Chameleons, which are ectotherms (which used to be called “cold-blooded”), conserve energy by using just their tongues to catch prey. (istockphoto)

When I was a kid, I was taught that the animal kingdom could be divided into two groups. Warm-blooded animals, such as mammals and birds, were able to maintain their body temperature regardless of the surroundings. Cold-blooded animals, such as reptiles, amphibians, insects, arachnids and fish, were not. So while cold-blooded animals did not always have “cold” blood, their body temperature could vary dramatically depending on the environment.

Scientists no longer use these terms because they don’t adequately describe the variations in temperature control found in nature.

Endotherms are animals that keep their body temperature stable as a result of their metabolism, a word for the chemical activity in their cells. Cells are like tiny machines that make chemicals for energy and growth. However, like all machines, they lose some energy as heat. Endotherms have developed systems involving fat, sweat glands, fur and feathers to retain heat or release it to the environment.

Ectotherms are animals that don’t have the ability to retain the heat generated by their metabolism. When it’s cold outside, the metabolism of ectotherms slows down, as does their ability to move. That’s why reptiles, butterflies and other ectotherms can be found “sunning themselves” in the morning. Doing this raises their body temperature and allows the chemical activity in their cells to speed up.

Muscles work better if they’re warm, so one advantage that endotherms enjoy is the ability to spring into action at a moment’s notice. This is important for animals that forage throughout the day as well as for predators that need stamina, or strength over a long period of time, to catch their prey.

Some animals, such as bumblebees, have characteristics of ectotherms and endotherms. (istockphoto)

Ectotherms usually feed during the day, when the warmth of the sun enables their muscles to function better. Nocturnal ectotherms and ones that live in colder regions commonly employ “wait and trap” techniques that don’t require much energy. For example, a chameleon uses very little energy while it sits waiting for an insect to get within striking distance of its sticky tongue.

One disadvantage of being an endotherm is that it takes a lot of energy to keep your body temperature steady regardless of the environment. That’s why mammals and birds need to eat frequently. Ectotherms, on the other hand, can go for long periods without eating. If there’s no food around, their metabolism can slow down because they don’t need the extra energy to maintain body temperature. (That’s why adult snakes can go months without eating.)

Although most endotherms appear “warm-blooded” and most ectotherms appear “cold-blooded,” some animals display characteristics of both groups. They are called heterotherms. Here are a few examples:

At rest, a bumblebee’s temperature varies, like that of a traditional ectotherm. However, worker bees can’t fly if their body temperature isn’t high enough. Before takeoff, the bees repeatedly flex their flying muscles. This generates heat in their upper body and enables them to fly.

Some species of bat and squirrel slow their metabolism when they’re resting. As a result, their body temperature, which is warm while active, can drop markedly. This is similar to what happens to endotherms that hibernate in cold weather.

Scientists recently discovered that the opah, a deep-water predatory fish, keeps its blood warmer than the surrounding water. It does this by rapidly flapping its pectoral fins and “saving” the heat generated by this muscle activity through specially designed blood vessels in its gills.

So, my dear endotherm (that means you): The next time you’re dying for a bedtime snack, you’ll understand why.

Bennett is a Washington pediatrician. His Web site,, includes past articles and other cool stuff.