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

教授：好的，我们接着学习动物行为知识中动物如何做出决定的部分，今天我们将把重点放在那些比较很艰难的决定上。

Professor: OK, let's continue our discussion about animal behavior by talking about decisions that animals face, complex ones.

动物，甚至包括昆虫在内，都似乎展示出一种非常复杂的决策过程。

Animals, even insects, carry out what look like very complex decision-making processes.

不过，问题在于，这究竟是怎样一个过程呢？

我的意思是，没有人真的会觉得，比如，一只蜜蜂会权衡利弊，究竟是到这朵花上还是到那朵花上授粉比较好。

I mean, no one really thinks that, say, a bee goes through weighing the pros and cons of pollinating this flower or that flower.

但是另一方面，问题在于动物如何解决那些复杂的、似乎需要做出决定的问题？

But then, how do animals solve complex questions, questions that seem to require decision making?

我们当然希望这些问题的答案能与自然选择拉上关系。

The answer we'll propose, of course, is that their behavior is largely a matter of natural selection.

比如，我们来看看海狸寻找食物有关的案例吧！

As an example, let's look at foraging behavior among beavers.

海狸是素食类动物，主要吃植物，但同时他们又利用树木和树枝在溪流和湖泊中来建造自己的住所。

Beavers eat plants, mostly trees, and they also use trees and tree branches to construct their homes in streams and lakes.

当它们开始觅食和找材料建住所时,它们不得不离开自己的家去陆地上,那里会有它们主要的捕食者。

So when they do forage for food and for shelter materials, they have to leave their homes and go up on land, where their main predators are.

所以，它们在寻找食物时会面临很多选择。

So there are a number of choices that have to be made of foraging.

比如说，嗯，它们必须得知道哪些树是它们应该推倒的。

So, for example, uh, they need to decide what kind of tree they should cut down.

一些树木的营养价值高于其他，一些树木更适合用来做建筑材料，而还有一些则两种用途都适合……嗯，比如杨树。

Some trees have higher nutritional values than others, and some are better for building material, and some are good for both, um, aspen trees.

海狸们把杨树桩剥皮为食，用杨树枝修筑居所。

Beavers peel of the bark to eat and they also use the branches for building their shelters.

这样的话，杨树同时承担了两项任务。

But ash trees, beavers use ash trees only for construction.

另一个涉及到决定的问题是海狸们何时去找食。它们应该在更为温热的、需要消耗更多能量的白天出去觅食呢？

Another decision is when to forage for food, should they go out during the day time when it's hotter outside and they have to expend more energy?

抑或是天气冷一些、捕食者更为活跃的晚上去？

Or at night when the weather is cooler but predators are more active?

好的，还有两个更重要的问题，它们真的很关键，很重要，记住了吧？

OK, but there are two more important issues, really the most central, the most, uh, important, ok?

第一，我们假设当一棵大树有许多枝干，所以海狸能够从它身上获得的木材总量相当于三棵小树。

First, let's say a beaver could get the same amount of wood from a single large tree when it has lots of branches as it could get from three small trees.

如果它选择那棵大树，它就必须把大树上的大块头木材扛回窝里去，但把一大块木头移动40到50码是个不简单的活儿，会耗费大量的精力。

If it chooses one large tree, it'll have to carry that large piece of wood back home, and lugging a big piece of wood forty or fifty yards is hard work, takes a lot of energy.

但这项决定的好处是，它只需要搬一次就能把木头搬到水里。

Of course, it'll have to only make one trip to get the wood back to the water.

不过，如果它选择了小树并且获得等量的木材呢，它把木头搬回去时可以节省一些力气，但需要来来回回走三次。

On the other hand, if it goes for three small trees instead, it'll take less energy per tree to get the wood back home, but it'll have to make three trips back and forth for these three trees.

