Chapter 11 Chapter Nine The Battle Between the Sexes

If 50% of the genes in the body are the same, if there is a conflict of interest between the parent and the offspring, how intense will the conflict of interest be between spouses who have no blood relationship with each other?The only thing they have in common is a fifty percent genetic investment in their children.Given that both fathers and mothers are concerned with the welfare of their own halves of their children, it may be beneficial for both to cooperate in raising these children.One parent is better off if he or she pays a smaller share than the other parent of the costly investment of resources in each child; this is because he or she has more resources to spend with the other other children of a mate, so that his or her genes have a greater chance of reproduction.So we can say that each spouse tries to take advantage of the other, trying to force the other to invest more.As far as the individual is concerned, the desirable calculation is to wish to dominate as many members of the opposite sex as possible (I don't mean physical pleasure, although the individual may enjoy doing so), and let the spouse with whom it mates raise the children big.As we shall see, the males of some species already do this, but there are also males of other species who assume equal obligations to their mates in the rearing of their children.Trivers specifically emphasizes that the relationship between sexual spouses is one of mutual distrust and mutual exploitation.This view of the interrelationships between sexual couples is relatively new to individual ecologists.We used to think of sex, mating, and the courtship before that as mostly cooperative ventures for the common good, or even for the good of the species!

Let's go straight back to the fundamentals and delve into the fundamental nature of masculinity and femininity.We discussed the nature of sex in Chapter 3 without emphasizing its asymmetries.We simply admit that some animals are male and others are female, but we do not go any further into what the words male and female mean.What is the essence of masculinity?What is the fundamental definition of female?We as mammals see nature define sex in terms of a variety of traits, such as having a penis, bearing children, breastfeeding with special mammary glands, certain chromosomal traits, and so on.For mammals, these standards for judging individual sex are understandable, but for animals and plants in general, such standards are no more reliable than wearing long trousers as a standard for judging human sex.Frogs, for example, neither male nor female have penises.In this way, the terms male and female may not have the meaning commonly understood.After all, they are but two words.If we don't think they're useful for telling the sex of a frog, we might as well not use them.We can arbitrarily divide frogs into sex ones and sex two if we like.However, sex has a basic characteristic, which can be used to indicate the male and female of all animals and plants.This means that male sex cells, or gametes, are much smaller and more numerous than female gametes.This is true whether we are discussing animals or plants.If individuals of one group have large sex cells, we may conveniently call them females; if individuals of another group have small sex cells, we may conveniently call them males.This difference is particularly pronounced in reptiles and birds.A single egg cell is large enough and contains enough nutrients to feed a developing young for several weeks.Even in humans, an egg, although microscopically small, is still many times larger than a sperm.We shall see that from this fundamental difference we can explain all the other differences between the sexes.

Certain primitive organisms, such as fungi, do not have male and female problems, although some type of sexual reproduction does occur.In a system called isogamy, individuals do not differentiate between the two sexes.Any individual can mate with each other.There are no two different gametes sperm and eggs but all sex cells are the same, are called isogametes (isogametes).Two isogametes fuse together to produce a new individual, and each isogamete is produced by meiosis.If there are three isogametes A, B, and C, then A can fuse with either B or C, and B can fuse with either A or C.This would never happen in a normal sexual system.If A is a sperm, it can fuse with either B or C, then B and C must be eggs, and B cannot fuse with C.

