Nature’s Mating Dance: Do Females Prefer Honest Males?

[Music] if you're watching this on YouTube you might have noticed that this episode is a week delayed but if you want to get early access to our episodes consider becoming a paying member if you enjoyed this episode please subscribe and share it with your friends thank you for all your support I'm sometimes engaged to give lectures on ships in places like Galapagos I can't date this particular occasion I suspect it was one of the Expeditions organized by the center for inquiry CFI the charity which later merged with my own Richard Dawkins Foundation this lecture was on sexual selection and sex ratio Theory why are the Sexes evenly balanced even in species like elephant seals where a small minority of males monopolizes the females right I thought what I do now is as I say talk a little bit about sex ratio Theory and uh perhaps about sexual selection if there's time um those of you well I suppose we all probably saw the sea liines out on the beach on the SE seaon Island and our guides pointed out the big bull sea liines patrolling their area of Beach and each uh dominant male had a harim of females and elsewhere there were Bachelor males who never got a look in and in some species of um seal and celan this is Extreme the the inequality this the sheer unfairness of some of the males getting all the action and other males getting none is uh remarkable now it probably occurs to many people of an economical Turn of Mind why are there so many males if only a small minority of males does all the population in in California elephant seals for example it's something like uh 90% of the males never do any copulating well why doesn't natural selection favor a shifted sex ratio so that most of the animals are female and just enough males to service those females and this is the darwinian theory of sex ratios Darwin himself didn't solve it it was solved Again by the great ra Fisher the Ronald Fischer uh who uh wrote his great book in the 1930s and here is how Fisher solved the problem of sex ratios why are sex ratios 50/50 or close to 50/50 even in those species where a minority of one sex usually the males does all the mating here's how the reasoning goes every baby born has exactly one one mother and one father obvious everyone knows that but it has remarkable consequences put yourself in the position of an animal a parent a mother or say say a mother trying to decide whether to have a son or a daughter now of course we're not saying that animals actually do decide whether to have a son or a daughter the kind of convention that darwinian adopt is to put ourselves in the position of an animal and imagine that the animal is doing its best to maximize the number of its genes that survive into future generations to maximize the number of its great great great grandchildren there another way to put it so put yourself in the position of a of a mother trying to decide whether to have a son or a daughter and imagine it's a mother elephant just a seon a mother a mother seon if you have a daughter you can be pretty sure that your daughter will give you grandchildren because your daughter is almost bound to be the me be a member of the harim of some male or another but if you have a son the chances are your son won't give you any grandchildren at all because he's almost he's likely to be a bachelor but if by luck he happens to be a a dominant male then he'll give you dozens of grandchildren and so the average of no grandchildren at all from the unsuccessful son and uh lots and lots of grandchildren from the the successful son works out exactly the same as the average expectation of reproduction that that you would get if you had a daughter um now normally we're not dealing with seaons normally we're dealing with animals like us which um which uh don't have this extremely biased um system where a minority of males does all the mating the principle applies there as well of course since every child has one father and one mother if you're trying to decide whether to have a son or a daughter if the sex ratio were anything other than 50/50 You' be better off having a child of the minority sex if if there is a minority sex you're better off having a child of that Minority sex not because it'll be easier to get a mate that's not the right way to look at it but because since every child is born of one mother and one father the total number of descendants that are going to be born in that species the total number of animals alive A Thousand Years hence is going to be derived from all the males and females that are at present in your in the population and if there are say more females than males then the total share of that posterity which each male on average can expect to have is higher by simple virtue of the Rarity of males so if there are twice as many females as males on average a male has got to have twice as many descendants in the remote future than a typical female that of course doesn't mean that every individual does but on average the the contribution to the animals of the future of a of a typical individual will be uh proportional to its Rarity value in the population at present so natural selection stabilizes the sex ratio if the sex ratio were anything other than 50/50 natural selection would bring it back to 50/50 now now if you remember from school biology you'll know that in mammals sex determination is done by ex chromosomes and Y chromosomes so you may be saying to yourself well this is all very well but animals don't have a choice I mean you know you there there's a mechanism which which causes there to be 50/50 and it's the xxxy uh system but that's just um a detail that's that's not the fundamental darwinian question the fundamental Darwin question is why in the first place did a mechanism arise that produced a 50/50 sex ratio and of and there are ways of biasing the sex ratio anyway the the xxxy system doesn't produce a perfect um 50/50 there are all sorts of ways in which animals can and do um uh influence the sex of their offspring perhaps by feeding Sons more than daughters or feeding daughters more than Sons or something of that sort Fisher took