EEB 122: Principles of Evolution, Ecology and Behavior
|Transcript||Audio||Low Bandwidth Video||High Bandwidth Video|
Principles of Evolution, Ecology and Behavior
E&EB 122 - Lecture 32 - Economic Decisions for the Foraging Individual
Chapter 1. Introduction [00:00:00]
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.
Chapter 2. The Marginal Value Theorem [00:05:30]
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 ∆y/∆x. Right? ∆y is the change that you get, or the amount of payoff you get per amount of time you spend searching; so it’s the payoff per unit time. So drawing a line that way, as a tangent, maximizes the payoff per unit time. You don’t have to write down any equation; it’s just geometry. Okay? And that actually is the cool thing about the marginal value theorem.
Now I want you to imagine you are walking through a field in the Alps, and there are some cows grazing in the field, and you look off to the side and you see two biologists down on the ground, looking at a cow-pie, and you wonder what the hell is going on; why do they have their noses down in a pile of cow dung? And the answer is they are looking at this guy. Okay? And one of the biologists is Geoff Parker and the other one is me; because I had this happen to me with Geoff in the Alps. Okay?
So a cow–let me go back–a cow has come along and it has dropped a pile of dung here, and it has been rapidly colonized by a bunch of dung flies, and because of that, within about a minute, a male will find a female and they’ll go into copulation.
And then the issue is how long should the male stay on that female before he jumps off and goes to find another one? You can measure how many of the offspring of that female will get fertilized if he stays on. And this is the proportion of eggs that he will fertilize, if he stays on. Okay?
So it’s starting to look pretty much like a marginal value theorem problem. You can set up the analysis as search and guard time, plus time spent in copula. So this is his- how long it takes him to search and guard a female, and this is how long he spends in copula on her, and this is his payoff.
And now look at what the Y axis is. It’s directly fitness; that’s a direct fitness payoff. So actually this is the purest form of the biological implementation of marginal value theorem; finding a mate and fertilizing some eggs.
Now the interesting thing is that he jumps off about ten minutes earlier than predicted. Any ideas on why he might jump off ten minutes earlier than predicted? It’s actually a method of risk minimizing or spreading of risk. He cannot predict when the next fresh cow-pie is going to hit the ground, and that’s going to be the next open resource which gets colonized by females, and he needs some time to find it, because he wants to be the first one there. If he can be the first one there, he’ll get the best female. So he has to jump off this female a little bit earlier than this simple analysis would predict, just to hedge against the problem of trying to be first to the next cow-pie.
This problem is actually almost quantifiable. Of course you are measuring some rather strange things. You’re sitting there with your stopwatch watching as the cow-pies arrive on the surface of the pasture, right? But it’s a completely analyzable problem.
Chapter 3. Inferring the Fitness Measure Used by Foragers [00:12:05]
Now, so that’s the marginal value theorem applied to mate guarding. Now let’s look at an experiment that you can do with honeybees. And this was designed by Paul Schmid-Hempel, who is a very clever Swiss biologist. And what Paul did was he built a model in which he could predict how long bees would spend flying between flowers, and how many flowers they would visit before they came back to the hive. Okay?
And he had two different measures that they might be using. So this is the optimal relationship between how long it is flying between flowers and how many flowers you visit before you fly back to the hive. In one model he had calories per minute, which is the usual rate. Okay? In the marginal value theorem, when you maximize that slope, if you’re measuring that payoff curve in calories what you’re measuring then is maximal calories per minute, in the marginal value theorem.
Paul thought well maybe they have another fitness measure. They might be using calories gained per calories spent. So he built the two models, and he had quite different predictions, and then he manipulated them by gluing a little wire onto their back and adding a tiny little weight. So he made them–you know, he had a series of treatments where they weren’t weighted and then they had a little weight and then they had a lot of weight.
And this is what they did. This is the data, okay? So it really looks like he was able to get them to tell him which fitness measure they were using while they were foraging. That’s a very, very clever experiment. And this is the kind of thing that you can do in behavioral ecology. It’s possible to construct situations in which the decisions that the animals are making are so precisely constrained that you can get them to give you an answer.
I am waving over all the details in the mathematical models. Okay? That’s graduate student stuff. But I think the essential point is that you can do an experiment that will get you to tell you what an animal is actually using as a fitness measure.
