136 お 4 ″ん研励ど g ど ra 々パ theory 0f ageing (page 42 ) , women in the natural state became gradually less efficient at bringing up children as they got older. Therefore the life expectancy of a child of an old mother was less than that of a child 0f a young mother. This means that, if a woman had a child and a grandchild born on the same day, the grandchild could expect to live longer than the child. When a woman reached the age where the average chance 0f each child reaching adulthood was just less than half the chance 0f each grandchild 0f the same age reaching adulthood, any gene for lnvesting in grandchildren in preference t0 children would tend t0 prosper. Such a gene is carried by only one in four grandchil- dren, whereas the rival gene IS carried by one in tWO children, but the greater expectation 0f life 0f the grandchildren outweighs this, and the 'grandchild altruism gene prevails in the gene pool. A woman could not invest fully in her grandchildren if she went on having children of her own. Therefore genes for becommg reproductively infertile in middle age became more numerous, S1nce they were carried in the bodies 0f grandchildren whose survival was assisted by grandmotherly altruism. This is a possible explanation Of the evolution Of the meno- pause in females. The reason why the fertility 0f males tails 0 仕 gradually rather than abruptly is probably that males do not mvest SO much as females in each individual child anyway. Provided he can sire children by young women, it will always pay even a very Old man tO invest in children rather than in grand— children. SO far, in this chapter and in the last, we have seen everything from the parent's point of view, largely the mother's. We have asked whether parents can be expected tO have favourltes, and in general what is the best investment policy for a parent. But per- haps each child can influence hOW much his parents invest in him as against his brothers and sisters. Even if parents dO not to show favouritlsm among their children, could it be that chil- dren grab favoured treatment for themselves? Would it pay them to do so? More strictly, would genes for selfish grabbing among children become more numerous in the gene POOI than rival genes for accepting no more than one's fair share? This matter has been brilliantly analysed by Trivers, in a paper 0f 1974 called 君 4 尾〃ト

grandchildren have, genetically speaking, equal reason to behave altruistically to each other, srnce they share ↓ of each other's genes. But if the grandchildren have the greater expectauon of life, genes for grandparent to grandchild altruism have a higher selective advantage than genes for grandchild to grandparent altruism. lt is quite possible for the net benefit of assisting a young distant relative tO exceed the net benefit Of assisting an 01d close relative. (lncidentally, it is not, of course, necessarily the case that grandparents have a shorter expectation of life than grandchildren. ln species with a high infant-mortality rate, the reverse may be true. ) TO extend the actuarial analogy, individuals can be thought of as life—insurance underwrrters. An individual can be expected t() lnvest or risk a certain proportion Of his own assets in the life of another individual. He takes into account his relatedness to the other individual, and also whether the individual is a good risk' in terms Of his life expectancy compared with the insurer s own. Strictly we should say reproduction expectancy' rather than 、 li expectancy', or tO be even more strict, general capacity tO benefit own genes in the future expectancy'. Then in order for altruistlc behaviour tO evolve, the net risk to the altruist must be less than the net benefit to the recipient multiplied by the relatedness. Risks and benefits have to be calculated in the complex actuarial way I have outlined. But what a complicated calculation to expect a poor survival machine tO dO, especially in a hurry! Even the great mathematical biologist J. B. S. HaIdane ()n a paper of 1955 in which he anticipated HamiIton by postulating the spread of a gene for saving close relatives from drowning) remarked: . on the tWO occasions when I have pulled possibly drowmng people out of the water ()t an infinitesimal risk tO myself) I had no time to make such calculations. ' Fortunately, however, as Haldane well knew, it IS not necessary tO assume that survival machines d() the sums consciously in their heads. Just as we may use a slide rule without appreciating that we are, in effect, using logarithms, SO an animal may be pre-programmed in such a way that it behaves ifit had made a complicated calculation. This is not so difficult to imagrne as lt appears. When a man throws a ball high in the air and catches it again, he behaves as if 103

