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Foraging Behavior Among Beavers

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What is the lecture mainly about?
  • A. Different foraging strategies among animals

  • B. Methods beavers use to gather building materials

  • C. Decisions beavers make about where to live

  • D. Choices beavers face when foraging

显示答案 正确答案: D

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

    FEMALE 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, uh, 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’s cooler but predators are more active?

    OK, but there are two more important issues—really the most central, the most, uh, 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’ll 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, 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’ve thought, the bigger the tree, the farther the beaver would be willing to travel for it.That’d make sense, right?If you’re going to travel far, make the trip worth it by bringing back the most wood possible.But actually, the opposite is true.Beavers will cut down only large trees that’re close to the water.They’ll 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’ll 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.

  • 旁白:听下面一段生物学课上的演讲。

    教授:好的,我们接着讨论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复杂的决定上。动物,甚至包括昆虫在内,似乎展示出一种非常复杂的决策过程。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。但是动物们如何解决那些复杂的、需要决策的问题呢?我们当然希望这些问题的答案是,他们的行为在很大程度上是自然选择的影响。比如,我们来看一个海狸觅食行为的案例吧!

    海狸是素食动物,主要吃树木。同时他们又利用树木和树枝在溪流和湖泊中建造自己的住所。所以当他们寻觅食物和居所材料时,他们需要离家到陆地上,而那里有许多它们的天敌。所以,它们在觅食时会面临很多选择。

    比如说,嗯,它们需要决定哪些树是需要的。一些树木的营养价值高于其它,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。海狸们把杨树桩剥皮为食,用杨树枝修窝。这样的话,杨树同时承担了两项任务。但是白蜡树,只被用作建材。

    另一个涉及到决策的问题是海狸们何时去觅食。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,还是应该在天气冷一些、但天敌活动更活跃的晚上去呢?

    好的,还有两个更重要的问题;它们真的很关键,很重要。好,首先,我们假设海狸从一棵有许多枝干的大树上获得的木材总量,与它从三棵小树上获得的木材总量相当。这时,海狸会如何选择?如果它选择一棵大树,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 码不是个简单活儿,会耗费大量的精力。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。不过,如果它选择了三棵小树来获得等量的木材呢,每次把木材搬回家都可以节省一些力气,但需要来来回回走三次。而且理论上来讲,离家越频繁,越有可能被天敌发现。所以,哪一个选择更好呢?搬一棵大树还是搬三棵小树呢?

    另一个与刚才提到的树木大小有关的关键问题是它们会在距离水多远的地方获得木材?在大树提供较多木头这一前提下,它们愿意走很远的距离搞到一棵大树么?海狸们如果要获得杨树的话,当然要比获得白蜡树走得更远。这反映出了两种树木的相对价值。那么木头的大小会有影响么?它们会为了一棵大树走得更远吗?

    现在我们可能会觉得树木越大,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这的确能说通,是吧?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。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。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。一般而言,它们离家越远,它们推倒的树也就越小。

    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天敌的几率增加。这样看来,海狸在觅食时对减少遭遇天敌这件事并不怎么感兴趣,它们觅食时更愿意保存能量。这也就解释了它们为什么主要选择晚上出来觅食。好的,那么,为什么它们这样的行为表明了比起暴露在天敌面前,它们更在意能量的保持呢?

    没人觉得海狸有权衡利弊的能力。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,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,而是以最大量繁衍后代这一结果为目的的。我们接下来讨论这项结论是如何解释事实的。

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  • 本题对应音频:
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    题型分类:主旨题

    音频定位:

    1. OK, let’s continue our discussion about animal behavior by talking about decisions that animals face …complex ones.

    2. As an example let’s look at foraging behavior among beavers.

    选项分析:lecture首句提到要通过动物决策来讨论动物行为,首段末句点明文章要讨论河狸觅食时需要作出的选择,与D对应。

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