You are here
EEB 122: Principles of Evolution, Ecology and Behavior
- Sexual Selection
Sexual selection is a component of natural selection in which mating success is traded for survival. Natural selection is not necessarily survival of the fittest, but reproduction of the fittest. Sexual dimorphism is a product of sexual selection. In intersexual selection, a sex chooses a mate. In intrasexual selection, individuals of one sex compete among themselves for access to mates. Often honest, costly signals are used to help the sex that chooses make decisions.
|Low Bandwidth Video
|High Bandwidth Video
Principles of Evolution, Ecology and Behavior
E&EB 122 - Lecture 13 - Sexual Selection
Chapter 1. Introduction [00:00:00]
Professor Stephen Stearns: Okay. Tomorrow is Darwin’s birthday. It’s 200 years since he was born, on the same day as Abraham Lincoln. Astrologers have made a lot of the fact that both men were deeply opposed to slavery. But today we’re going to talk about sexual selection. So this is a Valentine’s Day lecture. [Laughter]
Sexual selection is actually a component of natural selection. When Darwin looked at the extravagant plumage of the birds that you just saw, he thought there has got to be something special going on here. [Crew talk] When he saw all of these extravagant behaviors, he thought there must be something else going on, besides natural selection, and that’s because Darwin thought that natural selection, in some sense, was the survival of the fittest.
We now know that natural selection really is the survival of those that reproduce the best, and that sexual selection is a component of natural selection that is associated with mating success. So basically sexual selection is a case in which mating success is trading off with survival.
And to bring this home to you, as you sit here in your 18 to 45-year-old state–there are various ages in the audience–I’d like you to consider the fact that the ratio of male to female mortality, in the United States, starts to diverge early in life, and by the time one has hit late teenage and early twenties, the ratio of male to female mortality is climbing rather strikingly. This is from all causes; this is from external causes; this is from internal causes. And actually this divergence here is enough to account for the different life spans of human males and females, which differ by about four or five years.
Now that is sexual dimorphism, and sexual dimorphism in mortality rates, and it appears to be associated with blockheaded risk-taking behavior. It appears that males behave differently at those ages than females do. Now we don’t know whether that’s evolved and genetic, or whether it’s culturally influenced, but I invite you to consider those alternative possibilities and think how one might test them.
From animal studies, we know that the more polygynous the species, the greater the difference in male and female lifespan. There isn’t any difference in monogamous species. So if you look at swans or other monogamous birds, the males and the females live the same amount of time, but the more polygamous a species is, the shorter the male’s lifespan relative to the female’s.
So how does sexual selection work? It might account for that pattern. How does it work? Well if a change in a trait is going to increase lifetime reproductive success, by improving the ability of an individual to attract or to control a mate, or to achieve fertilization, it can be favored by selection, even if it lowers survival probability. This is a Woody Allen film about sex and death.
Now sexual selection will change traits that influence mating success until the improvement in mating success is balanced by costs in other fitness components; then the response will stop. This will not go on to the point where there’s wholesale slaughter and suicide. Although in some few cases there is sexual cannibalism during copulation–in the Australian Redback Spider– and the male does appear to insist on committing suicide. So that would be probably the most extreme case. But that’s not normal, that’s really an extreme case.
The normal case is that if there is sexual selection going on, then the sexes will diverge in their behavior and their morphology, and their traits and their behavior will be modified in ways so that they are getting better mating success, but they are incurring costs, in terms of survival.
Now this line of thought explains why organisms will take risks to mate–and often they will take extreme risks to mate– and it explains why juveniles develop secondary sexual characters only on maturation. That’s because the secondary sexual characters, which are the things that we think of as causing the two sexes to look different in any species, bear with them costs, and those costs can be imposed from a variety of sources. The costs may be that if you look like an adult, you’re going to elicit competitive behavior from other adults–you might get beaten up–or it may that it simply makes it much more difficult for you to escape a predator. So the costs can come in from all sorts of places.
The main questions about sexual selection are how did it originate? And we are now pretty well satisfied that before there was anisogamy, there could not be any sexual selection. There had to be gametes of different sizes, and there had to be individuals specialized on producing small ones and big ones before you could start getting things that functioned as males and females, and then evolve to begin to look like and behave like males and females.
