Energy budgets reflect adaptations for regulating body temperature

Both ectotherms and endotherms influence their body temperatures by altering four avenues of heat exchange between their bodies and the environment (Figure 39.9):

  1. Radiation Heat moves from warmer objects to cooler ones via the exchange of infrared radiation (what you feel when you stand in front of a fire).

  2. Convection Heat exchanges with a surrounding medium such as air or water that flows over a surface (the wind-chill factor).

  3. Conduction Heat flows directly between two objects at different temperatures when they come into contact (e.g., an icepack on a sprained ankle).

  4. Evaporation Heat is transferred away from a surface when water evaporates on that surface (the effect of sweating).

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Figure 39.9 Animals Exchange Heat with the Environment An animal’s body temperature is determined by the balance between internal heat production and four avenues of heat exchange with the environment: radiation, convection, conduction, and evaporation.

The total balance of heat production and heat exchange can be expressed as an energy budget, based on the simple fact that if the body temperature of an animal is to remain constant, the heat entering the animal must equal the heat leaving it. The heat coming in is usually from metabolism and radiation (Rabs, for radiation absorbed). Heat leaves the body via the four mechanisms listed above—radiation emitted (Rout), convection, conduction, and evaporation. The energy budget takes the mathematical form

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The energy budget is a useful concept because any adaptation that influences the ability of an animal to control its temperature must affect one or more components of the energy budget. The energy budget gives us the ability to quantify and compare the thermal adaptations of animals. One interesting observation is that all of the components on the right side of the energy budget equation—that is, the heat-loss side—depend on the surface temperature of the animal. In fact, if the environmental temperature is above skin surface temperature, convection and conduction are avenues of heat gain rather than loss. One way surface temperature can be controlled is by altering the flow of blood to the skin.