100 G ど〃ど川〃〃ん ~ 第 in practice is tO multiply by the number Of ancestors. First cousins, for instance, have tWO common ancestors, and the genera— tion distance V1a each one IS 4. Therefore their relatedness IS 2 >< ()Y = 第 If 月 is B's great-grandchild, the generation dis- tance IS 3 and the number of common ancestors' is I ( お himself), SO the relatedness is 1 x ( 当ア = I GeneticaIly speaking, your first cousln is equivalent to a great-grandchild. SimiIarly, you are Just as likely to 'take after' your uncle (relatedness = 2 x (})3 as after your grandfather (relatedness For relationships as distant as third cousin ( 2 x (})8 = ・長 ) , we are getting down near the baseline probability that a particular gene possessed by 月 will be shared by any random individual taken from the population. A third cousln not far from being equivalent tO any Old Tom, Dick, or Harry as far as an altruistic gene IS concerned. A second cousin (relatedness = ) iS only a little bit special; a first cousin somewhat more so ({). FuII brothers and sisters, and parents and children are very special (}), and 1 ) just as special as oneself. Uncles identical twins (relatedness and aunts, nephews and nieces, grandparents and grandchildren, and half brothers and half sisters, are intermediate with a relatedness Of 上 . NOW we are ln a position tO talk about genes for kin—altruism much more precisely. A gene for suicidally savrng five couslns would not become more numerous in the population, but a gene for saving five brothers or ten first cousms would. The reqmrement for a suicidal altruist1C gene tO be success— ful is that it should save mo 代 than two siblings ()r children or parents), or more than four half-siblings ()r uncles, aunts, nephews, nleces, grandparents, grandchildren), or more than eight first cousins, etc. Such a gene, on average, tends tO live on in the bodies of enough individuals saved by the altruist t0 com- pensate for the death 0f the altruist itself. If an individual could be sure that a particular person was his identical twin, he should be exactly as concerned for his twin's welfare as for his own. Any gene for twin altruism is bound tO be carried by b0th twins, therefore if one dies heroically t0 save the Other the gene lives on. Nine-banded armadillos are born in a litter of identical quadruplets. As far as I know, no feats of heroic self-sacrifice have been reported for young armadillos, but it has

170 お 4 なん研励ど覊工 go for 01d men. Whatever their shortcomings, they have at least proved they can survive, and she is likely to be allying her genes with genes for longevity. However, there is no point in ensuring that her children live long lives if they do not also give her lots of grandchildren. Longevity is not prima facie evidence of virility. lndeed a long-lived male may have survived precisely 厖〃 he does not take risks in order to reproduce. A female who selects an 01d male is not necessarily golng to have more descendants than a rival female Wh() chooses a young one WhO ShOWS some Other evidence of good genes. What other evidence? There are many possibilities. Perhaps strong muscles as evidence of ability to catch food, perhaps long legs as evidence 0f ability t0 run away from predators. A female might benefit her genes by allying them with such traits, since they might be useful qualities in both her sons and her daughters. TO begin with, then, we have to imagine females choosing males on the basis of perfectly genuine labels or indicators which tend t0 be evidence Of good underlying genes. But now here is a very interesting point, realized by Darwin, and clearly enunciated by Fisher. ln a society where males compete with each other tO be chosen as he—men by females, one Of the best things a mother can dO for her genes is tO make a son WhO will turn out in his turn tO be an attractive he—man. If she can ensure that her son IS one Of the fortunate few males whO wins most Of the copulations in the SOCiety when he grows up, She will have an enormous number Of grandchildren. The result 0f this is that one 0f the most desirable qualities a male can have in the eyes 0f a female is, quite simply, sexual attractiveness itself. A female WhO mates with a super— attractive he—man IS more likely tO have sons whO are attractlve tO females of the next generation, and who will make lots 0f grand- children for her. OriginaIIy, then, females may be thought of as selecting males on the basis of obviously useful qualities like big muscles, but once such qualities became widely accepted as at— tractive among the females Of a species, natural selection would continue tO favour them simply because they were attractlve. Extravagances such as the tails 0f male birds 0f paradise may therefore have evolved by a kind 0f unstable, run-away process. ln the early days, a slightly longer tail than usual may have been selected by females as a desirable quality in males' perhaps