The mechanisms of sexual selection are basically two: competing for mates and choosing mates. So there’s force and subtlety involved in this process. There’s a lot of evidence for it. This is, as you might imagine, since we are among the more sexual primates–we’re not quite as sexual as bonobos, but we are a relatively highly sexed primate–it’s not unusual that this is an area of biology that has attracted an awful lot of attention. You can find thousands of papers on sexual selection.
The strength of sexual selection is actually on the day of mating determined by the operational sex ratio; that is, the local ratio of males that are ready to mate with females who are ready to mate. Okay? So that’s just a brief outline of some of the main points.
Chapter 2. Competing and Choosing [00:06:52]
Now about competing and choosing. Competing and choosing have different consequences. It is the limiting sex that can be choosey–okay?–and normally individuals of the other sex compete. Now it is quite possible that if one sex is limiting, and the other sex is competing for the attentions of that sex, that the individuals among the limited sex can actually compete with each other for the attentions of the suitors. There’s nothing ruling that out. It’s just that I’m describing here the processes and forces that are stronger in the two sexes. That doesn’t mean that other things aren’t going on; there’s arguably a lot going on.
Generally speaking, mate competition will be stronger in the sex that has the greater reproductive potential, and it’s the one that should be competing for the sex with the lesser reproductive potential. And usually males have greater reproductive potential, females have lesser reproductive potential; so males compete and females choose, generally speaking. You’ll see that there are interesting exceptions.
Now, what should a female choose? I’ll already tell you this is a big topic in evolutionary psychology. David Buss, at the University of Texas, wrote a big review paper on this a few years ago. It’s highly controversial. It’s fun to read this stuff. It’s difficult to come to a hard science conclusion on it. Basically what Buss says is that males choose young females that look healthy and have high reproductive potential, and females choose males that have access to resources and are likely to contribute to child rearing. Okay? And he says this is cross-cultural; all cultures, all times, that’s what people do. Obviously that could be controversial, and there are a lot of refinements on that picture, and there’s a lot of evidence that could be better. But that’s what Buss claims. That’s for humans.
What about animals, where you can actually do experiments and do manipulations and ask them to tell you why they are choosing mates? Well here are some hypotheses. A female could look at a male and say, “Does he control important resources? Will that one be a good parent? Will it supply food efficiently?” Or, looking at it, “Is that potential mate healthy? Is it free of parasites, and is it advertising its ability to resist parasites and pathogens with a costly signal that gives me an indication that in fact that thing is honest and is not trying to deceive me?” Or, “Does that potential mate have traits that are attractive to sexual partners? Will I have a sexy son?”
Now this sets off a very interesting co-evolutionary process between the preference genes that are sitting there being expressed in the female’s brain and the expression of that in the form of some kind of sexy morphological trait in the male’s morphology. And these genes for preference and genes for attraction come together in the offspring, and that has very interesting consequences.
Well what do we know about this? André Dhondt and his students, who at that time were in Belgium–and André is now at Cornell–did an interesting study on Blue Tits. Now the point about Blue Tits, that makes them useful for this, is that they frequently have extra-pair copulations. That means that Blue Tits indulge in adultery a lot.
And because of genetic fingerprinting, you can go out and you can determine the paternity of offspring in this brood, where the male is having his primary residence, and in neighboring broods, and when the male flies out, perhaps to have an extra-pair copulation with a female in a neighboring territory, another male might fly in and have an extra-pair copulation with his female. And you can trace the consequences of this. Okay?
We define as unattractive guys that don’t get much action, and we define as attractive guys that get a lot of action. Okay? So that’s an operational definition of attractive. And if the unattractive males turned out to be smaller and they died younger than attractive males–so female Blue Tits are able to pick up on something about the physical health of the males that they’re looking at–the offspring of attractive males, in their own home brood, in their own home nest, lived longer than the offspring of unattractive females.
Now you could explain that by direct benefit rather than by females looking at some signal in the male’s morphology or behavior that he had especially good genes, if the attractive males had better territories and were better parents. That we don’t know a lot about. If the extra-pair offspring of the attractive males–so the babies that were laid in other nests, in other territories and were reared by other people; so they were foster-children in other territories–if they also had better survival, that would be hard to explain in terms of direct phenotypic reward, and would then indicate good genes.
