NARRATOR：

Listen to part of a lecture in a biology class.

FEMALE PROFESSOR：

So, to review... who remembers how animals are classified, in terms of body temperature? Mike?

MALE STUDENT：

[hesitant] Um, endotherms and ectotherms?

FEMALE PROFESSOR：

Right. All animals are considered either endotherms or ectotherms.

"Therm"...that means "heat."

And the main thing that distinguishes endotherms from ectotherms is the source of body heat.

So an endotherm- "endo" meaning "internal"-an endotherm's body heat mainly comes from inside its body.

It can generate its own heat internally, with its metabolism.

And an ectotherm- "ecto" meaning "external"- an ectotherm gets its body heat mainly from outside its own body.

Meaning, from its environment. Mostly from the sun's radiation.

So we've got endotherms- mammals and birds are the classes that fall under this category-and ectotherms-that's pretty much everything else... including reptiles, amphibians, insects...

Now, body temperature is important.

And if an animal's environment gets very hot or very cold, something needs to happen in order for the animal to maintain its body temperature within its normal range.

In endotherms, this is mostly physiological-the body changes its rate of heat production.

OK, well... humans are endotherms.

What does your body automatically do when it gets really cold?

MALE STUDENT：

Shiver?

FEMALE PROFESSOR：

Right. Shivering-in fact, any muscle movement-increases metabolism, the process that produces heat and keeps your body temperature up when your surroundings get cold.

And then there's what's known as brown fat. Like other kinds of fat, it stores triglyceride.

But brown fat is unique because it chemically produces lots of heat even without muscle movement.

That's especially beneficial for small mammals in colder climates.

And when an animal gets too hot... well, have you ever seen a dog cooling off by taking short, quick breaths?

And humans... we sweat, of course... perspire, which also gets rid of body heat.

These are automatic physiological responses too.

Yes, Allie?

FEMALE STUDENT：

[checking comprehension] So in endotherms, it's not really about behavior... about doing things.

FEMALE PROFESSOR：

Well... a human, you know, might put on a winter coat, or jump in a swimming pool.

Or, elephants... Elephants might splash themselves with cold water when it's warm out.

But, for the most part, no... it's not what we endotherms do that keeps our temperatures within range.

Unlike ectotherms.

FEMALE STUDENT：

Well, what about ectotherms? Like... frogs, they must have metabolism too...

FEMALE PROFESSOR：

Sure they do. It's just that metabolism in ectotherms is so much lower.

I mean, the metabolic rate of an endotherm-say a mouse- is at least six or seven times that of an ectotherm of a similar size, like a frog or a lizard.

An ectotherm doesn't generate nearly as much heat internally, so its body temperature will tend to equalize with the temperature of its surroundings.

And that's where behavior comes in.

Imagine a lizard, OK? Living in the desert. Now, a desert gets very cold at night and very hot during the day.

So what's the lizard do to maintain its body temperature?

Well, on a cold morning, it can warm itself by going to a sunny spot and lying in the sun.

And later, if it gets too hot, it can seek out a cool place in the shade.

It's by means of such behavior that an ectotherm like this lizard regulates its temperature.

But you put that same lizard in a temperature-controlled chamber... and gradually drop the temperature, say, twenty degrees... and here, of course, the lizard can't go off to lie in the sun...

So, what happens?

MALE STUDENT：

Well, the lizard's body temperature drops too.

FEMALE PROFESSOR：

Right. And this really slows down its metabolism- which depends on temperature.

Even that twenty-degree drop in body temperature, though... the lizard can survive that, no problem, and come out just fine when it warms up again.

Ectotherms can do that.

MALE STUDENT：

But an ectotherm probably wouldn't survive in a place where the temperatures got too low, right?

FEMALE PROFESSOR：

[lightly affırming] Ever heard of a frog being chased by a polar bear?

MALE STUDENT：

[chuckle, understanding]No.

FEMALE PROFESSOR：

Well, there you are.

Now, a mouse in the same situation, in the same temperature chamber... it's just the opposite of the lizard.

When the temperature goes down, the mouse's metabolism goes up.

Like any endotherm, it starts producing more heat to keep its body temperature stable.

Same as it would do outside in a field.