托福official67听力lecture1 Animal Behavior Patterns原文解析+翻译音频

[00:00.00]Professor: Now we've said that animal behavior patterns involve an interplay between an animal and its environment. [00:11.16]In particular, behavior patterns can change as an animal becomes adapted to its environment. [00:16.88]We can see the results of this in the way that animals have developed different types of feeding behavior. [00:22.20]Animals are generally classified as a generalist feeders and specialist feeders. [00:29.21]Now, um, these terms are used in a variety of ways. [00:33.36]At times, they're used to refer to an animal's range of habitats. [00:37.88]I've also seen them used to describe the number of foraging techniques that are used by an individual or species.
[00:43.80]Um, but for our purposes, we're going to use them to refer to the range of food items that are eaten by a species. [00:51.86]So a generalist feeder is a species that eats a wide variety of foods during its lifetime. [00:57.81]And a specialist is a species that feeds on a highly restricted number of foods. [01:03.27]A good example of a generalist would be a rat. [01:06.58]Rats are notorious for eating just about anything that's available, whether it's a plant or an animal. [01:12.29]At the other end of the spectrum, we have the koala which eats only the leaves of eucalyptus trees.
[01:19.06]Okay, now there are advantages to each. [01:22.49]For a generalist, If 1 type of food isn't around, it can feed on another that's more abundant. [01:28.38]On the other hand, the advantage of being a specialist is that the animal is well adapted to its food. [01:34.58]You could say that specialists are feeding masters, their consumption, even their digestive system is just so finely tuned. For instance, take the koala. [01:45.40]The eucalyptus leaves that it feeds on, don't contain much protein. [01:49.48]What's more, the leaves contain compounds that are poisonous to most species? [01:54.46]But the koalas digestive system has adapted to handle this. [01:58.46]It's liver deactivates, the poisonous compounds, and its digestive system is designed to extract the maximum amount of nutrients from the leaves.
[02:07.66]So it's not a question of which strategy is better. [02:11.60]The real question is, what determines how selective animal species are in what they eat? [02:18.02]Well, the answer may lie in a concept called optimal foraging. [02:22.89]The idea is that natural selection favors animals whose feeding behavior has the lowest cost to benefit ratio. [02:31.54]That is you want to get the best healthiest food you can. [02:35.55]And you want to do it as quickly as possible. And as easily as possible.
[02:39.88]Now, in a natural environment, no animal forage in an absolutely optimal manner. [02:46.16]But, well, I read some research on a little rat called the kangaroo rat, which lives in deserts in North America. [02:53.20]The kangaroo rat is a generalist which eats plant matter like seeds, leaves, and fruit. [03:00.05]It lives in deep burrows which provide protection from the desert heat and from predators like hawks or snakes. [03:06.34]And it goes foraging at night for food.
[03:08.69]For plant matter, under the protection of darkness, even though it's a generalist. [03:14.44]If the only thing available is seed, if a choice of seeds is available to it, it picks up seeds that provide more energy than those it leaves behind. [03:23.56]And it carries them back to its burrow. [03:25.77]Once it's there, it might eat only the richest seeds from the ones that brought back. [03:31.39]So it selects seeds outside in a way that lets it spend as little time as possible foraging in that risky environment, and then sorts out its food later when it's safely in its burrow.
[03:43.36]Oh, yes, Kenneth. You have a question?
[03:46.32]Male student: But what about the koala? It gets by uh, by being even more selective, you'd think you would be really risky to rely completely on eucalyptus leaves. [03:55.88]Wouldn't it be better to diversify like the kangaroo rat, and eat, at least one or two other things?
[04:01.35]Professor: Well, what do you think the koala would say? What do I gain by being so selective? [04:08.84]In terms of optimal foraging, it's clear that eucalyptus trees provide an abundant source of food. [04:15.23]Other animals can't eat the leaves and think of how little energy the koala uses. [04:20.80]It spends only about 3 hours each day eating to get the energy it needs. [04:25.81]So being a specialist allows it to get the nutrients it needs with minimal expenditure of energy.