135 On the other hand, if the choice 1S not such a stark life or death choice, her best bet might be to prefer the younger one. For instance, suppose her dilemma is whether to give a particular morsel of food t0 a little child or a big one. The big one is likely t0 be more capable 0f finding his own 応 od unaided. Therefore if she stopped feeding him he would not necessarily die. On the other hand, the little one who is t00 young to find food for himself would be more likely to die if his mother gave the food to his big brother. Now, even though the mother would prefer the little brother to die rather than the big brother, she may still give the 応 od to the little one, because the big one is unlikely to die anyway. This is why mammal mothers wean their children, rather than going on feeding them indefinitely throughout their lives. There comes a time in the life of a child when it pays the mother tO divert investment from him into future children. When this moment comes, she will want tO wean him. A mother who had some way of knowing that she had had her last child might be expected tO continue tO invest all her resources in him for the rest of her li , and perhaps suckle him well into adulthood. Never- theless, she should 'weigh up' whether it would not pay her more tO invest in grandchildren or nephews and nieces, since although these are half as closely related to her as her own children, their capacity tO benefit from her investment may be more than double that of one of her own children. This seems a good moment to mention the puzzling phenomenon known as the menopause, the rather abrupt termm- atlon 0f a human female's reproductive fertility in middle age. ThiS may not have occurred t00 commonly in our wild ancestors, SInce not many women would have lived that long anyway. But still, the difference between the abrupt change of life in women and the gradual fading out of fertility in men suggests that there IS something genetically 'deliberate' about the menopause—that it is an 、 adaptation'. lt is rather difficult tO explain. At first sight we might expect that a woman should go on having children until she dropped, even if advancing years made it progressively less likely that any individual child would survive. Surely it would seem always worth trying? But we must remember that she is also related to her grandchildren, though half as closely. For 、 rar10us reasons, perhaps connected with the Medawar

114 G 砌 4 んゆ more tO say about liars and cheaters and exploiters in following chapters. ln a world where Other individuals are constantly on the alert for opportunlties to exploit kin-selected altruism, and use it for their own ends, a survival machine has to consider wh() it can trust, who it can be really sure of. 4 、召 is really my baby brother, then I should care for him up to half as much as I care for myself, and fully as much as I care for my own child. But can I be as sure 0f him as I can of my own child? How do I know he is my baby brother? 4 、 C is my identical twin, then I should care for him twice as much as I care for any of my children, indeed I should value his life no less than my own. But can I be sure of him? He looks like me tO be sure, but it could be that we just happen to share the genes for facial features. No, I will not give up my life for him, because although it is 第 0 ル / ど that he bears 100 per cent of my genes, I absolutely ん加ル that I contain 100 per cent of my genes, SO I am worth more tO me than he is. I am the only individual that any one of my selfish genes can be sure of. And although ideally a gene for individual selfishness could be displaced by a rival gene for altruistically saving at least one identical twin, two children or brothers, or at least four grandchildren etc. , the gene for individual selfishness has the enormous advantage of 灯 4 Of individual identity. The rival kin—altruistic gene runs the risk Of making mistakes Of identity, either genuinely accidental, or deliberately englneered by cheats and parasites. We therefore must expect individual selfishness ln nature, tO an extent greater than would be predicted by considerations Of genetic relatedness alone. ln many specles a mother can be 1 ore sure Of her young than a father can. The mother lays the visible, tangible egg, or bears the child. She has a good chance of knowing for certain the bearers of her own genes. The poor father is much more vulnerable to deception. lt is therefore to be expected that fathers will put less effort than mothers into caring for young. We shall see that there are Other reasons tO expect the same thing, in the chapter on the BattIe of the Sexes (Chapter 9 ). SimiIarly, maternal grandmothers can be more sure of their grandchildren than paternal grand- mothers can, and might be expected tO show more altruism than paternal grandmothers. This is because they can be sure Of their

