MCDB 150: Global Problems of Population Growth
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Global Problems of Population Growth
MCDB 150 - Lecture 1 - Evolution of Sex and Reproductive Strategies
Chapter 1. Introduction [00:00:00]
Professor Robert Wyman: I want to start today with some stories from the newspapers: to set a tone. It was over the last few years.
Tehran: one afternoon Fatima Eskandari opened the front metal gate of the shelter she runs for runaway girls in central Tehran and was confronted with two men, armed with knives and rifles. “Who are you?” She asked. “We are the uncles of a girl named Ranach, we are told that you are keeping Ranach,” and indeed that was true. Ranach was 16 years old, and she was inside, recovering from the bruises that she suffered at the hands of these same uncles. She had run away from home to escape them. The uncles had driven from Sanandaj in the northwest corner of Iran, hundreds of miles away; they demanded to see their niece. The uncle said she had shamed the family by leaving home a few days before. They had come to behead her. They were very open about this. There was no secret. [NY Times 11/5/00]
Staten Island, New York: A 17-year-old girl worked as a cashier in a convenience store. The store owner said that the girl was stealing from the register and he was going to fire her. The girl went to her father and said that the store owner had groped her. The father flew into a rage, grabbed a baseball bat and a gun, went down to the store, and killed two people. [NY Times 12/14/05]
Islamabad, Pakistan Zahida Perveen, 21-years-old was pregnant in 1998 when her husband, Mehmood Iqbal, bound her hands and feet with a rope. First he shoved a rod in one of her eyes, blinding her, then cut off her nose and ears. He SUSPECTED her of seeing another man. [New Haven Register 1/8/01]
Tuscaloosa, Alabama: Felicia Scott already had two sons, but she had an obsessive desire to have another baby. She convinced her boyfriend to help her get one, so they went out, shot a pregnant woman, and cut the full term fetus from her womb. [NY Times 9/28/98]
Saudi Arabia: New York Times–A young woman was raped by seven men. She pressed charges. The Saudi Court sentenced HER to 200 lashes–200 lashes is almost a lethal dose. [NY Times 1/4/08 p1]
Dominican Republic: Crucita Medina is 18, she’s been married for a year in which her husband Jose beat her constantly. She had the courage to separate from him but she met with Jose after their separation when he asked her to talk. He took her to a desolate street on his motorcycle and they had an argument. He grabbed a container he had filled with - what they call the devil’s acid, - a mixture of gasoline, hydrochloric acid, car battery acid, urine and other chemicals. He threw it at Crucida. The liquid disfigured her permanently. It burned her face, her arms, the right side of her chest, and a portion of her legs. She is still trying to bring her ex-husband to justice. [IPPF Reaching Out 22: Spring ‘02]
Gosarigaon, Bangladesh: I’m almost done with this. The village elders met under a lychee tree to put a value on a Payara Bigum’s grotesquely ruined face. A young man had become obsessed with her; she was married and he was turned away. He took his revenge with sulfuric acid to erase the beauty that had once enchanted him and to empty her life of happiness. Her cheeks melted, her right eye was blinded and hollowed to a crater. The husband had to bribe the prosecutors before they would even take up the case. Eventually the perpetrator’s family had to pay a fee of $3,000.
In one year, the year I have statistics for, 1999, there are 174 acid attacks in Bangladesh and those are the ones that were officially reported. Of course, probably the vast majority of these never get reported. The article mentioned a 13-year-old girl who was attacked as she slept. Some victims die, some are forced to marry their attacker, another was forbidden to come home until she allowed her husband to take a second wife. [NY Times 6/24/00]
Well these stories are obviously at the extreme of human behavior, but the purpose of giving you all that is to explain that human sexual behavior has extremely deep roots, very emotional, and very hard to change or manipulate. Male/female relationships are very difficult for humans to cope with. I think it shouldn’t particularly surprise you that human sexuality is not particularly driven by rational calculation.
The few stories that I’ve pulled out of the newspapers are just the tip, clearly just the tip, of an iceberg, of a very widespread phenomenon. In the long course of human history across cultures you see are the–I gave you everything from Brooklyn to Bangladesh–these very similar sorts of things happen.
Females, through the long course of history and in most cultures, where most humans have lived, females are treated very badly. There’s a huge amount of battering. Battering is the prime human version of violence, and females are so discriminated against that statistics indicate that there’s something now, right now, something like a hundred million missing females. That these females are either aborted before they’re born, killed by infanticide, pretty much as soon as they’re born, or neglected so that they don’t get the food, or they don’t get the medical care that their brothers get. There’s a dearth of something like a hundred million women in the world today. These are extreme incidents but it’s an extremely common thing.
One of the purposes of this course is to try to get you to understand what is causing all of this. From a biological point of view this abuse of females is extremely weird. Males, as you know, can only reproduce via a female and so–and evolution is–the name of the game is reproducing, so almost all species–what kind of female do the males want? They want the healthiest, most well protected, the best fed female, and you’ll see some examples of the extremes to which males will go to provide this so that that female can produce offspring for that male and carry on the evolution game.
In humans, we find that it’s very general that females are treated atrociously, and it just doesn’t make sense that human males should keep many of their females hungry, sick, and abused. In childbirth, if a woman does give birth to a child they can often be underweight, sickly, and it’s because the mother is not in great health. This is all a biological disaster the way human males treat human females and we don’t know–I mean we do–we have some idea, why humans do that and the first part of the course we’ll talk something about that.
