There are several ways to examine the behaviors of organisms when they forage or hunt for food or mates. These behaviors become more complex in higher organisms, such as primates and whales, which can hunt in groups. Foragers and hunters have been shown to examine the marginal cost and marginal benefit of continuing an action and then adjust their behaviors accordingly. They are also able to handle risk by hoarding resources.
Krebs, John R. and Nicholas B. Davies. An Introduction to Behavioral Ecology, chapter 3
April 15, 2009
Professor Stephen Stearns: Okay, let's get going. We're into the last segment of the course. We did evolution, and then we did ecology, and now we're going to do behavior. I think the sequence does make sense, because evolution helps to explain how the things we deal with in ecology evolved, and it also explains how much of what we see in behavior evolved.
But I want to say at the outset that the behavioral ecology view of behavior--which is basically expressed on this slide; so behaviors evolved--the evolved patterns that we see in behavior should reflect things that happen frequently to the organisms in their environment, and the way animals behave should reflect the consequences of behavior for lifetime reproductive success.
That is really only part of the biology of behavior. If you really want to understand it at all levels, you have to understand how behavior evolved phylogentically; so you need a comparative view of behavior. You need to understand this issue, which is how is it that behavior is adaptive; is it, or is it a maladaptation?
But then you also need to understand how behavior develops; that is, if we follow the organism from zygote to death--you'll see some patterns of that today--how is it that organisms learn? How is behavior acquired? That's a whole field in and of itself.
And then finally we need to understand the mechanistic underpinnings of behavior. So in that respect there are a lot of different ways you can go at it. You can go at it through neurophysiology; you can go at it through endocrinology. There are many different kinds of mechanisms that are involved in triggering behavior patterns. So what we're going to concentrate on in this course is primarily the behavioral ecology approach to it, which is well exemplified in the book that you've got by Krebs and Davies.
But these other issues are also very interesting biology, and I'm just indicating that if you get interested in behavior, there are lots of ways you can go at it, and there are entirely different paradigms you can use to analyze it.
So the five themes that we'll approach--and these are the next five lectures; so this is a sketch of how the course finishes. Today we'll talk about foraging and hunting. Then next time we'll talk about evolutionary game theory, which is one of the major analytical frameworks within which people approach behavior. We'll have a look at mating systems and parental care, and they are connected in interesting ways. We'll take a look at alternative breeding strategies, which are frequency dependent breeding strategies, often best analyzed with evolutionary game theory. And then we'll close with the evolutionary and ecological analysis of selfishness, altruism and cooperation, in animals and in humans.
So those are the five themes that I have selected out of behavioral ecology to emphasize in this course. It's an introductory course and, you know, frankly it would be great to give you an entire semester just on behavior, because it's such an interesting topic. But I will signal that we do have other courses in the department on it, and if you get interested in them, they might be fun to take. Okay?
So I'm going to start with foraging by bringing back in something that you guys presented on that Friday, which is the marginal value theorem. And this time I'm going to apply it not to whether you should fill your plate up, your tray up, in the dining hall full, if you're going to the far end, or just with a little bit if you're going to be close to the counter, but to the issue of how long you should guard your mate, and indicate that, in fact, the same intellectual framework applies in both cases.
Then I'll give you an example where we can actually do a clever experiment to get the foraging organism to tell us what fitness measure it is using; and that's often a very satisfying kind of experiment to do, if you can get the animal, which cannot talk, to tell you what it thinks it's doing.
Then I'll illustrate how two different birds deal with risk. Because a small bird, at the end of a cold winter day, is exposed to extreme risk of dying overnight--and I can tell you this is quite real. Over the course of a normal Connecticut winter, I am often picking up the occasional house sparrow, or robin, or whatever, which has died next to my house because we've had a cold night; so that risk is real. Then I'll discuss a little bit how predators shape crypsis and conspicuousness.
But then the sort of--the thing that you'll probably remember a week from now is the part of the lecture that deals with why hunt in a group? And at that point I'll show some chimpanzees hunting. And I want to warn you that is not something where you want to be bringing- eating the food that you've brought into the room, or anticipating lunch, okay, because this is pretty gory stuff.
Okay, marginal value theorem. The important thing about the marginal value theorem first is that it's dealing with foraging in space. And it's assuming you're starting in one point, which would normally be your home, your nest, your refuge, your den, and you are going out to another point where you are looking for food. And you have options. You could either go to this place or you might go to some other place, when you go out to get food.
So you have to travel to get to that patch of food, and then you have to search in the patch, and then once you start getting food in the patch, you accumulate it--and this is a cumulative curve--in a way that expresses diminishing marginal returns. So the harder- the longer you're in the patch, the harder you have to look, basically because you've already eaten some of the stuff in the patch. Okay?
So there are a number of things to remember about this kind of diagram. One is the X axis is time. Two is it's split up into travel time and search time, and at the point that you start searching is where you draw your payoff curve here, because that's the point at which you're going to draw this cumulative payoff curve. The vertical axis is some kind of payoff, and it's assumed to have a relationship to fitness. Okay? So it's going to be food, or it could be mates.
And probably the clever thing, the most clever thing about this--and I well remember when Rick Charnov first drew this thing; he and I were grad students together, and he was in his office and he was drawing this thing on a piece of paper. So I saw it before it was published. The clever thing is the nice geometrical solution to the optimality problem.
And the way to think of that is this: here is the measurement of time, and the question is at what point should you stop searching in this patch and go on to another one? And if you imagine all the possible lines that you could draw, that fan out from this axis, it turns out that the one which is tangent to that curve has the highest slope. Okay?
So the slope--I mean, you can see that, just geometrically from looking at it. Okay, this is the line which is going to have the highest slope of all of the possible lines that you could draw that are within this envelope. And you can't go above this line. The reason you can't go above this line basically is that you're not getting any food above that line. This line is defining the rate at which you can conceivably accumulate food, just by the ecological constraints of that patch. And so this is the maximal point for the slope.
And then you ask yourself, what is the slope? Well the slope is â