Soaking Up the Sun

Imagine that you and a friend are taking a nature hike along a creek. It’s a sunny, spring Saturday morning. The warmth of the sun feels good on your skin, but the air is still chilly. Good thing you’re wearing a jacket.

At a bend in the creek, 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 red‑and‑blue skin shimmers in the sunlight. The creature is as still as a statue, and you wonder: Is it sleeping?

As if to answer, the lizard tilts its head toward you and blinks. Its tail sweeps back and forth slowly, but it doesn’t move off the rock. The lizard almost seems to be sunbathing.

That’s exactly what it’s doing!

That’s exactly what it’s doing!

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 and salamanders, 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.

Like the lizard in the forest, this desert lizard basks in the sun to warm itself up.

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

During the night, the lizard’s body temperature dropped. It became as cool as the surrounding air. A low body temperature makes an ectotherm’s muscles move slowly. The lizard became sluggish.

After about an hour of soaking up the morning sunshine, the lizard’s temperature will rise enough to give this leggy reptile the energy it needs to scurry from place to place. It will become fast enough to hunt and to escape predators.

Heat From the Inside

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

Unlike the lizard, these furry rodents have no need to soak up sunlight to get going in the morning. That’s because squirrels and other mammals are endotherms. An endotherm is an animal that produces enough 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. The inside of your body stays at the same warm temperature of about 37o C (98o F), whether you’re sleeping soundly in bed, playing outside on a hot day, or swimming in a cold lake.

Endotherms are sometimes called warm‑blooded. The term fits because an endotherm’s blood is always warm, like the rest of its internal body. Ectotherms are sometimes called cold‑blooded. This term is misleading, though, because as an ectotherm warms up, so does its blood.

What’s In a Name?

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

Ectotherm comes from the ancient Greek words ektos, which means “outside” and thermos, which means “warm.” An ectotherm must get warmth from outside its body.

Endotherm comes from the Greek word endo (“within”). An endotherm gets warmth from within its body.

See? Makes total sense.