That’s on the individual level, and evolution works on an individual level, but if you think on a species level, missing a hundred million of your breeders, that does not sound like a great tactic for survival of the species. I spent some time, when I started getting interested in this whole topic, reading the anthropological literature, the sociological literature, the feminist literature, and basically I thought all those explanations were a crock. I don’t think they came close to an answer. In my own studies I had to go back to the very beginning and understand what sex was really all about.
Chapter 2. Facts about Sex and Reproduction [00:08:19]
I’m going to tell you a few important things about sex and reproduction. The first one is that reproduction turns out to be very difficult. One day I was sitting over in our botanical garden under a beautiful big oak–huge oak tree that they have, and the ground was just covered with acorns under this tree, and I asked one of the forestry people, because that’s the kind of things that they know, I asked them how many acorns has this tree produced? They said, well the wind–this spread is so much and this line, and you know it’s so deep, and they came up with probably 750,000 acorns a year. I thought wow, so I checked the literature and found that people that collect these notes for commercial purposes, a good oak tree can produce five hundred pounds of acorns a year, and many sources say that an oak tree produces millions of acorns in its lifetime.
Wow, a lot of reproduction right, there’s got to be sex before it. Then I asked myself, how many of these million acorns survive to make a tree like their parents? What’s the answer? Basic biology question, you might know it. Well, what if each tree on average, taking an average of tree made two acorns, how many acorns would there be–how many oak trees would there be in the–made two acorns that made trees–how many oak trees would there be in the next generation? Twice as many as now; what if they only did–and so that can’t continue because then the Earth would immediately fill up with oak trees and we’d be this deep in acorns. Similarly, if the trees produced less on average, less than one, what will happen to the oak species? It goes extinct. So, over the long time, on average, each oak tree has only one acorn that grows up to be an acorn producing tree itself. That’s an amazing fact–it puts out millions of acorns and only one survives.
Of course this is true not only for trees but fish, which spew out–fish females–huge numbers, hundreds, tens of thousands, hundreds of thousands of eggs; or males of many, many species that spew out billions of sperms that, in a sexual species, the way mammals are, two parents on average, two children. If two parents for a species average more than two children, the world fills up with squirrels or whatever animal we’re talking about. If they have less than two over a long run, they’ve gone extinct.
Most species, in fact, in the history have gone extinct. The average of current species has got to be 1:1 or 2:2 depending on how you count, but for most species it’s less than that because most species have gone extinct. So, the answer is, it is brutally difficult to reproduce. If any given individual in a species produces a lot of children, more than one or two, depending on how you’re counting, then that means that some other individuals are having less, again, unless the species are humans, which are about the only species that are increasing continuously over the long time.
As an example, they now, by this genetic testing have tested: how many descendants does Genghis Khan have? The answer is 16 million descendants, this one guy, so that’s an awful lot. Guess how he did it? By killing a lot of men and raping their women. So he has that many descendants because an awful lot of men, all the men he killed, don’t have any descendants. That’s just Genghis Khan, the Mongol Horde had a lot of males and I think they spread their seeds quite widely, and a lot of males and conquered people just didn’t survive.
Of course every army in the world has done–essentially all–have done a huge amount of raping, and it’s a wonderful instrument of gene flow in the human population. Because reproduction is actually so difficult, species have evolved all kinds of fantastic mechanisms for trying to be successful at reproduction, so that’s Point One.
Point Two is that sex is not fair. It’s not only difficult but it’s not fair. Sex goes way back in evolution and even bacteria do it. This is–the biologists here are laughing. This is the bacteria that are in your colon and they have sex in this manner, and so right now billions of sexual acts are taking place in your unspeakable parts, and this is the way they do it. Now, in something like this, actually the exchange of DNA can be either way and this is–even though the hairy guy on the left is producing that long pilus, as it’s called, there’s more or less equality of sexuality here.
But when it comes to higher organisms you get quite a different story. You all know the story the chicken and the egg. Well the way I tell it is the poor chicken, the chicken has to build this big egg, it has to eat like crazy, put all that protein and fat into the egg which are–if you’re not fed in the barnyard–which are hard to come by, and then once the chicken is hatched guess who sits–once the egg comes–out guess who sits on the egg? The female. Guess who protects the young? The female. So that’s chickens. You can go all the way up to humans; females have to spend nine months pregnant and then in most societies and most times females are the ones that have the burden of responsibility for the children.
Meanwhile in all this, males just produce a little speck of protoplasm, insert it into the female, say, ‘bye-bye, I’ve had my quickie and I’m off,’ and–not in all species–but in most species, the male has a very minimal part in reproduction and the female has this huge burden. Now why does that come about? You’d think evolution would require that females just wouldn’t play this game in evolution and they would require a more equal distribution of the labor.
Well it turns out it is–starts–as a division of labor. The first animals to evolve, first of anything, were probably–were almost certainly in the sea, and stuck down on the bottom or floating around and so they couldn’t move. How do you mate if you can’t get up and find your mate? Well you just spew out your sperms into the ocean; you spew out your eggs into the ocean, and you hope that they meet. Not a very efficient mechanism, but at that stage, the male has produced zillions of sperms, and the female has to produce zillions of eggs. If you’re an animal and you have to produce so many of anything, gametes in this case, each one is going to be very tiny. Two tiny things meet and fuse; you’ve still got a tiny thing. The odds of survival of a tiny thing are not very great.
