Most organisms depend upon maintaining a consistent temperature, regardless of the external temperature surrounding them. This process can be accomplished either by changing behavior to become cooler or warmer and keep temperature levels consistent, or through internal processes of the organism. Sometimes both behavior and internal processes of a complex organism work in concert to maintain temperature. The ability to maintain temperature is called thermoregulation.

In humans, our ability to maintain body temperature is mostly internally regulated. If we get too hot, our body starts to produce sweat, a natural cooling device. Shivering is actually a beneficial process when our bodies get too cold, because it works to heat the body.

Humans and many other mammals also change their behavior in order to keep body temperature consistent. This is especially the case in extreme hot or cold temperatures. For example, a person swimming in a cold pool may start shivering. He then uses behavioral thermoregulation by getting out of the pool, wrapping himself in a towel and sitting in the sun. This hastens the process of getting warm, something that would not be so easily achieved by remaining in the pool.

In very warm weather, humans also tend to employ behavioral thermoregulation by using devices like air conditioners, wearing less clothing, and taking cool showers, baths or swims. This form of behavioral thermoregulation allows us to feel cooler. Unless the temperature is extremely high, our body temperature is not likely to rise more than a few tenths of a degree. Our bodies employ physiological thermoregulation in order to keep our body temperature consistent. At room temperature, our body temperatures display little variance, even though normal room temperature is much lower than human body temperature.

Some animals rely strictly on behavioral thermoregulation in order to keep their body temperatures consistent. These are often called “cold-blooded” organisms, or more appropriately, ectotherms. They maintain body temperature by relying on external sources of heat or cold, and do not possess physiological thermoregulation, as do mammals and birds.

The average lizard, for example, will use the sun’s heat in order to keep body temperatures consistent. However, the lizard must learn to recognize when the sun’s heat is too hot, and get into the shade, or underground so as not to overheat. Further, when the lizard becomes too cold, he will then find a heat source again, in order to keep his body temperature stable.

Some creatures have adapted by being able to function at very different temperature levels. Poikilotherms do not have thermoregulation in the traditional sense. Instead, their body temperatures stay consistent with ambient temperature, and still function.

Some animals with extremely high metabolisms are also able to slow down their metabolism when ambient temperatures are extremely cold. Bears that have this type of metabolism and hibernate are said to have bradymetabolism. They are essentially both warm-blooded and cold-blooded, at different times of the year. They differ from the traditional definition of thermoregulation in this sense, though warming and cooling processes are controlled by their bodies’ internal mechanisms.