So this is an example to show you how you can apply natural experiments, in the field, to try to settle the question of which hypothesis for sexual selection is correct–is it good genes, is it direct benefits?–and show you that some of the evidence had been collected by I think–2002, I think, is about when this came out. There may be more out there now and you can go find out. At any rate, this is something that can be done now with DNA fingerprinting and with small birds.
Chapter 3. Competition with Sexual Dimorphism [00:13:11]
There is a sexual selection maxim, which is kind of a default condition, and that is eggs are expensive and sperm are cheap, so females are limiting and are going to be choosey, and males have potentially higher reproductive success over a lifetime than a female, but probably also higher variance in reproductive success. And the consequence of this is that the lifetime reproductive success of the females is basically limited by the number of offspring they can produce; the lifetime reproductive success of males, by the number of females they can fertilize. That’s really asymmetrical.
That means that in this default condition the females become a limiting resource, and that sets off competition among males for mates. It allows females to choose partners. And so usually females are choosier than males, and males are more promiscuous than females. But there are a lot of exceptions. Right at the end I’ll show you a couple of very beautiful polyandrous birds where females hold harems of males, and their morphology has been changed so that in those species of birds they are the bright, dominant, colorful ones, that look like males. So this is a general principle, and you should note that sometimes sperm are actually more expensive than eggs; it’s not always true that sperm are cheap. Here’s a case.
This is a case in which the sex that is choosey changes in a plastic fashion as a function of how much food they have, and that’s because in Katydids males contribute nourishment to the females in their spermatophores. So a female is not only getting sperm from a male, she’s getting food from the male. Okay? Now if food is scarce, then female Katydids are actually reproductively limited by how many male spermatophores they can get. And at that point males back off, they don’t court so much, they get to be kind of coy, and the females fight over the males and the males become choosey.
However, when the food is abundant, the males are reproductively limited by the availability of females; females are getting a lot of food from sources other than spermatophores; males court and females are choosey. And you can take these and you can just flip them back and forth, from one mode to the other, just by how much you feed them. So that’s actually a nice test case, because here you’re taking the same individual organisms and changing them with an experimental manipulation between two different courtship modes; and it gets directly at this idea.
Now, what about competition for mates? There is a lot of armament out there in Nature. You’ve seen some of it. You’ve seen the antlers on deer. You’ve seen the tusks on narwhales. You’ve seen the tusks on elephants. You’ve seen that a male silverback gorilla weighs 450 pounds, and a female gorilla weighs about 120. There are major differences in body size of males and females in many species.
So these are set off–the evolution of that difference in body size is really driven by contests, scrambles and rivalries. And during these contests females can be sitting there, looking at the males go at it, fighting for access to them, and they could be choosing males for their competitive ability. So the fact that the males are fighting isn’t ruling out female choice, it’s making possible perhaps another kind of female choice. That does explain large, well-armed males, and the most striking examples of this dimorphism are in pinnipeds.
So here are two southern male elephant seals, okay? Bull elephant seals. By the way, these guys are very superior divers. They will go out offshore, oh 100 miles or so, and dive down to a depth of oh between 1000 and 3000 feet, to fish for squid, and they’re very good at it; they’re kind of like little mini whales cruising around out there. And a male elephant seal will spend about nine months a year storing up a lot of food, because then he’s going to haul up on a beach and try to protect a harem on the beach, and try to chase off other males and fight vigorously. And during that three months he doesn’t eat. He gets extremely grouchy.
Now it is possible for these guys to control a harem of about forty or fifty female seals on the beach. And it’s not easy to run down a beach if you’re a 4000 pound male elephant seal. You are built for swimming sleekly through the water, chasing squid, not for humping along the beach with your four flippers. Okay? But these guys do, for three months, and they get all beaten up, chasing off juvenile males that are coming in and trying to sneak copulations with their harems.
And, because of this spatial situation, if you’re a juvenile male, you can actually give the females a little chance to make another choice. So the juvenile males hang out sort of on the boundaries between the harems of the big dominant males, and try to sneak copulations. This sets off a situation of controlled chaos that goes on for three months, and everybody gets exhausted fighting.
If you look at the ratio of male body length to female body length, and look at the harem size that a pinniped can control–what you have here is data on different pinniped species. This, I think, is a harbor seal, which is virtually monogamous; so harbor seals tend to mate for life. But an elephant seal will be about 1.6 times as long as a female, and that means it’s how much heavier than a female? What do you do with that 1.6 to estimate the difference in weight? Can’t hear you.