Evolution doesn’t like that idea of the male and the female both spewing out lots and lots of these tiny gametes. What happens eventually is that the two sexes separate, that the female’s job is to make a big enough egg, with enough nutrients in it, so that the organism can survive, and the male’s job is to go find that egg. What do males have–what have the sperms evolved? Tails, to take them swimming. They find–their job is to find the egg, so there still has to be lots of those and they have to expend a lot of energy swimming. It’s still an equal amount of investment.
Then the whole animal–once evolution proceeds, the whole animal can now get up and walk over and find a female, or vice versa, and now they can copulate or do some sort of insemination very close together. This now does not require zillions and zillions of sperms. If you’re–if the eggs they already laid, like a fish in a nest, the male just lays the sperms right on top of the eggs, and you need some surplus over the number of eggs but nothing to compensate for the amount of energy she’s putting into her eggs. You start getting an inequality. As soon as animals can find each other and mate in a more spatially enclosed way, then you start getting inequality.
Evolution goes down that path because it’s turned out to be a very, very effective path, and what you get is that females make a few eggs, her eggs are very expensive, they are expensive and rare. Males still make many, many sperms; his sperms are plentiful and cheap. This is what we call a sexual dimorphism that males and females are taking different evolutionary routes. Once they take different evolutionary routes then different reproductive strategies come into play for males and females.
A male, through the first billion years of evolution has been producing a lot of sperm, and now all of sudden he figures out how to swim over to a female and he doesn’t need all those sperms but he’s still–evolution is conservative, it’s still–he’s still producing all these sperms. What is evolution going to do with all those sperms? One thing they can do is–he can evolve backwards and just make fewer sperm. That would save him energy, it would be good for him a little bit, but it wouldn’t really get him an awful lot more children, which is the name of the evolution game. Now what he can do with those excess sperms, find another female. That his limit–there stops being a limit on the number of females that he can inseminate he spreads his sperm to as many females as he can find.
This dimorphism that sperm are cheap and plentiful, while eggs are expensive and rare, leads to different strategies of reproduction in males and females. It also sets up some odd situations where, for instance, males are expendable. If one male can fertilize a lot of females, females don’t need a lot of males around. For instance, there’s a certain female wasp that lays its egg in a caterpillar. Wasps are one of the major predators of caterpillars; they lay the egg in, the egg hatches and starts eating up the caterpillar. All of the eggs that are laid in a single caterpillar compete with each other for the food, that is, the caterpillar. What happens is evolution has arranged that the females hatch first and they eat up all their brothers except one, and then that one male can fertilize all his sisters and things go on. The males are expendable in that case, and if the females just ignored them and let them live, the males would be competing with the females for food, the females would grow less big and healthy, and they would have less eggs and evolution doesn’t like that kind of a system.
We’ll–all of these things, if you think, it’s not very difficult to find human examples of this. Who goes off to war and gets killed? Males, and very often you don’t very any reproductive–any change in rate; the females manages to get inseminated, in humans. In lots and lots of species, the number of males is really not a critical factor in the amount of reproduction that goes on.
Chapter 3. Reproductive Competition and Reproductive Strategies [00:20:55]
Males can inseminate many females, but females want to worry about the survival of each egg, and similarly again, this is pretty obvious in humans that a female, if she gets pregnant, is going to be spending nine months pregnant, and then breast feeds and won’t get pregnant again, so it’s at least a year and probably at least a year and a half before that female can get pregnant again. We almost always bear only one child with an occasional twin or so forth, so a very low rate of reproduction where a male doesn’t have any such kind of limit.
This does not mean that females are monogamous, because there are many other reasons why a female might want to have many mates. First of all, we’ll see, she might want to get resources from many males. You’ll see she sometimes will not mate with a male unless that male gives her some resource. She might want to mate with a variety of males because she can’t tell which one has the best genes. She many want to mate with many males if her environment is unpredictable, and even if she can tell something about his genes, she doesn’t know what the environment for her children are going to be like, so she wants to shake up the deck and try–have a variety of children with a variety of different genes. She might want each male to think that he’s the father of the children so that he doesn’t–that he protects and doesn’t kill the children. There are species who do that. There’s something called sperm competition where she may want to have a whole variety of different sperms from different males in her and then the sperms compete.
Even though this sort of comic book presentation says, all males are promiscuous and males want to go around and have a lot of sex, and the females want one, there’s many species with many reasons why a female may also want to be promiscuous. It used to be believed that many species were in fact monogamous. Birds are a very good example: what you observe is that a male and a female meet at the beginning of mating season, they stay together all mating season, she maybe sits on the eggs, and he brings worms or some combination thereof, and people thought, ‘Well this is great. This is monogamy and what wonderful animals they are.’ Now we can do DNA testing and it isn’t so–apparently–so they call that now social monogamy and distinguish it from sexual monogamy. While social monogamy is as we’ve always believed it to be, sexual monogamy is almost nonexistent. In species–when they measured–it turns out 10% to 70% of the progeny had been sired by someone other than what they called the resident male.
