EEB 122: Principles of Evolution, Ecology and Behavior?

Lecture?32. Economic Decisions for the Foraging Individual

https://oyc.yale.edu/ecology-and-evolutionary-biology/eeb-122/lecture-32

We did evolution, and then?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.?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.?

• You have to understand how behavior evolved phylogentically; so you need a comparative view of behavior. How is it that behavior is adaptive; is it, or is it a maladaptation?

• You also need to understand how behavior develops. If we follow the organism from zygote to death,?how is it that organisms learn? How is behavior acquired?

• And then finally we need to understand the mechanistic underpinnings of behavior.?You can go at it through neurophysiology or?endocrinology. There are many different kinds of mechanisms that are involved in triggering behavior patterns.?

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.?

First,?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 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.

• X axis is time. 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.?So it's going to be food, or it could be mates.

• The clever thing is the nice geometrical solution to the optimality problem.?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.

• 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.?The slope is the payoff per unit time. So drawing a line that way, as a tangent, maximizes the payoff per unit time.?

Now two comments on the problem of how to deal with risk; and this is the small bird in winter problem.?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.?

• 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. 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?and drops its weight.?

But there's another way of dealing with it, and that is that if you look at this other relative of the Great Tit, such as?Marsh Tit; it also looks a bit like a Coal Tit; it's a little bit smaller, has a black head.?

• You put it in a high variance environment or a low variance environment, this one?doesn't change its weight at all. It just packs on as much weight as it can, by evening.

• 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?these two?species are?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.?

Now what are some of the consequences of predatory behavior for the prey??One of them is so-called aposematic coloration; and that is if you're carrying around something that is going to poison your predator, you want your predator to know that.?

• The monarch butterfly caterpillar, which gets its cardiac glycosides from milkweed, will cause tachycardia in the birds that eat it. Your heart jumps say from a pulse rate of 80 or 100, 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.

• One can do experiments with this kind of thing as well. This is a situation in which regular domestic chicks?were given different colored baits.?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.?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. 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. The praying mantises that look like flower petals?sit on flowers, and grab things when they come in; they're very nasty.

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.?

• 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.?These are males that are flickering, and then they get a response from a female.

• However,there are some that mimic the light signals of another species; some females that are sitting there saying--they're faking it. So she goes blink, with the signal of the other species, he flies in, and she chews him up. 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.?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?has its mouth open and?rips out a chunk of gill and goes running off.

One of the interesting issues with predation and parasitism and mimicry has to do with the 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--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.

Why don't the hosts throw out the cuckoo egg? There are really?three 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.?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.

• 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?and it is still kind of puzzling why the hosts don't throw out more cuckoo eggs. It's an interesting problem.?

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.

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 let's look at an analysis of why they choose to hunt in a group.?

• 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 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.

• 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.?

• 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 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.?

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?young chimps are learning how to hunt.?

• 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.

• 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.?

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.