Chapter 4. Dealing with Risk [00:14:18]
Okay, now two comments on the problem of how to deal with risk; and this is the small bird in winter problem. And this was a nice experiment. This is a Great Tit, which is a European form of chickadee, and this is an experiment which is done in an aviary, and this is a variable environment. Okay?
So when the experiment starts, the food supply starts becoming unpredictable in time; and that’s just a manipulation that the experimenter is imposing on the animal. And as the food starts getting unpredictable, the bird starts getting fat. That tells you a couple of interesting things right there. It says normally the bird doesn’t like to be that fat, but it’s going to get fat because it sees that its food supply is getting very unpredictable.
And then the way that you build one control into this experiment is then to switch it at this point into a constant food supply so that the environment just becomes nice and predictable and the bird relaxes, shrugs its shoulders and says, “Oh, I can get away with getting back to a normal weight,” and drops its weight. So that’s one way of dealing with it.
But there’s another way of dealing with it, and that is that if you look at this other relative of the Great Tit–okay, this is a Marsh Tit; it also looks a bit like a Coal Tit; it’s a little bit smaller, has a black head–and you put it in a high variance environment or a low variance environment, this one, it doesn’t change its weight at all. It just packs on as much weight as it can, by evening.
And by the way, at that latitude it’s getting dark at 4:00 in the afternoon in the winter. So it’s going up to just about peak weight. And you can see how much it’s losing by the next morning. What it does though is it stores seeds, and if you put it in a high variance environment, it greatly increases the number of seeds that it stores.
So one of these species–they’re very closely related by the way; it’s interesting that there doesn’t seem to be much phylogenetic component to this; one of them decided to pack it on as body mass that it carries around with itself, and the other one decided that it was going to store seeds. It may have something to do with the risks of predation. Big, fat birds don’t get away from predators quite as easily as nice slender little birds.
Chapter 5. How Predators Shape Crypsis and Conspicuousness [00:16:58]
Now what are some of the consequences of predatory behavior for the prey? Well one of them is so-called aposematic coloration; and that is that if you’re carrying around something that is going to poison your predator, you want your predator to know that.
You don’t want–you know, if you put a bunch of ham sandwiches out in Saybrook Commons, and you’ve got cyanide in five of them, and it’s in the interest of those five cyanide-laced ham sandwiches not to be eaten, then you want a big warning label on it that says, “Do not eat.” Right? Well that’s what these are, these warning colorations.
If you go out and you pick up a warningly colored millipede, and you shake it in your hand, it will smell like bitter almond. And it’s perfectly safe, by the way. There’s not enough cyanide in it to get hurt by sniffing it. So you’re perfectly safe picking up and shaking a millipede, except you may feel a little moral compromise at the act of shaking an invertebrate nervous system. Okay? But they will emit cyanide all over your hand, and it does smell like bitter almond.
And of course the monarch butterfly caterpillar, which gets its cardiac glycosides from milkweed, will cause tachycardia in the birds that eat it. Tachycardia is a condition where your heart jumps say from a pulse rate of 80, in a human–let’s say if you’ve been exercising a little bit you might have your pulse at 80 or 100 or something like that; with tachycardia you go up to 250 or 300 and you pass out, because your heart starts fluttering and it can’t pump anymore.
So that’s what eating that will do to a Blue Jay; it will have cardiac arrest. So it would take a few of them to do that to you. So I suggest that if you want to try this one, don’t eat more than one. Okay? If you really want to get into it, you’re probably getting threatened if you eat maybe five or six of them, or make a milkshake out of them or something like that.
One can do experiments with this kind of thing as well. This is a situation in which chicks, just regular domestic chicks, were given different colored baits. Okay? And in both of these situations they were given seeds that had been stained green or blue, and they had been soaked in quinine; and chicks do not like seeds that are soaked in quinine. So they were both distasteful. The only difference here is that in one case the green and blue seeds are on a green background, and here they are on a blue background.
And what you can see is that the ones that match the background continue to get eaten, and the ones that stand out from the background start getting avoided. Okay? So there’s a bit of learning; oh, I don’t like to taste these things. But then they avoid the ones that they see most easily. And that’s what’s going on over here. It’s this process that has produced these colors.
