CONCEPTS IN BIOLOGY
PART V. THE ORIGIN AND CLASSIFICATION OF LIFE
23. The Animal Kingdom
23.3. Temperature Regulation
The temperature of an organism affects the rate at which its metabolic reactions take place. Some animals do little to regulate their temperature, whereas others expend considerable energy to maintain a constant temperature.
Poikilotherms are organisms whose body temperature varies with the environmental temperature. When the water or air temperature changes, so does the temperature of the organism. All microbes and plants and most animals, including insects, worms, and reptiles, are poikilotherms. This is significant because, at colder body temperatures, poikilo- therms have lower metabolic rates; at higher body temperatures, they have higher metabolic rates. A low metabolic rate means that all of their body functions, such as movement and rate of digestion of food, are slowed. However, an organism with a low metabolic rate requires less food.
Homeotherms are animals that maintain a constant body temperature that is generally higher than the environmental temperature, regardless of the external temperature (figure 23.3). These animals—birds and mammals—have high metabolic rates, because they use metabolic energy to maintain their constant body temperature. This means that homeotherms have higher food demands than poikilotherms but are able to remain active at low environmental temperatures. Among homeotherms, the smaller animals have higher body temperatures and higher metabolic rates than the larger animals. This is in part because small animals lose heat faster than large animals, because small animals have a larger surface area compared to their volume.
FIGURE 23.3. Regulating Body Temperature
The body temperature of a homeotherm remains relatively constant, regardless of changes in environmental temperature. The body temperature of a poikilotherm varies considerably, depending on the environmental temperature.
Sometimes, the terms ectotherm and endotherm are used to describe the same relationship from a slightly different point of view. Ectotherms are organisms whose body temperatures depend on the external temperature, and endo- therms are animals that have internal heat-generating mechanisms and can maintain a relatively constant body temperature in spite of wide variations in the temperature of their environment.
However, poikilotherms or ectotherms are not necessarily totally at the mercy of their environment. Many poikilotherms use behavioral means to regulate body temperature. One simple method is to position the body so that it absorbs heat from its surroundings. Many kinds of reptiles and insects “sun” themselves. By placing themselves in the Sun or on rocks or other surfaces that have been heated by the Sun, they can raise their body temperature above that of their environment. Similarly, many poikilotherms can generate heat by contracting their muscles. In cold weather, bumblebees often beat their wings prior to flying, raising their body temperature. Body temperature is important, because the rate at which the wings can beat is determined by body temperature.
23.3. CONCEPT REVIEW
4. How does the body temperature of a poikilotherm affect its metabolism?
5. List an advantage and a disadvantage of maintaining a constant body temperature.