One article I read claimed that there’s only one species where it is known for sure that they are 100% sexually monogamous, and in that case the male and female physically fuse together, so neither can go anywhere.
Males have the job of finding and gaining access to females. They have basically two strategies. One is to let their sperms compete. In that case they make just more and better sperm, and the sexual system in a species like that will be promiscuous, that the females will mate with many males, they’ll have a lot of semen in their vagina, or spermatheca or sac for this, and then the sperms will compete with each other. Some scientists believe that in humans also, we have a variety of sperm, including killer sperm, and other kinds of sperm that go in and facilitate this. These killer sperm are supposed to kill sperm that have a different genotype, and it’s very controversial about whether this is true or not. The data starts when you look at sperm they have–human sperm–they have very different shapes. And one version is, well, all these other shapes are just damaged, they’re just bad, they’re not effective and there’s some evidence for that. The other story is that, no, these are doing different jobs then fertilization, and it’s a hot area of research.
That’s strategy Number One, to engage in sperm competition, and you’ll see if we get time that in our related species, Bonobos, chimps, our group of species generally engages in sperm competition. What you get is very large testes, so you can determine this by taking the measure–the weight of testes–as a fraction of total body weight. If it’s very large, you know there’s sperm competition going on.
The other strategy of course is males can compete with each other for control of the females. This happens of course in motile–motile animals–sperm competition is the original thing where you spew zillions of sperms and then the best ones find–are the ones that find the females, but once they’re motile, the males can come into contact with each other and they can fight in some way and control a territory, for instance. Coral reef fish control territories in the coral and the females cruise around, find a male who’s got a good territory, then come and mate with them.
Well generally the males will actually fight with each other for dominance and then use that dominance to gain access to females. How does this–what are some examples of how this all works out? The females have also two basic strategies. One is to get the males to provide resources other than sperm, and those resources allow her to build big healthy eggs.
One of the really cute examples of this is in the jungle, protein, nitrogen, is very, very scarce. The soils are thin and the rains wash everything away so it’s really hard to get nitrogen. Guess what’s a great source of nitrogen? Dung. Any animal takes a dump in the rain forest, immediately there’s all kinds of–especially insects that come and are going to utilize that nitrogen. It’s a really scarce resource. Beetles have–are ones that are very good at this and there’s a whole group of beetles called dung beetles. What do they do? They–as soon as they detect this–by odor probably, they come in and start rolling it up into little balls, because it’s much too big for them. Let me show you a picture of–this is a dung beetle, and what he has done–there’s a big pad–a big animal pad nearby and he’s cut off with his cutting claws this big ball, and that’s what he’s going to present to a female.
A whole lot of dung beetles come in, they make these balls, then in the species I’m talking about they put it on their back and sort of parade around with it. Meanwhile, the females are on the outside, and they’re watching all this go on and what males do they choose? The ones with the biggest balls. She wants resources and that’s her resource.
Now the most extreme case of this is in praying mantises. So a praying mantis is a beautiful animal, as you may know, here’s one of them, but they’re very solitary. They have to actually catch insects. They sit on the ends of branches and–with these claws–and they wait for an insect to fly by, and then they grab it and eat it. It’s not an easy way to make your living. They’re poised; they’re very quiet, and they want to capture a meal. They have just a few milliseconds of something buzzing by.
Now a male comes up and wants to mate with this female but she’s ready to eat, so he’s got the problem of approaching her and not getting eaten. In evolution they sometimes get eaten and that has led to a very interesting form of reproductive strategy. That what you see is that the male will come on and you can see–now that’s the male on top and look how much smaller he is then the female. In many species the male is bigger, but he’s smaller, and that’s a sign that the males are not fighting with each other. You’ll see later that if the male is bigger that means they’re–almost always means they’re fighting. She’s big because he only has to make sperm, she has to make eggs.
Notice he has his head still on, she has her head on. That’s the way it starts, and you think that something normal is going to take place, when actually in fact, she reaches around and grabs him, she’s bigger and stronger than him, pulls him off and starts eating his head. What then happens is she allows him to go back, but now you can see she’s still got her head, there’s no head there. The way the insect nervous system is put out; all the circuits to coordinate the copulation are down here. What the head actually does is inhibit that, unless it goes and says–well most situations–no this isn’t right to start copulating, but only in the very special situation where it’s a female, when she eats off his head that releases those circuits from inhibition and he copulates her to completion. Now–and then she can–when he’s done she can eat the rest of him. Now what’s really interesting is–and he doesn’t object, he doesn’t try to get away.
What’s going on? The key thing is that this is a sparse species. How many of you have seen a praying mantis? How many of you have seen a lot of praying mantis? One person, you must study them or something. They’re not hard–they’re not easy to find, there’s not many of them. If a male is lucky enough to find a female and get in this situation, man he’s in heaven, and it’s unlikely if he leaves her he’s going to find another one. That’s just an unlikely event. How does he maximize his reproduction? Well in my one chance I’ve got to have that female produce as many eggs with my genes in it, so he wants her to produce a lot of big, very healthy eggs and so his body is the food for her, including a lot of protein so that she can make a whole lot of eggs.
In evolution it’s worked out that males who sacrifice their bodies to this have more offspring then males who don’t, so you see the evolution from him being prey just because he’s prey to it being an actual part of the whole sexual situation. The next time someone tells you that evolution is the survival of the fittest, what are you going to tell them? Fitness has–survival has nothing to do with the price of cheese. That’s only one of the ways.