Of course it can go in the other direction. If you, in fact, are not distasteful, and you want to avoid being eaten, then often natural selection will change your morphology in such a way that it’s rather difficult to see what you are. Okay? The head is here; that’s the end of the right wing, that’s the end of the left wing; there’s a nice wing vein running down the middle.
Some of these things are just remarkably precise in resembling a dead leaf or other things. I think some of the ones that I like the most are the praying mantises that look like flower petals, and sit on flowers, and grab things when they come in; they’re very nasty. Here’s another one that’s very nasty; same kind of thing, okay. So this is aggressive mimicry. These are the light signals which are given out by different species of so-called fireflies–in fact, these are beetles–and they have a light organ, and you can see that there’s a species specific signal pattern.
Now normally you would think, okay fine, they’re just dividing up the frequencies. They don’t need to have an FCC to regulate which frequency they use. They’re flying through the night. They see a signal over there and they can say, “Oh, that is another one of my species, I’ll go check it out and potentially mate with it.” So you might think, oh, that’s all just normal mate behavior.
However, these are males that are flickering, and then they get a response from a female. Okay? So this would be male-male, female, male-male, female kind of thing. And so there are some that mimic the light signals of another species; some females that are sitting there saying–they’re faking it. Okay? A male blinks, the female can see, oh, that’s not a male of my species, therefore I can safely eat him. So she goes blink, with the signal of the other species, he flies in, and she chews him up. In so doing she gets two things. She gets calories out of him, but in some cases she also is absorbing a defensive chemical that will protect her from birds, bats and spiders. So she gets a double dose. She gets both calories and she gets defense from doing this.
This kind of aggressive mimicry is reasonably widespread, and it has been evolved convergently a number of times. The one that always got me as a kid is the saber-tooth blenny. You know about cleaning wrasses that come into the mouths of big fish and clean the parasites off of them and so forth, and the big fish have evolved, because this is a beneficial thing, to kind of relax. And so you will see giant groupers and barracudas opening their mouths and letting these little fish swim through them and clean off their teeth.
Well, there is another fish called the saber-tooth blenny that mimics both the color and the approach behavior of the cleaning wrasse; which is, by the way, a sigmoidal dance. It goes through the water like this. And so you see one of these things coming up and you kind of relax and say, “Oh, it’s just a cleaning wrasse, it’s going to be fine.”
And it comes up, and if it’s a human, it takes a chunk out of your thigh, and if it’s a fish that has its mouth open, it rips out a chunk of gill and goes running off. So this is the sort of thing that led Darwin to think that evolution just fills the world up with things that are taking advantage of every possible opportunity; and aggressive mimicry is a good example of this.
One of the interesting issues with predation and parasitism and mimicry has to do with the cuckoo. And I want to mention it because it really is an interesting series of puzzles. Okay? So here’s a cuckoo. Cuckoos, by the way, feed on caterpillar larvae, and so they tend to disappear from places where lots of insecticides are used. So they’re kind of a canary in a coalmine. If there have been cuckoos on the landscape and you can’t hear them anymore, it means that intensive agricultural practice has probably wiped out the entire large fauna of caterpillars; so they like to eat caterpillars.
And what they do is they go around and they find a nest, like a robin’s nest here, and they lay their own egg into the nest. And they have their whole developmental program set up in such a way that their baby will hatch earlier than the babies of the host species.
And its behavior is set up in such a way that the first thing that it does is–you know, it’s just a tiny little baby bird, and it’s just hatched out, but it has enough muscular coordination and enough behavioral complexity to take the other eggs and shove them out of the nest onto the ground, so it’s the only one left. And then it sits there.
And it’s got a very effective feeding behavior. It opens its mouth, it gives all of the morphological and behavioral cues that say, “Feed me, feed me, feed me.” And the parents work really hard, the parents of the other species work really hard to come in and feed it, and you get a baby cuckoo out of the nest, instead of a warbler or whatever. Okay?
Well why don’t the hosts throw out the cuckoo egg? There they are. They’re good parents. They come back to the nest. There’s an egg in it that they haven’t laid. Sometimes it looks quite a bit like their egg, sometimes it doesn’t; it all depends on which particular host species it is and how close that particular race of cuckoo is to matching the host species color.
