What makes some animals cold-blooded and others warm-blooded?

George and Doug reply:

First off, let’s get a few terms straight. “Warm-blooded” and “cold-blooded” are misleading labels. Modern biologists generally use two other contrasting pairs of terms to describe the thermal physiology of animals: homeotherm/poikilotherm and endotherm/ectotherm. (Don’t things seem clearer already?) Basically, a homeotherm is an organism that maintains its body temperature at a nearly constant level, while a poikilotherm experiences much larger fluctuations. The latter terms refer to the source of the body’s heat. In endotherms, most of the heat is generated internally, through metabolism, while in ectotherms, most of the heat comes from external sources, such as the sun.

These characteristics tend to be correlated–most homeotherms are endotherms, and most poikilotherms are ectotherms. But there are exceptions. For example, some desert lizards, although ectotherms (receiving most of their heat from sunlight) are behavioral homeotherms. In the morning they bask to bring their body temperature up to activity level; at mid-day they seek out shade to avoid overheating. By so doing they can keep their body temperature within fairly narrow limits. They are homeothermic ectotherms, at least during the daytime. (At night, when no sunlight is available, they are poikilotherms, their body temperature conforming to the environment.) Conversely, hummingbirds are endotherms that, in order to conserve energy, may go into torpor at night and allow their body temperature to drop. They are then poikilothermic endotherms (although during the day they are homeothermic).

So, is this helping? Patience, we’ll get there. Where does the heat used by endotherms come from? From internal combustion, which we call metabolism. Metabolism basically refers to the chemical reactions cells use to break down glucose into water and carbon dioxide and in so doing generate ATP (adenosine triphosphate), a high-energy compound used to power other cellular processes.

All organisms metabolize food and other inputs, but some make better use of the output than others. Like all energy conversions metabolism is rather inefficient, and around 60% of the available energy is converted to heat rather than to ATP. In most organisms, this heat is simply lost to the environment. However, endothermic homeotherms (the animals generally characterized as “warm-blooded”) both produce more heat and have better ways to retain and regulate it than other animals. They have a higher basal metabolic rate, and also a greater capacity to increase their metabolic rate when engaged in strenuous activity. They usually have well-developed insulation in order to retain body heat, fur in the case of mammals and feathers in birds. When this insulation is insufficient to maintain body temperature, they may resort to shivering–rapid muscle contractions that quickly use up ATP, thus stimulating cellular metabolism to replace it and consequently produce more heat. In hot environments, they generally use evaporative cooling to shed excess heat, either by sweating (some mammals) or panting (many mammals and all birds)–mechanisms not generally present in poikilotherms.

Although calling birds and mammals “warm-blooded” is generally correct, referring to other animals as “cold-blooded” can be quite misleading. For example, although bees and butterflies are poikilothermic, they can generate quite a bit of heat in their flight muscles. In one of the more dramatic examples, a flying honeybee drone has a body temperature over 130 F, much higher than the normal human body temperature of about 99 F. Likewise, the desert lizards we mentioned earlier commonly have daytime body temperatures higher than those of the mammals that live in the same place (although the reptiles’ heat is obtained from the sun, rather than through metabolism). Obviously, calling these animals cold-blooded is inappropriate.

George and Doug

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