Student: Cube it.
Professor Stephen Stearns: You cube it. That’s right. So that means that the male elephant seal is on the order of five or six times heavier than the female. So if he’s 4000 pounds, she’s about 850 pounds, something like that.
And that’s a pretty strong relationship in biology, for sexual dimorphism versus harem size, and that illustrates the importance of competition for mates in controlling the evolution of size differences between males and females.
Now the one that’s probably a bit more fun is mate choice. I had a graduate student in Switzerland who went to a behavioral ecology meeting in Sweden, at a time when mate choice was a hot topic in the literature. And it is a standard feature of scientific meetings that in the evening there’s a bar, and people are talking at the bar, and so she quietly went around and decorated males with various things, like feathers or whatnot, and then stood back and took notes to see how long they got to chat up a woman, depending upon what kind of decoration they were wearing. And she showed a very significant effect, and the ones that looked weirdest actually got the most attention. [Laughter] So, just a tip. Okay? [Laughter]
Now if you’re really choosey and you’re able to detect high quality territories or good genes or sexy sons or something like that, you can improve your fitness by being choosey. But remember to be choosey you have to take time. Choosiness is essentially shopping. Right? Shopping takes time. If you shop too long, you may miss the opportunity because the shelf will be empty. Okay?
So organisms should be careful, but not too careful. There’s kind of an optimal waiting time, and after awhile, as the season progresses, for a seasonally breeding organism, there’s going to come a time when mating with anybody is better than not mating at all. So this business of being choosey is constrained by time.
Now choice based on an immediate phenotypic benefit–that means I’m going to choose this mate because he’s got a great territory and I’m going to get a lot of food, or I can see that my babies will have a lot of food; that would be an immediate phenotypic benefit–that can explain a lot, but it won’t explain extravagant male morphology or leks, and very often extravagant male morphology and leks are associated with each other.
You just saw some in the video at the beginning. That Bird of Paradise was on a lek, and I’ll show you some more in a minute. So things that lek are peacocks, sage-grouse, Birds of Paradise. And a lek–it’s one of the few Swedish words which has been appropriated into English. Lek in Swedish means–has two meanings. Meaning number one is a sports place. So you can actually drive down a road in Sweden and see a little sign that says Lek, and you’ll see a sports field. But it also means the same thing that it does in behavioral ecology in English, which is a traditional display ground where males come year after year to advertise and try to attract females to mate with them. Okay?
And this is a mating system in which the males are then not going to go off and take care of the babies. The males sit there and display and fight with each other. Females come and mate with them, and the only thing that the female gets from the male is genes. These are some things that lek. This is a sage-grouse at Malheur National Wildlife Refuge in Oregon. This is a Bird of Paradise and this is a peacock. Now these males–these are all males–have pretty extravagant morphology, but they not only have extravagant morphology, they have extravagant behavior.
You saw what that Rifle Bird was doing, waving its wings back and forth. Anytime you see something like this, you can bet that if you were out there and you saw it in action that there would be feathers in motion; there would be dances going on; there would be really elaborate stuff that these birds were doing with their behavior to attract mates.
For example, the male sage-grouse in Malheur in Oregon go to the lek in January. In January, in Eastern Oregon, the temperature can be 20 degrees below 0. There’s snow on the ground. These guys are getting up in the dark, before sunrise, to get out there on the lek, to try to get a one-up on the competition. They puff up their breasts and they make a popping sound when they do so, and then after they pop they coo. So they go pop-pop, coo, coo, pop-pop, coo. You can hear them a kilometer away. And they make themselves as visible as possible.
Around the lek there are lurking coyotes. Overhead, cruising through the air, there are Golden Eagles. These guys are taking major risks to get up at five in the morning to go out and try to make love in the snow. You know? [Laughter] That’s a serious modification of male behavior. Talk about being a prisoner of your hormones. These guys are in jail. [Laughs]
So what kind of an experiment could you do in the field to try to decide what’s a female looking for? Well Malte Andersson had a great idea. He wanted to work on the African Widowbird. African Widowbirds have–the males have naturally long tails, and they control territories within which two, three, four, five females might nest.