Well there are really, I think, two reasons, but they may not be quantitatively sufficient to explain everything we see. One is the source-sink distinction. The hosts can’t adapt as fast as the cuckoo because the parasitized nests are sinks for the host, and unparasitized nests are sources for the hosts.
Most of the sources are coming out of nests that have not had cuckoos in them. If a cuckoo has gotten into the nest, it’s wiped out that reproduction. Okay? So the adaptation is to the source, which is to the condition without cuckoos, and not to the sink.
But there’s another issue, and that is if you’re just starting to evolve the behavior of throwing eggs out of your nest, and you’re not very good at it yet, you can make a serious mistake by killing one of your own kids. So there’s kind of a threshold there that you have to get over. You have to actually–this is a behavior where you actually have to be accurate and pretty good at it, before it pays off. Before you get good at it, you are indulging in some very costly behavior. Okay? So that’s another reason.
Another reason that we don’t see the hosts throwing the cuckoo egg out is that the cuckoos may be moving on to new hosts. So it may be that the cuckoos for say a hundred years parasitized robins, and the robins may slowly–slowly, because of these reasons–start to evolve a response to cuckoos, at which point the cuckoos just switch over and start parasitizing warblers.
And they do that for awhile, and they just keep moving around among the different species in their landscape, so that they’re always able to stay ahead, because their evolution is a little bit faster than that of the hosts. So that process is hard to observe.
And the egg mimicry isn’t very precise. You can see here, this isn’t a very good match. This is one kind of cuckoo egg. There are some that are a bit closer to robins. This might be appropriate for another kind of species. The egg mimicry isn’t very precise, and it is still kind of puzzling why the hosts don’t throw out more cuckoo eggs. It’s an interesting problem.
Chapter 6. Hunting in a Group [00:28:09]
Now I’d like to talk about hunting in a group. And this is a situation that is interesting, both because we can quantify the benefits of foraging styles, and we can see whether or not animals are actually doing what is quantitatively best for them. But it also addresses the whole issue of why animals should exist in groups. And since we are a group living and hunting primate, this is a very interesting thing for us to contemplate.
Now when an individual joins a group, it’s making a pretty fundamental decision. It’s basically deciding that the payoff it’s going to get, from the coordination of group hunting, is going to more than compensate for the fact it’s going to have to share the food; unless it’s an extremely confident dominant type, it’s going to have to share the food. Okay?
So what you see in wolves, coyotes, African hunting dogs and hyenas is that all of these things will hunt alone for little things and they’ll hunt together for big things. So go up to Ellesmere Island in the Arctic, pop yourself down onto a wolf study site, and go out and look at the wolves hunting. And if they are hunting for voles and mice, which are about this big, you’ll see individual wolves moving about the landscape, trapping them with their paws and munching them up.
However, if they decide to tackle something like a muskox, they will form up into a pack and go into coordinated behavior, with a division of labor and assigned roles, to bring down the muskox. And the muskox, of course, have a counter-adaptation, which is a group adaptation, and they form a protective circle, and they all face outward towards the wolves, and they defend themselves with their horns.
Similarly for coyotes, African hunting dogs and hyenas. With African hunting dogs, they will be going after usually small rodents or birds or things like that, on the ground, but they’re actually capable of bringing down a zebra. This is a dog which is this big. You know? A zebra is a horse, which is this big.
And five or six African hunting dogs, each of which weighs say about 75 pounds maybe, at max–50 to 75 pounds–can bring down a 500 pound zebra. So the interesting thing is that they switch facultatively from solitary hunting to group hunting, as group size- as the size of prey increases.
Now in chimpanzees–I’m going to show you a bit of the gory detail of chimpanzees in a minute. But before I do that I just want to show you some of the kinds of things that these capture. So we have wolves and coyotes and hyenas and African hunting dogs, and these are pictures where they have done a sophisticated, coordinated group hunt to bring down a big piece of meat. Now let’s see what chimpanzees do.
[Professor Stearns plays a video]
Professor Stephen Stearns: That’s Brutus. Brutus was born in 1952, August.
[Professor Stearns plays the video]
Professor Stephen Stearns: That’s an important point. They use seasoning, a little spice from the weeds.