the theory some steps further he thought in economic terms now imagine that the economic costs of making one sex say a son imagine that it costs more to make a son than to make a daughter perhaps you have to feed Sons more than you have to feed daughters now let's go back to the hypothetical situation of a mother quotes trying to discide decide whether to have a son or a daughter since it costs more suppose it costs twice as much to make a son as to make a daughter the real economic choice that faces this mother is not shall I have a son or a daughter but shall I have a son or two daughters because it because daughters are cheaper to make daughters cost half as much to make so for the same amount of of food resources whatever the whatever the cost is for the same amount of res sources whatever the Seine equivalent of money is you can either buy one son or two daughters under those conditions if the economic costs of making one sex is is um greater than than than the other then natural selection will stabilize the sex ratio not at 50/50 in numbers but at 50/50 as measured in economic cost so if it costs twice as much to make a son as to make a daughter natural selection will stabilize the sex ratio at twice as many females as males the the costs that we're talking about are the costs borne by the parent or more strictly by the decision-making entity by The Entity and it usually would be a parent whose genes are instrumental in deciding whether to have a son or a daughter or whether to feed a son more or feed a daughter more and in practice this will often mean that if you really want to do the Fisher calculation properly what you have to calculate in order to say what it is that stabilizes at 50/50 it is the total investment of effort food energy whatever it takes the total investment by the parent up to the end of the investment time up to the end of the period of childhood rearing when the baby leaves dependency on the parent if subsequent to leaving the parental care one sex grows bigger than the other say males grow which they clearly do in sein grow grow bigger if that happens after the end of Parental care then that does doesn't enter into the Fisher calculation that's something else right that's all I want to say about sex ratios um we we end at quarter two don't don't we think so if you'd like I could say a bit about sexual selection um we've seen some nice examples uh perhaps the most spectacular we've seen is the great big red balloon of the male frig birds and uh you'll be familiar with things like pheasants and peacocks and almost I mean very very often when you see very very bright colors birds of paradise um Darwin and Wallace the co-discoverer of uh natural selection were both extremely interested in these bright colors and Darwin developed uh what he thought of as a as a separate Theory the theory of sexual selection which he mentions in the Origin of Species but develops more uh um intensively in The Descent of Man his his uh later book which is actually The Descent of Man is not so much about humans it's more about sexual selection as Darwin realized it's not enough to survive the whole point about natural selection is reproduction and because individuals often compete with members of the same sex for mates of the opposite sex anything that causes an individual to win that competition will be favored by a form of natural selection if a male frig bird has a great big round tomato like um balloon on its chest and if females like that if females are attracted to that then uh genes for making great big red balloons will get passed on to the next Generation even if the Red Balloon makes the individual male less likely to survive as an individual so there'll be a trade-off and very often sexually selected characters do make the individual more likely to die perhaps it's it's more clumsy in Flight perhaps its conspicuousness um is is attracts Predators or something of that sort so Darwin developed this theory of sexual selection and he used it to account for peacocks and birds of paradise and pheasants and and and indeed humans he he made great play of it in discussing the evolution of humans Wallace hated the idea Wallace described himself as more darwinian than Darwin and in that this respect he was he was less darwinian than Darwin in another respect that in his old age Wallace started flirting with religion and and um mysticism and things but in the in terms of sexual selection Wallace was more darwinian than Darwin Wallace hated the idea of sexual selection he wanted everything to be useful for survival and reproduction but not attraction so for Wallace he was always reluctant to explain bright colors as due to sexual selection and he pointed quite rightly to the fact that we got plenty of bright colors if you open any one of us up it's bright red nothing to do with sexual selection bright red for a perfectly good physiological reason and Wallace was always eager to discover uh utilitarian reasons for the sort of bright colors that Darwin ascribed to um sexual selection this distinction between Wallace's views and Darwin's views by the way is due to the philosopher and historian of science Helena Cronin uh whose beautifully written book The Ant and the peacock I very strongly recommend now one of the problems with Darwin's idea of sexual selection and one of the things that Wallace hated about it was that it just assumed as a given that females like certain colors or certain shapes and uh Wallace could just about stomach the idea of females liking these shapes and colors and thereby providing the selection pressure to produce things like peacock's Tales but Wallace wanted there to be a utilitarian underpinning he wanted it to be an advertisement that this is a good fit male he wanted the peacock's tail or the uh frig Bird's red balloon to be some kind of a genuine indicator to females that this is a good male this male will be a good parent this male will be um will be a good male to mate with Darwin was perfectly happy to say no the females just like it it's all there is to it it's just just Aesthetics later on after both Wallace and Darwin were dead again ra fiser uh solve the problem