并且，从理论上说，它在住所之外的地方活动越得多，就越有可能被捕食者发现。

And presumably, the more often it wanders for home, the more it's likely to be exposed to predators.

所以，哪一个选择更好呢？一棵大一些的树还是三棵小一些的树？

So which is better: a single large tree or three small trees?

另一个关键问题与第一个树木大小问题有关，即它们会在距离水多远的地方获得木材？

Another critical issue, and it's related to the first, to the size issue, is how far from the water should it go to get trees?

在大树提供较多木头这一前提下，它们愿意走多远去找一棵大一些的树呢？

Should it be willing to travel a greater distance for a large tree since it'll get so much wood from it?

海狸们如果需要杨树的话，它们当然需要比获得灰树走得更远。

Beavers certainly go farther from the water to get an Aspen tree than for an Ash tree.

这种选择反映出了两种树木的相对价值，但木头的尺寸呢？

That reflects their relative values, but what about size?

它们会为了一棵大树走得更远吗？

Will it travel farther for a larger tree than it will for a smaller tree?

现在，我们当然会觉得如果树木越大，海狸为了获得这些树木的木材而进行远距离走动的意愿也就越大。

Now, I would've thought the bigger the tree, the farther the beaver would be willing to travel for it.

假设你们要走一段较长的路的话，你们会选择一棵与这段距离价值最为匹配的树木。

If you're going to travel far, make the trip worth it by bringing back the most wood possible.

但对于海狸而言，事实上，它们的选择恰恰与此相反。

海狸只会推倒距水较近的大树。

Beavers will cut down only large trees that are close to the water.

而跑到很远的地方去推倒一些较小的、能够很快拖回家的树木。

They'll travel far only to cut down certain small trees that they can cut down quickly and drag back home quickly.

一般而言，它们离家越远，它们推倒的树也就越小。

Generally, the farther they go from the water, the smaller the tree they'll cut down.

它们宁愿多走几次，每次只带回较少的木料，尽管这样会使它们面对捕食者的几率增加。

They're willing to make more trips to haul back less wood, which carries a greater risk of being exposed to predators.

这样看来，海狸在找食物时，与减小遭遇捕食者几率相比，它们更愿意保存能量，这也就解释了它们为什么会选择晚上出来觅食。

So, it looks as though beavers are less interested in minimizing their exposure to predators, and more interested in saving energy when foraging for wood, which may also explain why beavers forage primarily during the evenings.

好的，那么，为什么它们这样的行为表明了比起暴露在捕食者面前，它们更在意能量的保持呢？

OK, so why does their behavior indicate more of a concern with how much energy they expend than with being exposed to predators?

没有人会相信海狸有权衡这些因素利弊的能力。

No one believes a beaver consciously weighs the pros and cons of each of these elements.

有的人给出的答案是，它们的行为是与时俱进的。这种行为模式是由于时间的较大跨度这种束缚造成的。

The answer that some give is that their behavior has evolved over time, it's been shaped by constraint over vast stretches of time.

一切因素都指向了这样一个事实：海狸们的最佳觅食策略不是以最大食物量或木材量为最终目的的。

All of which comes down to the fact that the best foraging strategy for beavers isn't the one that yields the most food or wood.

而是以最大量繁衍后代这一结果为目的的。

It's the one that results in the most descendants, the most offspring.

我们接下来讨论这项结论是如何解释事实的。

What difference between aspen trees and ash trees does the professor point out?

What does the professor identify as the two central issues involved in beavers' foraging behavior?

What does the professor say about the cutting down of large trees?

According to the professor, why do beavers generally forage at night?