When two isogametes fuse with each other, each provides an equal number of genes and contributes an equal amount of food storage to the new individual.Sperm and eggs contribute the same number of genes to the new individual, but eggs far exceed sperm in providing food storage: in fact, sperm do not provide any food storage, but are dedicated to delivering their genes to eggs as quickly as possible That's all.Thus, at the moment of conception, the father invests less than his share of resources (fifty percent) in the offspring.Because each sperm is so tiny, a single male can make millions of them every day.This means that he has the potential to be able to produce a large number of babies from different females in a short period of time.This is possible only because each conception provides enough food for the new fetus.Thus, there is a limit to the number of offspring each female can have, but an essentially unlimited number of offspring that a male can have.This presents an opportunity for the female individual to take advantage of this condition.Parker and others have shown that this asymmetry may have evolved from an original isogametic state.Back when all the sex cells were interchangeable and roughly the same size, it's likely that some of them just happened to be slightly larger than others.The slightly larger isogamete may have some advantages over the normal isogamete in that it provides the fetus with a large amount of food in the first place, giving it a head start.So there may have been an evolutionary tendency to form larger gametes then.But the road won't be smooth.Isogametes, whose size is larger than necessary, will open the door for selfish exploitation when they begin to evolve.Individuals who make smaller gametes benefit if they can be sure to fuse their small gametes with the extra-large ones.This can be achieved simply by making the small gametes more mobile and able to actively seek out the larger ones.An individual that can produce small, fast-moving gametes has an advantage: It can produce gametes in large numbers, so it has the potential to produce more young.Natural selection favors the production of small sex cells that actively pick up and fuse with larger ones.So we can imagine that there are two diametrically opposed sexual strategies evolving.One is a heavy investment or honest strategy.This strategy naturally opens the way for small investments, exploitative or dodgy tactics.Once the divergence of these two strategies starts, it will continue like a runaway horse.Intermediates between these two volumes are penalized because they don't have the advantage of either of these two extreme strategies.The cunning gametes become smaller and more mobile.The honest gametes evolved to get bigger and bigger to compensate for the dwindling investment of the cunning gametes, who were always actively chasing them anyway.Each honest gamete would rather fuse with another honest gamete.But the natural selection pressure to repel cunning gametes (p. 47) is weaker than the pressure that drives them to exploit them: for the cunning gametes must win the evolutionary battle or lose a lot.So the honest gamete becomes an egg, and the cunning gamete becomes a sperm.

Males are thus insignificant creatures, and on a simple theory of species interest we would expect males to become less and less numerous than females.Since, in theory, one male produces enough sperm to satisfy the needs of a hundred females, we can assume that the ratio of males and females in an animal population should be one hundred: one.In other words, male individuals are more low-value and expendable, while female individuals are more valuable to the species.This is, of course, entirely true from the point of view of the species as a whole.As an extreme example, in a study of elephant seals, it was observed that four percent of the male elephant seals performed 88 percent of all matings.In this example, and many others, there is a large surplus of celibate males who have never copulated, and who may never get a chance to mate for life.But these extra males lead otherwise normal lives, devouring the population's food resources as relentlessly as other mature individuals.From the point of view of the interests of the species, this situation is a huge waste; these extra males are, so to speak, social parasites.This phenomenon is just another example of the difficulties encountered by group selection theory.On the other hand, the theory of the selfish gene has no trouble explaining that the number of males and females tends to equalize, even though males actually reproducing may be a small fraction of the total.It was Fischer who first made this explanation.

The question of how many males and how many females are born is a special problem in parental strategies.We have discussed the optimal family size for parental individuals seeking to maximize their genetic survival.In the same way, I can also discuss the most suitable sex ratio.Should you entrust your precious genes to your son or your daughter?Assume that a mother invests all her resources in her sons and thus has nothing left over to invest in daughters: in general, she contributes as much to the future gene pool as a mother who invests all her resources in her daughters Will it be bigger than that?Will there be more or fewer genes for sons than for daughters?Fischer demonstrated that, under normal circumstances, the optimum sex ratio is fifty:fifty.In order to understand this question, we must first have a little knowledge about the mechanism of sex determination.

In mammals, sex is determined genetically in such a way that all eggs can develop into either male or female individuals.The carrier of the sex-determining chromosome is the sperm.Of the sperm produced by men, half of them produce females, or X sperm; half of them produce males, or Y sperm.The two types of sperm appear to be indistinguishable from each other.They differ in only one chromosome.A gene for a father to have only daughters requires only that he produce only X sperm; a gene for a mother to have only daughters requires only that she secrete a selective spermicide or abort male fetuses. Can.What we are looking for is something equivalent to an evolutionarily stable strategy (ESS), although here strategy is more of a metaphor (we have used this in the chapter on aggressive behavior). simile).In fact, individuals cannot choose the sex of their children at will.But it is still possible that genes predispose individuals to have children of one sex.If we assume that such genes, genes that favor uneven sex ratios, are present, will they outnumber their alleles, genes that lean toward average sex ratios, in the gene pool?