134 the welfare 0f children. How should a young female, setting out on her adult life, invest her life's resources? What would be a wise investment policy for her t0 応Ⅱ ow ~ We have already seen from the Lack theory that she should not spread her investment t00 thinly among t00 many children. That way she willlose t00 many genes: she won't have enough grandchildren. ()n the other hand, she must not devote all her investment tO t00 few children— spoilt brats. She may virtually guarantee herself 襯ど grandchil- dren, but rivals whO invest in the optimum number Of children will end up with more grandchildren. SO much for even-handed lnvestment policies. Our present lnterest is in whether it could ever pay a mother tO invest unequally among her children, i. e. ln whether she should have favourites. The answer is that there is no genetIC reason for a mother tO have favourites. Her relatedness tO all her children is the same, 矛 Her optimal strategy is tO lnvest どイ〃 4 / in the largest number Of children that she can rear t0 the age when they have children 0f their own. But, as we have already seen, some individuals are better life insurance risks than others. An under—sized runt bears Just as many Of his mother's genes as his more thriving litter mates. But his life expectation iS less. Another way tO put this is that he 〃どど赤 more than his fair share Of parental investment, Just tO end up equal tO his brothers. Depending on the circumstances, it may pay a mother t0 refuse t0 feed a runt, and allocate all 0f his share Of her parental investment tO his brothers and sisters. lndeed it may pay her to feed him t0 his brothers and sisters, or t0 eat him herself, and use him t0 make milk. M0ther pigs d0 sometimes devour their young, but I dO not know whether they pick especially on runts. Runts constitute a particular example. We can make some more general predictions about hOW a mother's tendency tO lnvest in a child might be affected by his age. If she has a straight choice between saving the life 0f one child or saving the life 0f anothen and if the one she does not save is bound t0 die, she should prefer the older one. This is because she stands t0 lose a higher propor- tion of her life's parental investment if he dies than if his little brother dies. perhaps a better way tO put this is that if she saves the little brother she will still have tO invest some costly resources in him just t0 get him up t0 the age 0f the big brother.

156 お 4 ″んツツい seen. SuperficialIy, therefore, we might expect the daughter- producing gene tO go on spreading until the sex ratio was SO unbalanced that the few remaming males, working flat out, could Just manage. But now, think what an enormous genetic advantage is enpyed by those few parents who have sons. Anyone who invests a son has a very good chance of being the grandparent of hundreds of seals. Those who are producing nothing but daughters are assured of a safe few grandchildren, but this is nothing compared to the glorious genetic possibilities which open up before anyone specializing in sons. Therefore genes for producing sons will tend tO become more numerous, and the pendulum will swing back. For simplicity I have talked in terms of a pendulum swing. ln practice the pendulum would never have been allowed to swmg that far in the direction of female domination, because the pres— sure tO have sons would have started tO push it back as soon as the sex ratio became unequal. The strategy of producing equal numbers Of sons and daughters IS an evolutionarily stable strategy, in the sense that any gene for departing from it makes a net loss. I have told the story in terms of numbers of sons versus num— bers 0f daughters. This is to make it simple, but strictly it should be worked out in terms of parental investment, meaning all the fOOd and Other resources which a parent has to offer, measured in the way discussed in the previous chapter. Parents should / 〃 0 ど equally in sons and daughters. This usually means they should have numerically as many sons as they have daughters. But there could be unequal sex ratios which were evolutionarily stable provided correspondingly unequal amounts Of resources were lnvested in sons and daughters. ln the case of the elephant seals, a policy 0f having three times as many daughters as sons, but of making each son a supermale by investing three times as much 応 od and other resources in him, could be stable. By investing more 応 Od in a son and making him big and strong, a parent might increase hiS chances Of winmng the supreme Of a harem. But this is a special case. Normally the amount invested in each son will roughly equal the amount invested in each daugh- ter, and the sex ratl(), in terms Of numbers, IS usually one tO one. ln its long Journey down the generations therefore, an average