for Darwin for for sexual selection in the following typically ingenious way he said all right let's take female taste but don't let's do what Darwin did and just say well females just like it just like the red s let's make female taste itself be under genetic control and subject to natural selection in its own right so you have males with their red balloons and you have females with their brains that may or may not like red balloons and both the Red Balloons themselves and the female brains with their taste are themselves subject to Natural Selection now fer then did some intuitive mathematics he said he say typical Fisher it is easy to see that under such conditions there will be this is not his his exact words there will be an an ever increasing runaway process whereby the uh red he didn't talk about Red Balloons but where whereby the the male characterist istic the the red balloons and the female taste for the same thing run away together and increase in an inflationary manner together because both of them are being naturally selected at the same time fiser said it is easy to see nobody else could see that it was easy to see until uh the 1970s when mathematical biologist started to uh do what to to reconstruct what must have been Fisher's ma mathematics Fisher didn't bother to uh to actually write down and it works approximately like this every baby born whether male or female inherits the genes from its father for having a red sack and the genes from its mother no I'll put that more generally every baby born inherits of of either sex inherits from its father the genes that made the father look like he does and the genes that made the mother like what the father looked like it doesn't matter what it was it could have been a red sack could have been a blue sack it it could have been a sky blue pink sack but whatever it was every chick born of either sex inherits from its father genes for having that characteristic and from its mother genes for liking that characteristic and the the genes in the in the boy chicks don't manifest themselves as taste and the genes in the girl chicks don't manifest themselves as red Sachs but each of them have the same genes and that's the key to the theory if if you plug that into the mathematical model that under at least some circumstances gives rise to a runaway process an in an exponentially inflating process whereby the characteristic in in our case the red the red sack and the uh the preference for that characteristic grows and grows and grows in inflating Manner and it could have been anything it could have been a green sack it could have been a blue sack uh it could have been a a purple tail doesn't matter what it is what what whatever it starts off has that will be hooked onto by this runaway process and Will Go On into inflating until uh it's brought up short by utilitarian pressures the sheer weight of death um death pressures because the thing is becoming so big and so conspicuous and so dangerous that's one version of the model and there are various other versions that don't have that consequence that's Fisher's mathematization of the darwinian theory of sexual selection and it's probably true at least in some circumstances however Wallace you remember Wallace hated the idea he wanted it to be utilitarian and Wallace too has his modern Descendants the most dramatic of these theories is the so-called handicap theory of the Israeli zoologist amot zahavi everybody can see that these characteristics are handicapped I said almost certainly they lead to the death of the individuals in some cases carrying around a great red sack must be a handicap in all sorts of ways carrying around a peacock's tail must be a handicap and uh everybody else had thought that it evolves in spite of being a handicap and the Fisher model suggest that it evolves in spite of being a handicap zahav's model says it evolves because it's a handicap it's a handicap which advertises to the female as zahavi says look what a good fit strong male I must be because I've survived with this rig great balloon hanging off my chest for many years and I was one of them uh in the selfish Gene all just about every zoologist ridiculed zahav's idea it just seemed too counterintuitive it didn't seem to work mathematical models that people tried mayard Smith tried uh um and it didn't work then uh much later Alan graphin a very brilliant theoretical zoologist theoretical biologist at Oxford and I'm happy to say one of my graduate students developed a mathematical model which once and for all showed that zahav's model could work highly ingenious model the the theory should now be called the the zahavi graphin handicap Theory uh he showed that it does work I think I probably haven't got time to to go into that uh but um it looks as though the handicap Theory probably does work at the same time Bill Hamilton who featured in the last slide that I showed uh yesterday with the great big straw hat in the Amazon on in a boat on the on the Amazon devel a kind of version of it uh to do with parasites and the idea that males are advertising to females um that they are healthy and the idea here this is another wallian theory of sexual selection the idea here is that one of the most important things a female can do when choosing a male is to be in effect a good diagnostic doctor to choose a male who's healthy using all the cues that a doctor would use unfortunately females don't have the benefit of uh thermometers and blood tests and x-rays and things which is what ideally they would do so they do the next best thing which is to look at the male assess his plumage assess his coloration uh if they if there's any chance of seeing the color of his blood then that would be of benefit to the females at the same time and this is the paradoxical aspect of the theory which is rather related to the handicap Theory modeling has shown theoretical model mathematical models have shown that it is also of advantage to males to advertise their health honestly to females even if they're unhealthy doesn't mean literally if you're unhealthy advertise the fact that you're unhealthy what what it means is advertise your health regardless