Suppose that in the above-mentioned elephant seals a mutant gene has arisen which has a tendency for parents to have mostly daughters.Since there is no shortage of male individuals in the population, there is no difficulty for daughters to find a mate, and the genes for making daughters can spread.Thus, the sex ratio within the population begins to shift towards a surplus of females.From the point of view of the interests of the species, this situation is not problematic.We have already said that because only a few males are sufficient to supply the spermatozoa required by even a large surplus of females.On the face of it, therefore, we might think that the daughter-making genes continued to proliferate until the sex ratios were so unbalanced that the few remaining males were too exhausted to cope.But imagine the enormous genetic advantage enjoyed by the few parents who have sons!An individual who bears one son has a great chance of being the grandfather or grandmother of hundreds of seals.Daughter-only individuals are sure to secure several grandchildren, but they pale in comparison to the genetically spectacular prospects of son-only individuals.As a result, the genes for sons tend to become more numerous, and the sex ratio pendulum swings back again.

For simplicity, I will illustrate the problem in terms of the swing of a pendulum.In fact, the pendulum never swings that far in the direction of female dominance.Because once the sex ratio is out of balance, the natural selection pressure to have sons starts to push the pendulum back.The strategy of producing an equal number of offspring is an evolutionarily stable strategy, that is, any gene that deviates from this strategy suffers a net loss. My argument is based on the number of sons versus the number of daughters, for the sake of simplicity.But strictly speaking, it should be interpreted in terms of the theory of parental investment, that is to say, in terms of all the food and other resources that one parent has to provide, in the way we discussed in the previous chapter.Parental investment in sons and daughters should be equal.In general, this means that they should have an equal number of sons and daughters.But a similar degree of disequilibrium in sex ratios could be evolutionarily stable if the investment of resources in sons and daughters is correspondingly unequal.In the case of elephant seals, the ratio of daughters to sons is three to one, and three times as much food and other resources are invested in each son to make each son a superior male. Strategies may be stable.By investing more food in the son, making him big and strong, the parent might give him a better chance of winning the highest prize of a wife and concubine.But this is a special case.Usually, the investment in each son is roughly equal to the investment in each daughter, and the sex ratio is generally one to one in number.

Thus, an average gene spends about half its time in males and the other half in females during its long journey from generation to generation.Certain effects of genes are expressed only in individuals of one sex.These effects are called sex-limited gene effects.The gene that controls penis length only shows its effect in males, but it is also present in females and may have an entirely different effect on females.It makes no sense to think that males cannot inherit the tendency to develop long penises from their mothers. In whichever of the two individuals the gene resides, we may assume that it will take full advantage of all the opportunities offered by that individual.These opportunities can vary widely depending on the sex of the individual.As a convenient approximation, we can again assume that each individual is a selfish machine trying to preserve all of its genes.The best strategy for such a selfish machine is often quite different depending on its gender.For the sake of brevity, we have to go back to the old way of treating individual behavior as purposeful.As before, we have to remember that this is only a metaphor.In effect, the individual is a machine blindly programmed by its own selfish genes.