So what Malte did–by the way, he did this in Masai Mara, in Kenya–he shortened tails on some Widowbirds by simply cutting them off with scissors. On his control group he cut the tail off and glued it back together, so it didn’t change in length. And then on his experimental group he cut the tail off–he took the cut tails from the short-tailed ones and glued them on to make super long tails. Okay? So he had three groups. He had short-tailed controls and real long tails.
Now the ones with shortened tails only averaged half a nest on their territory, and the ones with the lengthened tails averaged nearly two nests on their territory. Individuals were assigned at random to these different groups. And so the data indicate that female Widowbirds were building nests on territories of males with longer tails. And then the question is, if that’s such a great thing, if you’re going to double your reproductive success with a longer tail, then why don’t you already have it? Darn it, why hasn’t evolution done that to you?
Well the answer is probably that natural selection is preventing a further increase in male tail length because females are preferring much longer tails than are found in natural populations. It’s fun to give this part of the lecture in German, because those of you who know German know the double meaning of Schwanz in German; it’s what you imagine.
Chapter 4. Honest, Costly Signaling [00:27:29]
Okay, there’s another hypothesis, and that is what is a female looking for? Well she’s looking for an indication of good genes, under this hypothesis, and that would mean that a female should prefer a male displaying an honest costly signal–notice here I’ve put honest and costly together–an honest costly signal that they contain genes for superior survival ability; for example, genes for parasite and pathogen resistance, even for different MHC alleles.
Now the vertebrate immune system is partially integrated into the vertebrate nervous system. The two systems can send each other information. If there was a way that your sensory system could pick up information on the composition of the MHC alleles, in a potential partner, and send it to your brain, that would affect your mate choice.
This leads into something that you may enjoy looking at. If you’re into ISI Web of Science, type in T-shirt Experiment, and look at the impact of body odor on how attractive a potential mate smells. Okay? It turns out that if you do that, and then you do the DNA sequencing to see whether or not the people who are reporting attractive or unattractive have similar or dissimilar MHC alleles, the ones that are reporting that a smell is attractive are the ones who have different immune genes, and the ones who are reporting that a smell is repugnant have similar immune genes.
And the way the immune system works in the offspring, to resist infectious disease, is by generating diversity within the body, and it can only do that if the genes are different. So you have to find a mate with different MHC alleles if you want to have disease resistant offspring. And there is some evidence in humans that in fact we do react to scent and that there is information in scent. By the way, this is well established again–it’s not so well established in humans because we can’t do manipulation experiments–but it is well established in mice, that mice do this.
And where people make mistakes and they do mate with people who have similar MHC genes, they get into a situation where there are multiple spontaneous abortions. So it appears not only that there is a level of selection at mate choice, but there is also a level of rejection of zygotes that are potentially not going to resist infectious disease. That’s work by Carole Ober at the University of Chicago, working on Hutterites. So interesting stuff here.
Now what kind of evidence have we got? Well if a male produced an ornament in order to advertise that he was resistant to disease, then you would expect that male fitness would decrease with increased parasite infection. So that would be an assumption behind it; that would be the selection that was driving it. The condition of his ornaments should decrease with increased parasite burden. So the less he was able to resist the parasites, the less dramatic an ornament he would be able to express. So that means that that ornament’s got to be costly.
Then there must be some heritable variation in resistance, or there wouldn’t be any response to selection; just go back to the first basic four conditions for selection: that must be there. And if this holds, then females should be choosing the most ornamented and the least parasitized males. And there are three cases that are pretty well worked out where that’s exactly what appears to be going on.
So what female guppies appear to be looking for are orange spots in their males. What female pheasants appear to be looking for are red irises around the eyes of the males. And what female barn swallows appear to be looking for are nice long symmetric tails; they like the symmetry of the male’s tail.
Now that would be a good genes argument. The male’s got a gene for parasite resistance, so I’m going to make with him. That’s a good gene. And that is where the Fisherian process of sexy sons would start. Okay? So I’m now shifting into the argument for the third hypothesis, which is you choose a male because you think that if you have a son by him, that son will get a lot of matings.
So preference for good genes will select for the preference itself, and that makes the preferred trait an object of selection, and it explains the evolution of ornamentation. You can see it like this. Suppose the reason a female guppy likes a male with an orange spot is that the only way he can make that orange spot is if he gets carotenoids out of the crustacea that he eats. So if he’s really good at finding high quality food, he can make a bigger orange spot. So it’s advertising his foraging ability. Okay? That’s a good thing.
