Body Temperature Regulation

Depending on the source of an organism's body warmth, it may be classified as either an ectotherm or an endotherm. An ectotherm is an animal that warms itself primarily by obtaining heat from the environment, perhaps by sunning itself. An endotherm is an animal that produces most of its own heat metabolically. Endothermy is more familiar to humans because they, like other mammals, produce their own heat through the metabolic breakdown of food. This heat is mostly a by-product of the inefficiency of the anabolism and catabolism occurring in the cells of the body. (Remember, approximately 60% of the energy stored in food is lost as heat during catabolism.) Of course, practically all endothermic animals depend on environmental heat at some point in their lives. This is a principle well understood by house cats and winter campers (rather unrelated species). All birds and mammals are endotherms. More surprising, organisms such as tuna fish, skunk cabbages, butterflies, honey bees, crocuses, and others regularly warm their bodies to an appreciable extent through the use of metabolic heat.

Ectothermic animals include most fish, amphibians, and reptiles as well as most invertebrates. Many such animals do, however, control body temperature through behavior. The well-known image of the lizard sunning itself on a rock provides a good example. The color, body shape, and the timing of the activities of these organisms all contribute to thermoregulation. Most ectothermic organisms are able to maintain body temperatures both higher and more constant than the air or water around them. Some alpine lizards, for example, can maintain uniform body temperatures 30^oC above their surroundings even in winter conditions. Desert lizards can maintain their body temperature to within 0.1^oC during the daytime and then control how much they cool off at night by how deeply they burrow. It now seems that all organisms, whether ectotherms or endotherms have at least some adaptations for thermoregulation. Thus the old idea of "poikilotherms" - animals whose body temperature remains equal to that of their surroundings - is outmoded.

A second way of classifying animals is according to whether or not they keep their body temperature more or less constant at all times. Of course, no organism maintains a perfectly uniform body temperature. Humans, for example, tend to have low body temperatures early in the morning and higher ones in the afternoon. However, some animals substantially lower their body temperature on a regular basis. We are all familiar with the winter hibernation of bears and squirrels who can lower their body temperatures considerably while they are hibernating. Other vertebrates, such as some bats and small birds, actually cool off for a period of time each day. This apparently is an adaptation to save energy that is especially important to these animals because of their small size and high metabolic rate. Animals that maintain more or less constant body temperatures at all times are known as homeotherms. Organisms such as bears and small birds that allow their body temperature to vary to an appreciable extent on a regular basis are known as poikilotherms.

As should be apparent to you by now, there are organisms that can fit into each of the four possible categories created by these two ways of subdividing the thermoregulation of organisms. The following chart summarizes this:

A final question to answer might be: "If endothermy, especially homeothermic endothermy, is so wonderful, why isn't every organism doing it?" One answer might be that it requires a complex set of adaptations and most organisms have not yet evolved them. A better answer is that in fact it's not necessarily a better strategy for many organisms. We tend to think of amphibians and reptiles as being primitive because they are ectothermic. On the contrary, their way of life is superior to that of birds and mammals in many environments. Consider the following fact: birds and mammals perform 80% of their metabolism just to maintain their high body temperatures! This means that the typical bird has committed itself to needing five times as much food energy to stay alive as does a reptile of equivalent weight. For this reason, ectotherms can survive in many habitats and take advantage of many food resources unavailable to the less efficient (from this standpoint) bird or mammal.

Desert Pupfish - Homeothermic Ectotherms

Some pupfish live in the water flowing from desert hotsprings. The spring water may be near boiling point when it first flows from the ground but cools as it flows away. These pupfish maintain very high and very constant body temperatures all year long by regulating where they are in the spring flow. The pupfish shown here is the Devil's Hole Pupfish (Cyprinodon diabolis), an endangered fish species that lives in a constant temperature spring. See preserved pupfish in the study center demo!

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