Now, the next strategy of females–the first is to get resources, the second strategy is for females to try to find the male with the best genes and so that she–her offspring have good genes and are very successful. There’s a whole lot of aspects to this and one is she has to be able to control which male is fertilizing her genes. The more separated the male and the female, when you spew out into the ocean eggs and sperm there’s no control whatsoever. The closer that you come–the male and the female come to each other the more control that the female has.
One of the interesting ways of looking at this is internal fertilization, as in humans. It is among other things a strategy for female control. Internal fertilization and internal growth of the fetus has a lot of advantages. It protects the fetus, allows the mother to provide nutrition and waste removal continuously, etc. There are also these aspects to the female control strategy. Generally, if a female won’t allow a mating, it doesn’t happen. If a female doesn’t allow it then rape has to happen. We’ll talk about rape later.
In humans, as you know, females have a vulva and that’s the Latin word for valve. She can open it, she can close it, and that’s the whole idea. If the female wants to mate she gets excited and she lubricates, and the purpose of the lubrication is to ease penetration and ease the sexual act. Behaviorally she’s also–aside from the beginning she’s receptive, she assumes the proper posture, and everything goes just fine. But if these things–if she doesn’t want to do it and these things don’t happen, there’s no lubrication, there’s no assumption of the proper posture which is necessary for internal fertilization, then it’s very hard for the male to intermit and probably sexual intercourse won’t happen.
In a real rape, say among humans, the vagina does not lubricate, and that’s why there’s so much lacerating and tearing of the vaginal walls. That’s why it’s so dangerous because it’s part of the human female strategy to control what males inseminate her is to not lubricate, but if she’s forced there can be a lot of damage done to her.
How does a female know which male has the best genes, which male to allow to inseminate her? She has to either watch a competition or see the male in some way, or know the outcome of some competition that she doesn’t see, by say, detecting a male’s position in a status hierarchy. In some birds, like flamingos, there’s a beautiful movie of flamingos doing this which I don’t have time to show, there can be 100 or so males can gather together and start dancing. It’s what’s called a lek, and many of you may have heard of it or seen it in National Geographic movies. The males dance back and forth, back and forth, they show their coordination, they show how perfect their feathers are, they show their stamina because they’re doing this for quite a long time, and again, the females stand around on the outside and try to notice a male who’s a good dancer, a good strong well coordinated dancer, and then she chooses him and goes off and mates. By choosing these males that are not creeping around, she presumes that she’s getting a male with a good–with good genes.
Another way that the females get to choose a good male is by choosing males who have–in species where males fight as part of male strategy then the females choose the winners. Males may fight, and you set up a dominance hierarchy, but there’s nothing that says the female has to choose the top dog, maybe she wants to choose the middle dog or a bottom dog, but in general in species, females choose the top dog. They know the result of–in a variety of ways–the dominance fights, and the female then chooses the top dog. In the male fights, the winner gets the female, or maybe gets the whole harem of females, and the loser may get absolutely nothing.
Well the females are very happy to join the winner’s harem. Why is that? Because once that starts in a species that means that male has some genes which allow him to win these fights. He’s big, he’s strong, he’s vicious, he has sharp teeth, he’s a violent character, and therefore he is successful in these battles and the female wants her young to also–wants being in this evolutionary sense of if she does that she passes on her genes–the female wants her offspring to have these characteristics of being able to win a fight, i.e., of being very violent. She will choose the most violent male, the winner of these battles to father her children because then they–then the odds are that they will also become these big, strong, violent males.
What happens, in this case, is in evolution males may start fighting and once the females select the fighter’s then both male and female reproductive strategy colludes in increasing the violence a little bit in every generation. It’s very interesting that males and females are colluding in the evolution of male on male violence.
Chapter 4. Sexual Coercion, Violence and Rape [00:38:24]
What else does evolution do? Well if the males are fighting each other and reproductive success depends on winning these fights, then males will tend to get larger. You get a large male, humans, chimpanzees, males larger than the female, and that helps them in the fights. It also opens up a second strategy for reproductive success–a second strategy for reproductive success and that is that the male is now bigger than the female and he can start coercing the female.
He may not have to fight the males; he can just coerce, fight with and coerce this smaller female. Then guess what happens? If that gets into a species that the males start coercing the females, what is an evolutionarily wise female to do? She chooses a male who is most successful at coercing females, because again, her children will then inherit those genes which will allow them to successfully coerce females and get more reproduction and have more offspring. Again, we see that not only are male and female reproductive strategies cooperating in an increase in male on male violence, they also cooperate in increasing male on female violence.
It’s really quite an interesting way of looking at things and explains–starts to explain some of this violence that we see in human interactions. Now the great–we belong to the great ape line of evolution, and it seems that our line of evolution seems to specialize in male on female violence. Consider rape, so a lot of political correctness about use of the word rape, but I’m going to do it in a straight biological sense. Rape is the coercion of an unwilling female into intercourse.
Outside of mammals there are only a very few species where rape occurs. Scorpion flies are one of the best known examples. Normal sex occurs when a male offers a female a dead insect or other food mass; she’s getting resources from him. After she accepts she allows copulation to proceed, and it goes smoothly. Sometimes, however, sex happens in quite a different way. A male without an offering can ambush the female and she clearly is trying to escape and the whole time, he’s got genital claspers to try to grab onto her, and she’s trying to get away and push him away, fly away. In your reading, there’s a description of this rape of scorpion flies so I don’t have to go further with it.