So that gene for foraging ability comes together in the offspring with the gene for the preference, and because the male has better foraging ability, the gene for the preference will hitchhike on the reproductive success of the gene for the foraging ability, and females will develop stronger and stronger preferences for a male with orange spots. So that’s how the process gets going, at the beginning. It’s thought that initially the preference develops because it’s a preference for a gene that actually affects reproductive success in your offspring.
What if now that guppy population moves into a new habitat that doesn’t have any crustacean in it, and it becomes difficult perhaps for the males to make carotenoids? But they still can, they can still make orange spots. But the females have the preference. The male is no longer giving off a signal that’s reliable in terms of good genes. All he is signaling is that he’s attractive to females.
Well now evolution has a new reason to maintain that selection. It’s a selection simply for the attractiveness of the offspring, because a component of reproductive success is mating success, and by choosing a male that has an orange spot, the female is also choosing mating success in her sons.
So that leads to runaway selection for sexy sons. So females are preferring males with higher fitness. Their preference genes get united in their offspring, with the male’s genes for higher fitness. The female preference genes then hitchhike on the male’s fitness genes. Once their preferences are established, they work on male traits that are otherwise neutral, or maybe even disadvantageous, except that they are preferred by females. So they lead to success in mating. So then, if all that happens, then mothers will gain in fitness by selecting fathers with heritable traits that make their sons attractive to females in the next generation.
In the threespine stickleback, the intensity of the son’s red coloration is correlated with the daughter’s preference for red. So what I’ve just told you about the genes for preference and the genes for the male trait coming together in the offspring happens to be true, in sticklebacks. But when you then ask, “What’s the deal with red color on the belly of the male stickleback?”, you discover that it’s not only genetically correlated with female preference; bright red males resist parasites. So by choosing them, females are avoiding parasitized males, and that’s also satisfying a good genes hypothesis.
So you can see that in this case, the data in fact do not distinguish between the two. It’s not like it’s either/or. It looks like, hey, both things are going on at the same time. Females are choosing sexy sons who are also parasite resistant.
Chapter 5. Selection through Perception and Polyandry [00:35:55]
Now there’s a third possibility, and that is let’s suppose this kind of thing has been going on in the past and females have developed certain sensory abilities, to perceive the potential mates. You guys are not particularly good at perceiving potential mates in the ultraviolet. Okay? Bees are, but you’re not. So your sensory capacity is limited to certain windows, both with your eyes and your ears and your taste buds and everything else. And the idea is that the sensory capacity inherited from ancestors would bias the traits. Females might just be selecting males they can see or hear especially well.
So one well worked out example is that the female eardrum in a Tungara frog is tuned to receive some frequencies better than others. Here is a male Tungara frog in Panama calling, and his call signal is being picked up by the ear of the female, and he can’t change his call signal out of the frequency range that that female ear can hear. If she is attracted to him, he has reproductive success. But unfortunately, that’s a perfectly fine frequency for a bat to hear. And in fact a fringe-lipped bat is a frog eating bat, and it does exceptionally well during frog breeding season by swooping in and munching up the males who are dutifully calling to try to attract their females.
Now the idea here is that the male can’t evolve out of a frequency range that the bat can hear because the female’s eardrum is constrained to be a certain size; that’s the sensory bias hypothesis. Now, so I’ve stepped through the main hypotheses that are thought to drive female choice. So female choice could simply be for things that she can hear or see particularly well, as well as for sexy sons or direct benefits or good genes.
Now what is the actual context in which this is going on? Well it’s when mating is happening, when choice is being made. And that’s where the operational sex ratio comes into play. It basically is determining the opportunity for selection, and it varies with mating system and parental care. So sexual selection, which produces striking differences in the behavior and morphology of males and females, is correlated in ecology and evolution with mating systems and patterns of parental care. These things all go along with each other to form syndromes of traits.
In monogamy, equal numbers of males and females have offspring, and each sex has one partner, and there’s not very much difference in the operational sex ratio and not much opportunity for sexual selection. In polyandry there are more males than females, that have offspring, and each female has two or more male partners. So there is great variance in reproductive success among females; there’s less variance in reproductive success among males. In the reverse pattern, in polygyny, more females than males have offspring, and each male has two or more female partners; that leads to harems and leks. And then there are things like polygynandry where each sex may have several partners.