[Professor Stearns plays the video]
Professor Stephen Stearns: So that’s in Christophe Boesch’s study site, which is in Tai National Park in the Ivory Coast. Those pictures were taken in about 1990, 1991. I had been there in February of 1989, and I had been out on a hunt with that same group. I had had lunch, which was by the way fruit, with them. I ate the same stuff they did, but I was eating fruit, not colobus monkeys.
Those guys are all dead now, the chimps. They died of Ebola and they died of poaching. However, a smaller group has replaced them. Of all of the chimps that were in that group in 1990, there is only one female who is still alive. But the group is back to about oh fifteen or twenty; at that point there were sixty in it.
So let’s look at an analysis of why they choose to hunt in a group. Okay? So the increase must more than balance the cost: sharing. They do it during the rainy season. During the dry season they crack nuts. So these chimps actually have a culture where they teach their offspring how to crack nuts with a hammer and an anvil. This is true west of a certain river in the Ivory Coast and it’s not true in the rest of Africa. And you can see that they hunt a lot more frequently in September and October. And I was actually there and saw the hunt in February; so they weren’t so frequent in February, I was lucky to see one. But these chimps hunt a lot.
And if you look at the hunting success as a function of the numbers in the hunting party–you can see on the top here a comparison of solitary and group hunts in the Ivory Coast in Gombe and at Mahale. So Gombe is Jane Goodall’s study site in Tanzania, Tai is in the Ivory Coast, and Mahaleis a Japanese study site. And you can see that they hunt quite a bit in Tai, and they do a lot more group hunting there.
And if you look at the impact of group size on capture success, you can see that the more chimps that are hunting in the group, the more likely it is they are to make a kill, the longer the hunt lasts, and the greater the degree of collaboration during the hunt. So this is team behavior, where individuals have roles, and they learn to play a team role, and they learn to do what is good for the team so that the team will have greater success.
If we look at success as a function of group size, you can see that the net–and by the way, this is now measured in net benefit in calories, and in order to calculate that, you have to be able to estimate how many calories a chimpanzee is putting out, if it’s running along the ground or climbing a tree or something like that. And so this is taken from exercise physiology, the estimates are taken from exercise physiology. And then you can figure out how many calories are there in a colobus monkey of a given size.
And it turns out that the right number to have–if you are a hunter you do better up until you get to a group size of five; after that you have to share with too many others and the payoff isn’t so great. If you are a bystander, it’s pretty much the same, the curve mimics that. And if you are a latecomer who’s coming in, then normally you don’t get too much of what is caught.
So in this particular hunt, Brutus led the hunt and he made the capture. So that was the chimp that you saw going up as the blocker; that was Brutus. Brutus, by the way, was the oldest male in the group, and he also had a couple of strategic innovations.
Brutus had figured out that in competition with neighboring groups–and chimps do have wars with neighboring groups–if you were in a state of being the weaker group and you were being confronted by a big one, what you would do is you would go over and you would display at the border.
They would come in, and then you would quiet down–and he got them all to quiet down–and the pack of males would run around the back and steal a female from the back of the other group. And this caused such chaos and disarray that they would normally be able to overpower a larger neighboring group by using this sneaking behavior. Just think of how much strategic thinking it requires to figure something like that out. And that was Brutus; he was a smart guy. And there he is right there.
So Macho was contending with Brutus to be an alpha male–there are two or three different ways you can define group hierarchy in chimps–and Macho was trying to be group leader. And he was actually a very collaborative hunter. It’s one way to sort of weasel your way into power is to collaborate a lot. And Rousseau never made it. And Rousseau actually got attacked by a leopard once, who managed to rip off half of his scrotum, and Rousseau survived that; pretty amazing.
So this is the function–it’s sort of a hill-shaped or hump-shaped function of success versus group size, with the best group size being around five. And then if you look at who shares in the capture, what you see is a breakdown where there is considerable sharing which is going on. And you can see that bystanders, who are often females, are eating meat often after a capture for nearly half an hour, and the captor usually gets most of it. But it’s very interesting how much gets shared.
And if you look at the kind of hunt that was involved, you can see interestingly, if you classify the hunts by kind of a primitive, not terribly well-coordinated hunt, which is a half-anticipation, through a single-anticipation hunt, where one of the chimps is actually successfully anticipating where the colobus are going and manages to get over and block them off, to a doubled-anticipation hunt, which is even more sophisticated, you can see that there is a reward in terms of amount of meat eaten for participating in a more sophisticated hunt. Okay?