of whether you're healthy or not and do it honestly and in a way this is an aspect of the handicap principle the handicap principle is a way in which the costs of doing something are an honest advertisement of the truth value great big padded shoulders to demonstrate your muscularity would soon be seen through they don't work you have to actually do something you have to actually Lisk weights or something like that in order to show that you really are a strong healthy male and uh similarly with the with Hamilton's Health Theory so in the Hamilton Theory we have natural selection on females to become better and better diagnostic doctors or vets rather and pressure on males to become more and more honest advertisers of their true state of health so something that brings blood to the surface as in crests as in uh the the blushing that um monkeys do on on on their bottoms um or blushing itself in humans blushing in the in the face anything that brings blood to the surface could be regarded as revealing deliberately revealing to females uh how healthy you are the long tale of a bird of Paradise in Hamilton's interpretation develops its great length because one of the symptoms of ill health in a bird might be diarrhea and if you have diarrhea it's likely that your tail will be dirty and the longer your tail is the harder it is to keep clean and so a male that has a long tail is advert a long clean tail a a long tail in good condition with nice bright feathers is advertising his health so natural selection enhances amplifies characteristics which show which demonstrate to the Keen eyes of the female uh what is healthy and what is not so we have two families of sexual selection theory around today the Neo darwinian fisherian theories and the Neo wallian uh theories of um advertisement of quality and uh I'm not coming down in favor of one or the other I think they they both may be true in different animals May both be true even in the same animals um and it I think it's important to to for us to learn about sexual selection as the other great um contributor to Evolution as well as ordinary natural selection which is um it's about survival and reproduction but not about sexual competition the other part by the way of Darwin's theory of sexual selection is not about males competing for females but about males directly competing with other males by by fighting them that was a very very brisk run through um sex ratio Theory and sexual selection Theory um we got about one minute for for any questions if anyone yeah I've been puzzling over the U can you wait wait for the mic I've been puzzling over the question of the Galapagos tortoises that we learned about at the darland research station in which the each individual embryo has a genetic ability to produce either not only a male but also a female and whichever sex it becomes uh is determined by temperature at which the embryo is incubated and the parent tortoises uh control the sex ratio by the death at which they bury their young and um so if uh say in the process of climate change uh it becomes more difficult to produce the one whichever one is produced by the lower temperature then and if they don't become extinct would you would expect this temperature preferences to Evol yeah I mean that's actually one one theory for what drove the dinosaurs extinct it might it might have um something like a global shift in temperature leading to them all coming out one sex um f first notice that in those species like like many reptiles in which uh sex is determined by temperature that makes it much easier for the whole sex ratio Theory to work because then the idea that that that a that a female can control the sex of her Offspring becomes more plausible because obviously it's more easy to manipulate temperature than to manipulate uh sex chromosomes sex chromosomes is another way of doing it um you probably know that in birds and in lepidoptera in butterflies and moths the the same kind of xxxy system is there but it's the other way around so whereas in mammals the XY is the male the XX is female in birds and in lepidoptera it's the other way around um there are lots of different ways in which sex is determined and whichever way sex is determined natural selection will be working above it to ensure that there's a 50/50 sex ratio of um uh of investment so um the in the case of temperature dependence it will still come out 50/50 or at least it should according to Fisher's Theory and so the threshold for switching from one sex to the other will be carefully adjusted by natural selection so that on an in an average year you get uh 50/50 but as you can see it's a very hidden Miss mechanism but natural selection will certainly be in there adjusting the temperature threshold um to to give an overall expected result of 50/50 as I understand surv not I'm wondering what role you think accident plays along with natural selection and sexual selection accident of course plays an enormous role in the um in in the the kinds of animals that populate the Earth why on Earth Steven Jay G or anybody else to think anybody is surprised at that I don't know I mean it's perfectly obvious that accident plays a huge role in in in the species that are around um um you know the dinosaurs were wiped out by a by a comet that hit the earth pure accident it's not a selective event has nothing to do with natural selection uh but it's clearly very important um he talks as though um he says things like if picaya in the Precambrian had gone extinct we might not be here well of course we wouldn't be here if picaya had gone extinct it easily might have gone extinct um we probably wouldn't be here if the third dinosaur to the left hadn't sneezed at a particular time I mean all sorts of things ramify and and have um complicated effects we are here by the thinnest of threads of luck uh every one of us is if you enjoy this episode you can show some support by subscribing to the podcast sharing it with your friends and leaving a review [Music]

Channel: The Poetry of Reality with Richard Dawkins

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Length: 32min 45sec (1965 seconds)

Published: Tue Apr 30 2024