In vertebrates, rapes occur in several species of ducks and in mammals there are only three species where rape is routine: elephant seals, orangutans and humans. Rape occurs occasionally in three other mammal species: chimpanzees, howler monkeys and gorillas when they’re captive; it has not been observed in the wild. This is really quite striking. Of the six species, mammalian species, in which rape has been observed, five of those are the great–are apes. I’m sorry one is a monkey–of the five species of primates where rape’s been observed, four are great apes like us.
Those statistics are way out of the range of chance. There is something special about ape evolution that has led to this emphasis on violent relationship between males and females. Most likely it is the extreme unavailability of eggs, so it’s an extremely rare–in the great apes it’s extremely rare to find a female who has an egg ready to be fertilized, why?
Well first, primates take many years to become sexually mature. In chimps and humans it’s around 12 to 13 years before a female can ovulate an egg. Then primate mothers have these long gestation periods, eight months in chimps, nine months in humans, and so just counting gestation and the recovery from childbirth, females can have at most one young a year, which means one egg available for fertilization in a whole year. Then the females lactate and you probably know about lactational amenorrhea that when a female is lactating and the baby is sucking on the nipple, hormones are released and the female does not lactate–does not ovulate again.
The average for a chimpanzee is about another four years in which the female feeds–breastfeeds the infant and so she doesn’t come into fertility again. Then they can sometimes stay with their young even longer time, so the average birth interval for chimpanzees is five and a half years. That means for any female there’s one egg every five and a half years ready to be fertilized. That’s Jane Goodall’s number, a Japanese group says it’s six years, for orangutans it’s eight years, and gorilla females have a baby about once every ten years. There’s an extreme dearth of eggs to get fertilized.
You have a chimp community, there’s say 40, 45-55 individuals, something like that, maybe 10 or 12–this is a big community–10 or 12 sexually mature males, about the same number of females, non-mature males and females, and of those then, given the very long period, there’s probably going to be one or two females in that whole year who are going to be fertile and in estrus and ready to get fertilized.
These males spend the whole year, in terms of their reproductive evolution, spend the whole year setting themselves up to have that one–to inseminate that one egg that’s available in that whole year. You can imagine there’s going to be a huge amount of competition, a huge amount of violence among the males. As we’ve seen that male/male violence then spills over into male/female violence.
Chapter 5. Evolution and Sex in the Great Apes [00:45:10]
Within this group–we belong to an order called primates, as you know, and within this group monkeys are fairly distant relatives but the rest of the great apes are fairly close. You can see there are a lot of similarities, these are some jokes about the singles bar, even in biology there’s some sense of humor still left. Now, this is another professor at Yale and I always ask students–you have too many advisees–so I asked them wouldn’t you rather go to this guy? He looks very fatherly and kind and everything. Well that’s an orangutan. Of the great apes, it’s as far a distance from humans as–the most distant species and yet looks pretty much–you can empathize with that as almost a human being. It doesn’t take a lot of evolution to go between these two.
There are five species of great apes. That’s a young guy. This is a time scale, about 15 million years ago there was one line of evolution coming up from a long time ago and the first event was that orangutans split off the tree, that the tree split and one branch became orangutans. Then about ten million years ago another–there was another split and gorillas went off, a group went off and became gorillas. Then about six million years ago another group came off and they evolved into humans. The most recent split is about two million years ago and gave two species, very similar species: chimpanzees and Bonobos.
The farther distant the split is the more there’s been time to evolve differently, so an orangutan who as that nice guy, is very distant from the other species. That’s our family tree and the difference–this is a fairly recent split here. The genetic difference between these three species, between any pair of those three species, is about 1.5%. If you draw out the DNA sequence for a chimpanzee and a human and look at the base pairs, out of every hundred, 98 or 99 will be identical and one or two will be different. We’re extremely similar genetically.
Now, what does this difference mean? 1.5% or 2%, we don’t have the foggiest idea of what it means. Obviously, you can look at a chimpanzee and say, hey it isn’t human, and their behavior is different and they can’t speak, and all kinds of things, but genetically there’s not that big of a difference. I wouldn’t draw any particular conclusions from the genetics yet. This is a rapidly evolving field and we’re going to learn an awful lot very quickly, and one doesn’t know what the conclusions are going to turn out to be.
One thing which may be solid is that if you look between a man–females have two X chromosomes and males have an X and Y chromosome, so you can ask, well what is the genetic distance between a human male and a human female? Guess what, 1.5%. Whatever you think of the difference, genetic distance of those species, you pretty much at this current stage of our knowledge have to think the same as the genetic difference between a human male and a human female. That’s just for your thinking.
Each of the great ape species has evolved a different social system to organize reproduction. All but one have an awful lot of violence associated with it. Orangutans are the least social of apes. The males and females generally stay apart, and the mother and a child are the only stable social unit. The offspring stay with the mother until adolescence, very much like the chimpanzee story and like the human story. It’s about ten years before the young separates.