They pay a tax, and they are willing to pay a tax to belong to the group. The tax that they pay is by sharing what they capture. So there is an anticipation of being willing to give up personal gain for group benefit. And there’s another thing going on here, and that is that quite a bit of the reward of bystanders is trading food for sex. So both taxes and prostitution appear to be present in chimpanzees.
Now what about learning? Well this is the frequency of ambushes that are used by hunters of different ages. Ambushing is, you know, a moderately sophisticated tactic. It’s not quite as sophisticated as a big group hunt, but it’s an indication that, you know, young chimps are learning how to hunt. And this is how old they are. And the interesting thing about this is–well of course they’re weaned at five; so they aren’t really going to start participating until they get there, and they don’t do too much hunting before they become teenagers.
But then let’s suppose they start playing here, they start going out and hunting here. It takes them until they’re thirty-five-years-old before they really hit their peak. It takes them about twenty years to learn how to be an effective hunter. That’s pretty amazing.
And if you look, if you break this down by half-anticipations and full-anticipations, what you see is the frequency with which full-anticipations are practiced in hunts is lower, and it takes a long time before they get up to the level of half-anticipations. So these are categories of sophistication in hunting, and it takes a long time to get more sophisticated.
Chapter 7. Conclusion [00:45:28]
So to sum up on the behaviors that are involved in foraging and hunting, I’ve really given you today two paradigms for how to think about it. One of them is the marginal value theorem, which tells you how you should decide when to stop hunting in a certain place, or when to stop copulating in a certain place, and go off to find either food or mates in another place; and that works in a spatial situation where you have to move some distance before you get to the patch.
And the other paradigm that I’ve given you is simply the cost-benefit analysis of how much caloric reward do I get out of hunting in a group versus hunting by myself? And you can see that if you do that cost-benefit analysis, it turns out that organisms don’t do it perfectly, but they do get better and better at approximating the optimal payoff.
So that tells us that hunting is basically microeconomics. It’s a very short-term, kind of selfish behavior, where the group hunting behavior, which looks like it might not be so selfish, in terms of caloric reward is quite selfish, and it looks like it’s long-term selfish. So the sharing is stabilizing relationships that are paying off in terms of future hunts and future sexual opportunities.
We’ve seen that predation is shaping both the behavior and the morphology of prey. We saw that with both conspicuous coloration, aposematic coloration, which is advertising the presence of poisons; and we saw it with crypsis, which is cryptic coloration, which is hiding and looking like something else.
We saw that aggressive mimicry illustrates an important evolutionary trend, which is that every available opportunity will eventually be seized by some species evolving into that niche. And we saw it with fireflies, and then I told you about cleaning wrasses in saber-tooth blennies.
And then finally I showed you the cooperative hunting, complex example in chimpanzees. It requires strategic thinking; it requires a kind of teamwork that really is only attained in fairly complex organisms. I’ll mention one other, which is deeply cool, and that is the bubble-nets of humpback whales. Who has heard of a humpback whale bubble-net? A few.
Humpback whales hunt in family groups in Alaska and down near Antarctica, and they hunt for fish that are in schools. They hunt herring. And a mama humpback will go down below a group of herring, and she’ll start up in a spiral, just at the right rate, so that the bubbles that she’s blowing out of her nose will all rise up in a curtain to the surface around the school of herring at the same time; so a curtain of bubbles, that looks like a net, is going up around the school of herring.
And at the same time she gives a few little signals with her complex calling behavior, and the whole family of whales will dive down, and they’ll come up as a group and they’ll open their mouths underneath the school of herring, and all of the mouths will emerge from the water at the same time. They can take out an entire school of herring with one bite apiece. They’re taking in maybe two or three tons of fish when they do this; like five or six whales doing it all at once, they’re right next to each other.
So there are very interesting examples where other species, that appear to have complex cognition and good signaling capacity, learn really rather sophisticated behavior. And I think the chimpanzee hunt is one of the most interesting. I’m sorry it’s so bloody, and I do hope you enjoy your lunch.
[end of transcript]Back to Top
|mp3||mov [100MB]||mov [500MB]|