Metabolic rate is also affected by an animal’s internal body temperature. Temperature affects the rates of chemical reactions, which in turn determine how fast fuel molecules can be mobilized and broken down to supply energy for a cell. Animals can be categorized by the sources of most of their heat. Animals that produce most of their own heat as by-products of metabolic reactions, including the breakdown of food, are endotherms. These animals usually, but not always, maintain a constant body temperature that is higher than that of their environment. As we saw earlier (Chapter 35), the maintenance of a constant core body temperature is a form of homeostasis and requires balancing heat production and heat loss. Mammals, including humans, and birds are endotherms.

By contrast, animals that obtain most of their heat from the environment are ectotherms. Ectotherms often regulate their body temperatures by changing their behavior—moving into or out of the sun, for example. Think of a turtle or some other reptile sunning itself on a rock: It is raising its body temperature. The temperatures of these animals usually fluctuate with the outside environment. Most fish, amphibians, reptiles, and invertebrates are ectotherms.

Endotherms are sometimes referred to as “warm-blooded” and ectotherms as “cold-blooded,” but these terms are misleading because, depending on environmental conditions, cold-blooded animals can have core body temperatures higher than warm-blooded animals. It should also be kept in mind that these thermoregulatory mechanisms are not completely distinct, but instead represent extremes of a continuum.

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Nevertheless, thermoregulatory mechanisms have profound implications for an animal’s metabolic rate. Endotherms have a higher metabolic rate than ectotherms. As a result, they are able to be active over a broader range of external temperatures than ectotherms. Both the activity level and metabolic rate of ectotherms increase with increasing body temperature. However, ectotherms have metabolic rates that are about 25% of endotherms of similar body mass and at similar body temperatures. Although they can achieve activity levels similar to those of endotherms when their body temperatures are similar, ectotherms cannot sustain prolonged activity. Therefore, ectotherms such as lizards or insects rely on brief bouts of activity, followed by longer periods of inactivity compared to endotherms.

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Endotherms benefit from having an active lifestyle. However, their high metabolic rate requires long periods of foraging to acquire enough food to keep warm and be active. By contrast, although ectotherms cannot sustain activity for long periods, they can survive much longer periods without food.

As we have discussed, the control of core body temperature, or thermoregulation, is a form of homeostasis and requires the coordinated activities of the nervous, muscular, endocrine, circulatory, and digestive systems. We summarize how these systems work together in endotherms and ectotherms in Fig. 40.5.