For most of the eight years between births, the mother has no interest in males at all, no interest in sex at all; she doesn’t really come very close to them. There are two kinds of orangutan males. There’s a large one and a small one. The big males are about 200 pounds, the females and the small males are less than half that size, about 90 pounds. The small male develops normally up to adolescence and then just stops developing further. Doesn’t develop the characteristics of the full male, doesn’t grow a beard, crests of the hair, throat pouches, etc. They remain looking like adolescents. But, they are completely fertile, have a normal complement of testosterone, they’re sexually totally capable, and they can stay in this adolescent stage for up to 18 years, maybe longer but that’s the most that anyone has observed them.
They probably don’t grow into a big male until the dominant male in the region has died or is too weak to defend himself. The females always want to–When they have a choice they always want to mate with the big males because the big males are more successful, and it’s the old story that females mate with males of a type that are already successful because that is good for the genes that her children get.
In the mating between a female and a big male, sex is very relaxed, it takes on a languorous quality, an erotic quality, there’s not a great rush. Matings can begin with oral or manual manipulation of the partner’s genitalia, and it can be initiated by either the male or by the female, and when they finally do engage in intercourse they do it often face to face, missionary style, and it takes about as long as it takes humans: an average of 11 minutes and up to a half an hour, just in case you want to compare.
Now these big males are ponderous, they can’t move fast, whereas, the females are lithe and they can go pretty fast, so you know the female is choosing the male, because if she didn’t want him, she could be gone and there’s no way that he could catch her.
What about the small males? They are not attractive to females but they have one advantage, they’re small and they’re fast, and they can run fast and they can catch females. That’s what mother evolution has done. They try to catch and rape the females. The females are usually, as I said, alone with their young and if they’re found by a single–by one of these small males they’re chased and sometimes they get caught, and then the females show fear and they struggle to escape, and the males sometimes strike them or bite them, and the females scream and the young–they’re dependent–the young scream, the females bite back, they hit, they pull the hair of the males while the copulation is going on and that lasts –not more than ten minutes.
How common is this? Different–orangutans are very hard to see. They live in a part of Borneo that’s hard to get to and they’re very hard to see. There are different studies coming to different conclusions. One ethologist found that about a third, 1/3 of orangutan copulations involve some degree of forcing of the female by the male. Japanese observers reported that 88% of the copulations were rapes and that these were of the severe kind. A Dutch observer judged half the copulations to be rape, so these are not rare events. They’re a standard part of the orangutan sexual strategy.
There’s a lot of examples where orangutans are close enough to humans that apparently a lot of those same sexual signals are passing. A woman primatologist who ran primatology research in Borneo talked about an orangutan who had lived with humans for a lot of his life, so he was very acculturated to humans. One day apparently he raped one of the female cooks, the orangutan, the male orangutan, raped one of the human female cooks at the camp. Apparently it was a complete rape with penetration and everything.
As you know, rape is a big embarrassment for the female, as well as the male, but in this case, and this was in Indonesia, the husband, very unusually, took it quite easily. The husband said that since the rapist was not human, the rape should not provoke shame or rage. He said, “Why should my wife will or I be concerned; it wasn’t a man.” There are all kinds of stories that I don’t have time to tell you about.
Okay, now why doesn’t evolution just keep the big males and the females together? Probably food density because food is hard to get and individuals have to forage alone to find enough food; if they foraged in pairs what they found would not be enough for two of them.
Now gorillas live in a region, and have an ecology that they have more food available to them, so they live in somewhat larger groups and in gorillas the females stick close to the male. Each male, big silverback male can have a harem of say three to four females.
In these harems, they spend most of their time just the few of them together. They’re quiet; they’re relaxed; they’re affectionate with each other; the troop is stable with the one silverback, the three or four females and whatever young they have. Very little aggressiveness of–between the males and the females, or female to female, just hardly happens. But, as I said before, if one male is controlling three to four females that means there’s two to three males that don’t have any mates whatsoever.
What is evolution going to do with those bachelor males? Again, it isn’t nice. The males–so the gorillas travel through the jungle, resting some, eating some, they eat fruits, they eat roots, they eat shoots. These bachelor males follow the troop on the uphill side and just wait until the silverback is not watching. Then they charge down, they can make a fair amount of noise, so often the silverback notices and goes over and beats the hell out of the guy and he retreats again.
Every so often he’s successful; the male is with another female or off–never very far away, but a little away. He goes down and what does he do? He charges downhill, so he gets up a lot of speed, he charges right at a female with a young, he grabs the young immediately smashes it on the ground and kills it; runs away and doesn’t try to do anything, but just kills the young of the female.
Now what does the female do? The female’s are– these are very smart, they’re great apes so they’re very smart; they recognize each other as individuals; they have long memories, so you would expect that this female would remember this male the rest of her life and fear him, hate him, avoid him. In fact the opposite happens. What happens is the female within a few days generally leaves the silverback that was supposed to be guarding her and goes with the single male and they go off and have a consortship. What the message that the male is delivering to the female is, ‘Look you’ve put a huge investment into this infant and now it’s wasted. That guy can’t protect you. He has too many females; he’s too old, he’s too big and slow, he can’t protect you. I can–you can stay with him, get pregnant again and I’m going to come down and kill your next baby too, so if you want to reproduce you come off with me.’
Of course none of this is verbal, this is an evolutionary story, I hope you understand that. They’re not mentating this stuff that I’m saying. What you notice is in a few days this female leaves and goes with the male that has just killed her young and starts reproducing with that one. That may be stable and last a long time, or another male may come by and in a few months separate him–separate them.
