Soaking Up the Sun

Imagine taking a nature hike with friends along a creek. It’s a sunny spring morning. The warmth of the sun feels good, but the air is chilly.

A large gray rock sticks out of the muddy bank. A flash of color catches your eye. You step closer and discover a lizard is lying on the rock. Its skin shimmers in the sun. It is as still as a statue. You wonder: Is it sleeping? The lizard tilts its head, but it doesn’t move off the rock. Why? The little lizard is sunbathing!

Energy from the sun helps this desert lizard dart and dash. 

Lizards, snakes, and other reptiles are ectotherms. Fish, insects, worms, and amphibians, like frogs, are ectotherms, too. An ectotherm is an animal that produces very little heat inside its body. To warm up, the animal has to get heat from its environment.

That’s why the lizard is sunbathing. The sunlight strikes its body, warming it. The rock absorbs heat from the sun, which warms the lizard’s underside.

During the night, the lizard’s body temperature drops. It becomes as cool as the air around it. A low body temperature makes an ectotherm’s muscles slow down.

After soaking up some morning sunshine, however, the lizard’s temperature rises. It will then have the energy to scurry around. It will become fast enough to hunt and to escape predators.

This desert lizard basks in the sun to warm itself.

Heat From the Inside

As you continue your hike, you hear a sound: Clickety, clickety, clickety. It’s the distinct sound of tiny claws on tree bark. Two squirrels are chasing each other around a tree trunk.

Unlike the lizard, these furry rodents don’t need sunlight to get going in the morning. That’s because squirrels and other mammals are endotherms.

An endotherm is an animal that produces enough body heat to keep its insides warm. The squirrels don’t have to wait on the sun. They can be active the moment they wake up. So can birds. They are endotherms, too. And so can you. 

That’s right! You are an endotherm. It doesn’t matter whether you’re playing outside on a hot day or swimming in a cold lake. The inside of your body stays warm at about 37oC (98oF). Endotherms are also called warm‑blooded. That’s because an endotherm’s blood is always warm. Ectotherms are called cold‑blooded. This term is misleading, though. As an ectotherm warms up, so does its blood.

What’s In a Name?

Ectotherm and endotherm may sound like strange words. Yet, they make sense when you break them down.

Ectotherm comes from the Greek words ektos, meaning “outside,” and thermos,meaning “warm.” An ectotherm gets warmth from outside its body.

Endotherm comes from the Greek word endo, meaning “within.” An endotherm gets warmth from inside its body.