听力原文： OK, can anyone answer this question? What are the most important things for animals in terms of their survival? Anyone? No? OK. Well, on a very basic level, the most important things for them are the same as the most important things for the average real estate agent: location, location, and location. That was a little bit of a ... uh ... joke, but there is also a lot of truth to it. Let me explain. Look, imagine you have a small area with a lot of different types of monkeys all living there. Thats a good example, actually, because trade between Diana and putty-nosed monkeys is the topic for today. The monkeys will arrange themselves into groups according to species. All the A monkeys are together and so are all the B monkeys. This allows them to work together to get food and protect themselves. But, the area is small, and this puts all of the groups into competition for food and space, which are both in limited supply. So, where each group is in that space is really important, because its related to how much food it can collect and how safe it will be. See what I mean? The most important things are location, location, and location. Right. Lets look specifically at the putty-nosed and Diana monkeys that live in a small area in the rainforests along the Ivory Coast in West Africa. They live in the same area and have a serious problem: both of them eat vegetation found high up in trees. This situation should make for a lot of small 'wars' between the groups. They are both in the same spot and they both want the same thing. What do they do? Well, with other monkeys, the Diana monkeys tend to establish borders. Basically, after a certain amount of fighting, the two groups stake out and patrol a group of trees for themselves. The advantage is that once these borders are established, everyone can spend their time finding food. Please dont think that the little countries dont allow some 'immigrants.' The Diana monkeys, for example, dont care about Campbell monkeys. They let them into their protected areas all the time. Why? Well, like I said, the Diana monkeys look for food high up in the trees, while the Campbell monkeys tend to forage for food closer to the forest floor. There is no competition between the groups, so there is no fighting. What about the putty-nosed monkeys? Lets review what we know. Diana monkeys will make borders with other groups that compete for the same food in the same area. But, the borders they make are porous; any group that eats a different type of food can cross into the Diana monkeys territory. What about our friends the putty-nosed? Well, I imagine youre all expecting some sort of answer like: they fight with the Diana monkeys. Well, youre wrong. There is no fighting between the two groups and no borders are established. Why? Its sort of the price they willingly pay for the service the putty-nosed monkeys provide. You could call it 'rainforest economics.' Between the two groups, a situation has developed where both groups are willing to give something they have in exchange for something they want. Look, when not worrying about other groups of monkeys, the Diana monkeys spend a lot of time trying to protect themselves from eagles by running away. Heres an important fact, though: Diana monkeys are not very good at knowing when to run. They have a hard time noticing the eagles, which can fly really quickly and silently. Putty-nosed monkeys, on the other hand, are really good at eagle detection. With better hearing and sight, they act like early warning radar. As soon as an eagle comes close, a putty-nosed monkey will let out a shrill yell that is heard and then repeated by other monkeys in its group. The Diana monkeys all run away to safety, with the putty-nosed monkeys staying where they are; eagles cannot eat the larger, stronger putty-nosed monkeys. There is no fighting between the two groups and no borders are established. Why? Its sort of the price they willingly pay for the service the putty-nosed monkeys provide. You could call it 'rainforest economics.' Between the two groups, a situation has developed where both groups are willing to give something they have in exchange for something they want. Why does the professor say this: You could call it 'rainforest economics.' What is the lecture mainly about?