Narrator: 
Listen to a part of a lecture in a biology class. 
Professor 
OK. Let’s continue our discussion about animal behavior by talking about decisions that animals face, complex ones. Animals, even insects, carry out what look like very complex decision making processes. 
The question is how. I mean no one really thinks that, say a bee goes through weighing the pros and cons of pollinating this flower or that flower. But then how do animals solve complex questions, questions that seem to require decision making. The answer we’ll propose of course is that their behavior is largely a matter of natural selection. As an example, let’s look at foraging behavior among beavers. Beavers eat plants, mostly trees. And they also use trees and tree branches to construct their homes in streams and lakes. 
So when they do forage for food and for shelter materials, they have to leave their homes and go up on land where their main predators are. So there are a number of choices that have to be made about foraging. So for example, um... they need to decide what kind of tree they should cut down. Some trees have higher nutritional value than others, and some are better for building material, and some are good for both... um...aspen trees. 
Beavers peel off the bark to eat and they also use the branches for building their shelters. So aspens do double duty. But ash trees, beavers use ash trees only for construction. Another decision is when to forage for food. Should they go out during the daytime when it’s hotter outside and they have to expend more energy, or at night when the weather is cooler but predators are more active? 
Ok, but there are two more important issues, really the most central, the most important, OK? First, let’s say a beaver could get the same amount of wood from a single large tree, one that has lots of branches, as it could get from three small trees. Which should it choose? If it chooses one large tree, it’ll have to carry that large piece of wood back home, and lugging a big piece of wood 40 or 50 yards is hard work, takes a lot of energy. Of course it’ll have to make only one trip to get the wood back to the water. On the other hand, if it goes for three small trees instead, it will take less energy per tree to get the wood back home, but it’ll have to make three trips back and forth for the three trees. And presumably, the more often it wanders from home, the more it’s likely to be exposed to predators. So which is better, a single large tree or three small trees? 
Another critical issue and it’s related to the first, to the size issue, is how far from the water should it go to get trees. Should it be willing to travel a greater distance for a large tree, since it’ll get so much wood from it? Beavers certainly go farther from the water to get an aspen tree than for an ash tree. That reflects their relative values. But what about size? Will it travel farther for a larger tree than it will for a smaller tree? 
Now I would have thought the bigger the tree, the farther the beaver would be willing to travel for it. That would make sense, right? If you’re going to travel far, make the trip worth it buy bringing back most wood possible. But actually, the opposite is true. Beavers will cut down only large trees that are close to the water. They will travel far only to cut down certain small trees that they can cut down quickly and drag back home quickly. Generally, the farther they go from the water, the smaller the tree they will cut down. They’re willing to make more trips to haul back less wood, which carries a greater risk of being exposed to predators. So it looks as though beavers are less interested in minimizing their exposure to predators and more interested in saving energy when foraging for wood, which may also explain why beavers forage primarily during the evenings. 
Now I would have thought the bigger the tree, the farther the beaver would be willing to travel for it. That would make sense, right? If you’re going to travel far, make the trip worth it buy bringing back most wood possible. But actually, the opposite is true. Beavers will cut down only large trees that are close to the water. They will travel far only to cut down certain small trees that they can cut down quickly and drag back home quickly. Generally, the farther they go from the water, the smaller the tree they will cut down. They’re willing to make more trips to haul back less wood, which carries a greater risk of being exposed to predators. So it looks as though beavers are less interested in minimizing their exposure to predators and more interested in saving energy when foraging for wood, which may also explain why beavers forage primarily during the evenings. 
OK, so why does their behavior indicate more of a concern with how much energy they expend than with being exposed to predators? No one believes a beaver consciously weighs the pros and cons of each of these elements. The answer that some give is that their behavior has evolved over time. It’s been shaped by constraints over vast stretches of time, all of which comes down to the fact that the best foraging strategy for beavers isn’t the one that yields the most food or wood. It’s the one that results in the most descendants, the most offspring. So let’s discuss how this idea works. 
Now I would have thought the bigger the tree, the farther the beaver would be willing to travel for it. That would make sense, right? If you’re going to travel far, make the trip worth it buy bringing back most wood possible. But actually, the opposite is true. Beavers will cut down only large trees that are close to the water.