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.