Again, these are not isolated incidents. Diane Fossey had data on about 50 infants; 38% of them died before they were three, and 37% of those were from infanticide. About one infant in seven dies from infanticide, and each of the female gorillas she studied had at least one infant that was killed by infanticide.
In the gorillas, the females are trapped in this vortex of male initiated violence. At any moment a male may come crashing through the forest and kill her young, and the best way for her to prevent this is to go off with that male. She needs protection, she lives in a world of baby killers and she needs some protection from them.
Chimpanzees–so we’re going down the list of species, and we won’t get time to finish, but chimpanzees have yet another solution to this primate reproductive problem of very, very scarce eggs. Unlike orangutans and gorillas, the males are not solitary, but related males spend their lifetime together as a community. Chimps live in groups of about 40 individuals, with a dozen or so adult males, and a similar number of adult females. As with the orangutans, the chimp females spend most of their time alone with their young, and they’re not separated from their young until the young are several years old, really into adolescence. First they hold them, they’re not physically separated for several years, and then for several years they’re not out of sight, and then for several more years they’re not out of voice contact, so it’s really very, very tight bonding between mother and children.
The males defend a rather large territory, numbers of square kilometers, in which they range and which the females range, and the males spend their time searching for food, patrolling the borders, the territory, and they’re often with other males. This patrolling is often–is a bunch of males together and they go around checking on the females to see which ones have come into estrus or if any have come into estrus.
As I mentioned, the females come into estrus only about once every six years after their last young was born. They have a 35-day cycle, very similar to humans, and are sexually receptive for about 15 of these days in each of their monthly cycles. Her fertility increases during these 15 days and she’s most fertile the last two or three days of this receptive period.
The females do an interesting thing, have you ever been to a zoo and looked at chimpanzee females? What do you see? Big red rump if they’re fertile. It looks rather disgusting to humans, and I’ve been looking for a picture of it, and amazingly it’s such a striking thing, I cannot find a really good picture of this. You go to a zoo it’s very, very obvious they–they’re called ano-genital swelling. They advertise their estrus. Oppositely to humans; humans keep their estrus secret; neither the male nor the female knows whether they’re in estrus. Chimpanzees advertise it, everybody knows.
Now why do they do that? Well what happens is when a female comes into estrus, the males have been waiting all year for this, the males congregate together and what do the males start doing? Competing for the female. They’re fighting, the dominant male arrives, and it’s clear whose dominant, so the females advertise as a way of inciting male violence. That they want to be able to choose the dominant male, the ones that are in great fighting form, so they say, ‘Hey I’m in estrus, all you guys come and fight, and I’m going to sit there and choose the best of you,’ which in this case means the most violent of you.
Unfortunately, in this situation, all these males are not only fighting with each other but they’re trying to get at the females, so the females are herded about. They have to run around to escape the clashes with the males. They regularly receive quite a lot of wounds when they’re chased they try to climb up trees to escape this and sometimes they fall out of trees, and they have their young with them, and the young sometimes cling to them and then if they–the mothers fall, the kids fall. It’s a very, very dangerous unpleasant time.
The violence is so great that when a male approaches a female, she doesn’t know whether he’s coming to be–to try to mate with her or to be violent toward her because as you’ll read in the reading how the males have a long history of being violent to the females. It’s a way of cowing them so that when they’re in this melee, when they’re in estrus and the male approaches them, they don’t resist. If they do try to resist he does a lot of violence on them. The male has to–when the male comes and wants to mate with female, he has to signal to her don’t run away, I’m not going to try to beat you up and he has–this is–orangutans, gorilla, chimpanzee with her young. This is the–this male is inspecting the female, not only is there a red rump but there’s odors, chemicals, and he’s trying to see what the status of her cycle is, and this is a male displaying–letting a female know that this is a sexual engagement.
Humans have been known to do the same thing. This is a picture from [New Guinea], the same sort of advertising, with pretty much the same message, and here’s an incredible photograph. I don’t know should I leave that on the board for you? I get in trouble leaving things like that on the–up for too long. Okay, now, so in this great melee, this great violent melee, there’s all the males there trying to get at this female. If a male does–let’s say that alpha male is off fighting with someone else and the female is alone for a minute, the male–any male that’s around rushes in. He’s not going to have a lot of time before the other males notice what’s going on and rush into separate them, because they’re all competing to inseminate that egg.
The males have a very short time in which to complete the copulation, and ejaculation occurs after just 15 seconds, with only 8.8 pelvic thrusts. I think those are very important numbers you should know, but the females make up in quantity what they don’t get in quality. They appear–and there’s a story behind this, but they appear to be quite promiscuous in their sex partners. In the community followed by Jane Goodall, in each estrus cycle, each female had at least one bout of intercourse with every male, so all the males are getting some chance at having intercourse. They average six encounters a day. Don’t get excited, it’s still only a minute and a half at their rate, and in each monthly sexual cycle they have about 100 or so bouts of intercourse, so there’s a lot of sexuality. Do females have orgasms? Not known; there are debates about it, but not known.
All right, I guess our time is running out. We will continue on Thursday and I’ll set up sections in between, and any questions? I stay after every lecture for as long as necessary if people want to come down and ask questions.
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