reasonably constant limits (typically 35-40 °C) and maintain an appreciable temperature gradient between themselves and the environment.

The thermal environment of an animal is complex. Heat exchange between an animal and its environment occurs by conduction, convection, radiation, and evaporation/condensation (Figure 9). Conduction is direct heat transfer between two solid objects in physical contact. The rate of exchange depends on the area of physical contact, temperature difference, distance the heat must diffuse, and thermal conductive properties. Convection is transfer of heat by fluid movement (liquid or gas), and depends on the surface area, the temperature differential between the fluid and the surface of the solid, and the thickness and conductivity of the convective boundary layer. Forced convection occurs if the fluid movement is a result of external forces (e.g., wind), while free convection is induced by the temperature of the object itself. Radiation transfers heat between two objects that are not in physical contact by electromagnetic waves. The higher the surface temperature of an object, the greater is the

Figure 9 Avenues of thermal exchange between an animal and its environment; conduction, convection, radiation, and evaporation. Photograph by P. Withers.

radiative heat loss. Animals both emit and absorb radiation. Environmental sources of radiation for animals are complex and include direct solar radiation, diffuse scattered radiation, reflected radiation, and infrared radiation from surrounding objects and the ground. The structural and optical properties of an animal's surface are important determinants of its radiative heat load. Evaporative heat loss can be substantial because the latent heat of vaporization is about 2200 kJg-1 (and condensation has an equivalent warming effect). Terrestrial animals lose heat via cutaneous and respiratory evaporation, and may have adaptations to reduce or augment this loss depending on environmental conditions.

Ectothermic animals have no physiological capacity to regulate their body temperature using internal metabolic heat production. They must either thermoconform to their environment, if their environmental temperature is relatively constant or if they can tolerate fluctuations in Tb, or use behavioral regulatory mechanisms to maintain a Tb that is somewhat independent of environmental temperature. For example, many ectothermic reptiles remain largely independent of ambient temperature by using thermoregulatory behaviors such as basking, shuttling between warm and cool microhabitats, and postural adjustments to keep Tb about 36-38 °C. Endothermic animals, such as birds, mammals, and some insects, have physiological control of body temperature (typically 35-42 °C). They utilize heat produced as a by-product of metabolism to maintain their high and constant Tb independent of ambient conditions. Insulating fur and feathers reduces heat flux between endotherms and their environment. Endotherms may also employ behavioral thermoregulatory strategies to reduce the energetic costs of endogenous heat production, especially when the gradient between Tb and Ta is large.

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