Narrator: Listen to part of a lecture in a biology class.

旁白：请听一段生物学课的节选片段。

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

教授：我们先复习一下，谁还记得我们是怎么通过体温来划分动物的种类的？Mike？

Student: Um, endotherms and ectotherms?

学生：嗯，恒温动物和变温动物？

Professor: Right! All animals are considered either endotherms or ectotherms.

教授：对的！所有的动物都可以被划分成恒温的和变温的。

Therm- that means heat, and the main thing that distinguishes endotherms from ectotherms is the source of body heat.

这两个术语的意思是体温是不是热的，还有，区分恒温动物和变温动物的主要依据之一就是体温的来源。

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

所以一个恒温动物，恒温的意思是内部产热的，一个恒温动物的体温主要是来自身体内部。

It can generate its own heat internally with its metabolism.

它可以通过自己的新陈代谢产热。

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.

而一个变温动物，变温意味着体温来自于身体外部，一个变温动物从身体之外获得热量，也就是从环境里获得，大多数是从太阳辐射获得的。

So we've got endotherms.

我们再来看看恒温动物。

Mammals and birds are the classes that fall under this category.

哺乳动物和鸟类是恒温动物类别下的两种生物。

And ectotherms, that's pretty much everything else, including reptiles, amphibians, insects.

变温动物就是另一类了，包括爬虫类，两栖动物类，还有昆虫。

Now, body temperature is important.

所以体温很重要。

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.

如果动物所处的环境变得太热或者太冷，动物为了保持自己的体温在正常范围内，需要采取一些措施。

In endotherms, this is mostly physiological.

在恒温动物这边，这些措施都是生理性的。

The body changes its rate of heat production.

它们的身体会改变产热的速率。

Okay, well, humans are endotherms.

好的，那么，人类就是恒温动物。

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

当你觉得冷的时候，你的身体会做什么呢？

Student: Shiver?

学生：颤抖？

Professor: Right, shivering.

教授：对的，颤抖。

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

实际上，任何肌肉的动作都会增加新陈代谢，新陈代谢这个过程可以产热，在周围环境变冷的时候帮助你保持体温稳定。

And then there's what's known as brown fat, like other kinds of fat, it stores triiodothyronine, but brown fat is unique because it chemically produces lots of heat even without muscle movement.

有一种叫做褐色脂肪的东西，就像其他的脂肪一样，储存着三碘甲状腺氨酸，但是褐色脂肪很独特，因为它可以不需要肌肉运动也产生大量的热。

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

这就对于寒冷环境里的小哺乳动物很有用。

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

那么当一个动物很热的时候，你们见到过一只狗通过短促的呼吸来散热吗？

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

对于我们人类，当然就是出汗了，出汗可以帮助我们散热。

These are automatic, physiological responses too.

这也是自动的、生理上的反应。

Yes, Sally?

什么事，Sally？

Student: So, in endotherms it's really not about behavior, about doing things.

学生：所以，在恒温动物身上，调节体温这件事不仅仅是关于行为的，是关于做什么事情的。

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.

教授：一个人类，你知道，可能会穿上冬天的大衣或者跳进游泳池的水里，或者一只大象，可以在热爆了的时候拿凉水撒自己身上，但是对于自然界的大多数动物，这个不是一个普遍的解决办法。

It's not what we endotherms do that keeps our temperatures within range, unlike ectotherms.

变温动物用来把体温维持在正常范围内的手段和我们不同。

Student: Well, what about ectotherms, like frogs?

学生：关于变温动物，比如说青蛙？

They must have metabolism too.

他们肯定也有新陈代谢。

Professor: Sure they do.

教授：那是自然。

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

只是变温动物的新陈代谢速率太低了。

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.

我的意思是，一个恒温动物的新陈代谢速率，比如说一只老鼠的，至少是相同体型的青蛙或蜥蜴的六七倍。

An ectotherm doesn't generate nearly as much heat internally.

一只变温动物并不主要靠身体内部产热。

So its body temperature will tend to equalize with the temperature of its surroundings.

所以它们的体温一般是和环境温度持平的。

And that's where behavior comes in.

这就到了动物习性发挥作用的时候了。

Imagine a lizard, okay, living in the desert.

想象一只蜥蜴，住在沙漠里。

Now, a desert gets very cold at night and very hot during the day.

沙漠在夜间变得很冷，在白天变得很热。

So what does the lizard do to maintain its body temperature?

所以蜥蜴要怎么来维持体温呢？

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.

在寒冷的早晨，蜥蜴可以通过去有阳光的地方晒太阳来温暖自己，晒了一会儿之后，如果它觉得热了，它可以找一个树荫下的阴凉地呆着。

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

就是因为这种行为，像这种蜥蜴一样的变温动物可以做到调节自己的体温。

But you put that same lizard in a temperature controlled chamber and gradually drop the temperature, say, 20 degrees, and here of course, the lizard can't go off to lie in the sun.

但是如果你把同样一只蜥蜴放在一只可控温箱子里，逐渐降温的时候，假设降个20度，当然了，蜥蜴不会在太阳下被热死。

So what happens?

所以会发生什么呢？

Student: Well, the lizard's body temperature drops too.

学生：蜥蜴的体温也会下降。

Professor: Right, and this really slows down its metabolism, which depends on temperature.

教授：对的，这就降低了它的新陈代谢速率，新陈代谢速率是基于温度的。

Even that 20 degrees drop in body temperature though, the lizard can survive that no problem, and come out just fine when it warms up again.

即使降低了20度的气温，蜥蜴也可以活下去，当气温慢慢暖和过来时，蜥蜴还是活的很好的。

Ectotherms can do that.

变温动物就可以做到。

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

学生：但是一个变温动物不大可能在气温非常低的地方生存下来，对吗？

Professor: Ever heard a frog being chased by a polar bear?

教授：没听说过一只青蛙被北极熊追吧？

Student: No.

学生：没有。

Professor: Well, there you are.

教授：这就对了。

Now a mouse in the same situation, in the same temperature chamber, is just the opposite of the lizard.

假设现在是一只老鼠在相同的境地下，在相同的可控温箱子里，和蜥蜴的情况相反。

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

当气温下降的时候，老鼠的新陈代谢变快了。

Like any endotherm, it starts producing more heat to keep its body temperature stable, same as it would do outside in a field.

就像是任何的恒温动物一样，它开始产更多的热量，保证体温不下降，就像是老鼠在自然环境中所做的一样。