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Narrator:Listen to a part of a lecture in a biology class.
獨白:聽下面一段生物學課上的演講。
Professor:OK. Let's continue our discussion about animal behavior by talking about decisions that animals face, complex ones.
教授:好的，我們接著學習動物行為知識中動物如何做出決定的部分，今天我們將把重點放在那些比較很艱難的決定上。
Animals, even insects, carry out what look like very complex decision making processes.
動物，甚至包括昆蟲在內，都似乎展示出一種非常復雜的決策過程。
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.
不過，問題在于，這究竟是怎樣一個過程呢？我的意思是，沒有人真的會覺得，比如，一只蜜蜂會權衡支持或者反對到那朵花上干活的各種因素。
But then how do animals solve complex questions, questions that seem to require decision making.
但是另一方面，問題在于動物如何解決那些復雜的、似乎需要做出決定的問題。
The answer we'll propose of course is that their behavior is largely a matter of natural selection.
我們當然希望這些問題的答案能與自然選擇拉上關系。
As an example, let's look at foraging behavior among beavers.
比如，我們來看看海貍的尋找食物有關的案例吧！
Beavers eat plants, mostly trees.
海貍是素食類動物，主要吃植物。
And they also use trees and tree branches to construct their homes in streams and lakes.
但同時他們又利用樹木和樹枝來建造自己的住所，
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.
依靠樹木高度遠眺陸地來躲避它們的主要獵食者。
So there are a number of choices that have to be made about foraging.
所以，它們在尋找食物時會面臨很多選擇。
So for example, um... they need to decide what kind of tree they should cut down.
比如說，嗯，它們必須得知道那些樹是它們應該推倒的。
Some trees have higher nutritional value than others, and some are better for building material, and some are good for both... um...aspen trees.
一些樹木的營養價值高于其他，一些樹木更適合用來做建筑材料，而還有一些則兩種用途都適合…嗯，比如楊樹。
Beavers peel off the bark to eat and they also use the branches for building their shelters.
海貍們把楊樹樁剝皮為食，用楊樹枝修筑居所。
So aspens do double duty.
這樣的話，楊樹同時承擔了兩項任務。
But ash trees, beavers use ash trees only for construction. Another decision is when to forage for food.
但是灰樹，只被用作建材。另一個涉及到決定的問題是海貍們何時去找食。
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?
它們應該在較為溫熱的白天，它們更富精力之時去呢，抑或是天氣冷一些、捕食者更為活躍的晚上去？
Ok, but there are two more important issues, really the most central, the most important, OK?
好的，還有兩個更重要的問題；它們真的很關鍵，很重要，記住了吧？
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.
第一，我們假設當一棵大樹有許多枝干，所以海貍能夠從它身上獲得的木材總量相當于三棵小樹，
Which should it choose?
這時，海貍會如何選擇？
If it chooses one large tree, it' 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.
如果它選擇那棵大樹，它就必須把大樹上的大塊頭木材扛回窩里去，但把一大塊木頭移動 40 到 50 碼是個不簡單的活兒，會耗費大量的精力。
Of course it'll have to make only one trip to get the wood back to the water.
但這項決定的好處是，它只需要搬一次就能把木頭搬到水里。
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.
不過，如果它選擇了小樹并且獲得等量的木材呢，它把木頭搬回去時可以節省一些力氣，但需要來來回回走三次。
And presumably, the more often it wanders from home, the more it's likely to be exposed to predators.
并且，從理論上說，它在住所之外的地方活動越得多，就越有可能被捕食者發現。
So which is better, a single large tree or three small trees?
所以，哪一個選擇更好呢？一棵大一些的樹還是三棵小一些的樹？
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.
另一個關鍵問題與第一個樹木大小問題有關，即它們會在距離水多遠的地方獲得木材？
Should it be willing to travel a greater distance for a large tree, since it'll get so much wood from it?
在大樹提供較多木頭這一前提下，它們愿意走多遠去找一棵大一些的樹呢？
Beavers certainly go farther from the water to get an aspen tree than for an ash tree.
海貍們如果需要楊樹的話，它們當然需要比獲得灰樹走得更遠。
That reflects their relative values.
這種選擇反映出了兩種樹木的相對價值。
But what about size?
但木頭的尺寸呢？
Will it travel farther for a larger tree than it will for a smaller tree?
它們會為了一棵大樹走得更遠嗎？
Now I would have thought the bigger the tree, the farther the beaver would be willing to travel for it.
現在，我們當然會覺得如果樹木越大，海貍為了獲得這些樹木的木材而進行遠距離走動的意愿也就越大。
That would make sense, right?
這的確能說通，是吧？
If you're going to travel far, make the trip worth it buy bringing back most wood possible.
假設你們要走一段較長的路的話，你們會選擇一棵與這段距離價值最為匹配的樹木。
But actually, the opposite is true.
但對于海貍而言，事實上，它們的選擇恰恰與此相反。
Beavers will cut down only large trees that are close to the water.
海貍只會推倒距水較近的大樹，
They will travel far only to cut down certain small trees that they can cut down quickly and drag back home quickly.
而跑到很遠的地方去推倒一些較小的、能夠很快拖回家的樹木。
Generally, the farther they go from the water, the smaller the tree they will cut down.
一般而言，它們離家越遠，它們推倒的樹也就越小。
They're willing to make more trips to haul back less wood, which carries a greater risk of being exposed to predators.
它們寧愿多走幾次，每次只帶回較少的木料，盡管這樣會使它們面對捕食者的幾率增加。
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.
這樣看來，海貍在找食物時，與減小遭遇捕食者幾率相比，它們更愿意保存能量，這也就解釋了它們為什么會選擇晚上出來覓食。
OK, so why does their behavior indicate more of a concern with how much energy they expend than with being exposed to predators?
好的，那么，為什么它們這樣的行為表明了比起暴露在捕食者面前，它們更在意能量的保持呢？
No one believes a beaver consciously weighs the pros and cons of each of these elements.
沒有人會相信海貍有權衡利弊的能力。
The answer that some give is that their behavior has evolved over time.
有的人給出的答案是，它們的行為是與時俱進的。
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.
這種行為模式是由于時間的較大跨度這種束縛造成的，一切因素都指向了這樣一個事實:海貍們的最佳覓食策略不是以最大食物量或木材量為最終目的的，
It's the one that results in the most descendants, the most offspring.
而是以最大量繁衍后代這一結果為目的的。
So let's discuss how this idea works.
我們接下來討論這項結論是如何解釋事實的。