126 お 4 襯 / ケが 4 / 〃 g family sizes are no longer limited by the finite resources which the individual parents can provide. If a husband and wife have more children than they can feed, the state, which means the rest of the population, simply steps in and keeps the surplus children alive and healthy. There is, in fact, nothing to stop a couple with no material resources at all having and rearing precisely as many children as the woman can physically bear. But the welfare state IS a very unnatural thing. ln nature, parents who have more children than they can support do not have many grandchildren, and their genes are not passed on to future generations. There is no 〃ノ for altruistlc restraint in the birth-rate, because there is no welfare state ln nature. Any gene for over—indulgence promptly punished: the children containing that gene starve. Since we humans dO not want to return to the old selfish ways where we let the children of too-large families starve to death, we have abolished the family as a unit of economic self- sufficiency, and substituted the state. But the privilege of guaran— teed support for children should not be abused. Contraception IS sometrmes attacked as unnatural'. S() it is, very unnatural. The trouble is, SO is the welfare state. I think that most of us believe the welfare state is highly desirable. But you cannot have an unnatural welfare state, unless you alSO have unnatural birth—control, otherwise the end result will be misery even greater than that which obtains ln nature. The welfare state is perhaps the greatest altruistic system the animal kingdom has ever known. But any altruistic system IS inherently unstable because it is open to abuse by selfish individuals, ready to exploit it. lndividual humans who have more children than they are capable Of rearing are probably too ignorant in most cases [ 0 be accused Of consclous malevolent exploitation. Powerful institu— t10ns and leaders whO deliberately encourage them to do so seem tO me less free from susplC1()n. Returning to wild animals, the Lack clutch-size argument can be generalized to all the other examples Wynne-Edwards uses: territorial behaviour, dominance hierarchies, and SO on. Take, for instance, the red grouse that he and his colleagues have worked 0 圧 These birds eat heather, and they parcel out the moors ln territories containing apparently more 応 od than the territory owners actually need. Early in the season they fight over ter-

daughter's children, but their son may have been cuckolded. Maternal grandfathers are Just as sure 0f their grandchildren as paternal grandmothers are, S1nce bOth can reckon on one genera— on Of certainty and one generatlon Of uncertainty. Similarly, uncles on the mother's side should be more interested in the welfare Of nephews and nieces than uncles on the father's side, and in general should be Just as altruist1C as aunts are. lndeed in a society with a high degree 0f marital infidelity, maternal uncles should be more altruistic than 'fathers' S1nce they have more grounds for confidence in their relatedness t0 the child. They know that the child's mother is at least their half-sister. The 'legal' father knows nothing. I d0 not know of any evidence bearing on these predictions, but I offer them in the hope that Others may, or may start looking for evidence. ln particular, perhaps social anthropologists might have interesting things tO Return1ng tO the fact that parental altruism IS more common than fraternal altruism, it does seem reasonable tO explain this in terms Of the 'identification problem'. But this does not explain the fundamental asymmetry in the parent/child relationship itself. Parents care more for their children than children do for their parents, although the genetic relationship is symmetrical, and certalnty Of relatedness is Just as great bOth ways. One reason that parents are in a better practical position tO help their young, being Older and more competent at the business Of living. Even if a baby wanted t0 feed its parents, it is not well equipped t0 d0 so ln practlce. There is another asymmetry in the parent/child relationship which does not apply t0 the brother/sister one. Children are always younger than their parents. This often, though not always, means they have a longer expectation 0f life. As I emphasized above, expectatlon Of life is an lmportant variable which, in the best of all possible worlds, should enter into an animal's 'calcula- tIOn' when it is 'deciding' whether tO behave altruistically or not. ln a species ln which children have a longer average life- expectancy than parents, any gene for child altruism would be labouring under a disadvantage. lt would be engmeering altruistic self—sacrifice for the benefit Of individuals whO are nearer tO dying of old age than the altruist itself. A gene for parent 1 15