There’s a little brown bird, an LBB, called the Dunnock. It looks a little bit like a hedge sparrow. It lives, among other places, in the botanical garden at Cambridge University in England, where Nick Davies has studied it for a long time, and it had the notoriety of being regarded as an exemplar of marital fidelity, until Nick studied it. And once Nick got in there with his DNA fingerprinting and did paternity analysis, he discovered that Dunnocks are polygynandous. So each sex is mating with several partners. A female sitting on a nest will have eggs in it that were inseminated by several males, and the father who’s bringing food to that nest has his genes in eggs that are in several other nests. So that’s polygynandry, and well exemplified in the Dunnock.
Now the effects of mate choice are thought to be especially strong in these two circumstances here. That’s where one sex has relatively little variance in reproductive success; the other has large variance in reproductive success. One sex is limiting; the other sex is competing. One sex is choosey; the other sex is fighting.
I told you that I would show you some polyandrous birds. This is a Wattled Jacana, and the Jacanas–we ran into them on the Amazon–a Jacana female will hold a territory within which three, four or five males will build nests, and she will go around and lay–they’ll fertilize her and she’ll lay eggs into each of their nests. But she doesn’t spend any time raising the babies, she just fights off the females on the neighboring territories. So males take care of the babies and mom fights for the territory; and that happens in the Wattled Jacana.
This is a Phalarope; this is a female Phalarope. They are basically related to the Sandpipers; so they’re shore birds. They live in the far Arctic. And a similar pattern. The male sits on the eggs. A female holds a territory within which three, four, five males have nests. And there’ve been some very interesting studies done on the mating physiology of Phalaropes.
When a female Phalarope is displaying, in her courtship display, and attracting males, her ovaries are expressing testosterone. You could imagine that the courtship display of the female Phalarope had evolved in an ancestral male, and that all of the control machinery for that was all set up, and all you had to do to get it expressed in the other sex was turn on the testosterone at the right time; which is what her ovaries do when she’s displaying.
When she then goes over to the nest to lay the eggs, her ovaries secrete estrogen, and she–her physiology is switching back and forth between a male display in competitive cycle, driven by testosterone, and a female lay egg and make yolk and lay egg cycle, which is controlled by estrogen.
So these sorts of secondary sexual traits are not only associated with mating systems and patterns of parental care, they also set off a cascade of physiological integration throughout the organism, and uncovering that leads one into a whole host of interesting physiological questions.
Chapter 6. Summary [00:43:35]
So to summarize sexual selection. It’s a component of natural selection in which mating success trades off with survival. It’s not a separate kind of selection, it’s part of natural selection. It accounts for many of the attractive ornaments of plants and animals. It raises the interesting issue of aesthetics and why our brains see things as beautiful that other things find attractive.
There is contest competition for mates which are the scarcer reproductive resource, and that will explain a lot of sexual size dimorphism–bull elephant seals, things like that–particularly in polygenous species. We’ve seen there that the degree of dimorphism is directly related to the degree of polygamy; the bigger the harem, the bigger the size dimorphism. There has been a lot of documentation actually of active choice of the non-limiting sex sometimes. So that’s going on.
And whether or not this indirect mechanism–this Fisherian runaway process, that leads to preference for sexy sons–whether that is really needed to explain ornaments in lekking species, it seems to be logically the only possible explanation left standing. It’s as though you have ruled–you’re Sherlock Holmes and you’ve ruled out all of the other hypotheses. But getting positive evidence for that has been difficult. Okay?
So this looks plausible. I think it’s likely, but at this point the positive evidence indicating that that has been under selection is still out there. Getting whole genomes of some of these organisms might solve it, because you can then look for signatures of selection, if you can identify the preference genes; that’s a long-term project.
So let’s go back to this. I’d like to leave you with this. This is you guys. Here you are; you’re probably somewhere around this age group. And the mortality rates of the males in the audience, on average–every insurance company in the country knows this; every time you apply for car insurance this difference manifests itself in your monthly bill–the mortality rates for male humans at this age are several times those of female humans, and they appear to be taking risks. So I’ll leave you with the unanswered question of whether that’s a product of sexual selection or not. And next time we will talk about speciation.
[end of transcript]Back to Top