Let's go back to the couple we mentioned at the beginning of this chapter.As selfish machines, both spouses want an equal number of sons and daughters.On this point they are not in dispute.At issue is who will bear the primary responsibility for raising each of these children.Every individual hopes to have as many surviving children as possible.The less he or she invests in any one child, the more children he or she will be able to have.The obvious way to do this is to induce your sexual spouse to invest more than his or her fair share of resources in each child, freeing you to reproduce with another spouse.This strategy is a strategy that both sexes yearn for, but it is more difficult for females to get what they want.Since she initially invests more than the male in her large, food-rich eggs, the mother is, from the moment of conception, under a greater obligation to each young child than the father is.If the child dies, she suffers a greater loss than the father.Rather, she will have to invest more in the future than the father in order to raise another new child to the same size as the dead child.If she tricks the father into taking care of the young while she elopes with another male, the father can retaliate by abandoning the young with relatively little loss.Thus, at least in the early stages of a child's development, if such abandonment occurs, it is usually the father who abandons the mother, not the other way around.Likewise, we can expect females to invest more in their offspring than males, not only initially but throughout development.So, for example, among mammals, it is the female who conceives the fetus in her own body, and after the baby is born, it is the female who produces milk to feed the baby, and the main responsibility for raising and protecting the baby also falls on the shoulders of the female.Females are exploited, and the main evolutionary basis for this exploitation is that eggs are larger than sperm. Of course, in many species fathers are indeed very diligent and faithful caregivers of their young.But even so, we must expect that under normal circumstances there will be some evolutionary pressure on males to invest a little less in each offspring and to try to have more offspring with other mates.What I mean by this is only that if the genes say: Hey, if you are a male, leave your mate earlier and find another female instead of leaving when my alleles want you to leave , such genes tend to succeed in the gene pool.This evolutionary pressure has real-life effects that vary in size and size across species.In many species, such as the bird of paradise, the females do not get any help from the males and raise their young entirely on their own.Still other species, such as kittiwakes, form male-female pairs that are models of mutual loyalty, cooperating with each other to share the responsibility of raising their young.Here, we must imagine.A certain evolutionary counterpressure is at work: Selfish exploitation of a mate is not only rewarding but also punishable.In kittiwakes, this penalty outweighs the benefit.In any case, it is only to the advantage of the father to abandon his wife and children if the wife is conditionally independent of others to raise the children. Trivers explores the various courses of action that mothers who have been abandoned by their spouses may take.The best strategy for her would be to trick another male into adopting her offspring, thinking it was his own.If the child is still an unborn fetus, it may not be too difficult to do this.Of course, the toddler has half her genes in him, and the duped father has none of them.Natural selection would have severely penalized males for such gullibility, and in fact, selected against males who, once mated with new wives, took the aggressive step of killing any potential stepson or stepdaughter.This phenomenon likely explains the so-called Bruce effect: Male rats secrete a chemical that, when pregnant females smell it, induces spontaneous abortions.It aborts only if the taste is different from that of its previous mate.This is how the male rat kills a potential stepson or daughter and makes his new wife receptive to his sexual advances.By the way, Ardley actually used the Bruce effect as a way to control population density!The same thing happens with male lions, who, when new to a pride, sometimes kill existing cubs, probably because they were not their own cubs. A male individual does not need to kill his stepson and stepdaughter to achieve the same goal.Before mating with a female, he can prolong his courtship, during which time he will drive away any approaching males and prevent her from escaping.In this way he could see if there were any little stepson or stepdaughter in her womb, and if so, abandon her.We shall see later why a female may wish to be engaged for a longer period before mating.Here we talk about why male individuals also want the engagement period to be longer.Assuming he can cut her off from all contact with other males, this helps to avoid unknowingly becoming the guardian of the offspring of other males. If the abandoned female can't trick the new male into adopting her offspring, what else can she do?A lot depends on how old the toddler is.If it's just conceived, she's actually invested in the whole egg, maybe more, but it's still in her interest to abort the fetus and find a new mate as soon as possible.In this case, the abortion would also be to the advantage of her new future husband, since we have already assumed that she has no hope of deceiving him.This can show that from the perspective of female individuals, the Bruce effect is at work. Abandoned females also have the option of staying the course and trying to raise the young on their own.This is especially beneficial to her if the toddler is already quite old.The bigger the child, the more invested in him, and the less she has to pay for the task of raising the child.Even if the toddler is still very young, it may still be to her advantage to try to preserve something from her initial investment, although she will have to work twice as hard to feed the toddler now that the males are gone.The child also has half the genes of a male individual. She can vent her resentment on the child and abandon the child, but it is not a pleasant thing for her to do so.There is no reason to vent your grudges on young children.Half of the toddler's genes belong to her, and she alone faces the current predicament. As paradoxical as it may sound, the appropriate strategy for a female at risk of abandonment is to leave a male before he abandons her.Even if she is already more invested in her young than a male, it may still be to her advantage.It is an unpleasant fact that, in a situation, whoever abandons the other first, be it the father or the mother, gains the advantage.As Trivers says, jilted spouses are often caught in a ruthless bond.This is a rather dire but very delicate argument.One parent might say something like: The child is now quite old enough to be raised by one of us.So it would be to my advantage for me to leave now, assuming I can be sure my spouse won't leave too.If I leave now, my spouse can work hard in the best interest of her or his genes.He or she will be forced to make a much more drastic decision than I am making now that I am gone.My spouse understands that if he or she leaves too, the toddler will surely die.So, assuming my spouse makes a decision that would be in the best interest of his or her selfish genes, I conclude that my own course of action is that it would be best for me to leave first.Because my spouse may also be considering the exact same approach as me, and may come to strike first and abandon me at any time!Therefore, I should especially leave first.Such parents will actively abandon each other.This self-monologue, as before, is merely illustrative.The point is that natural selection favors discarding the other's genes first, simply because natural selection favors rejecting the other's genes later. We have already described some of the actions a female may take once abandoned.But in all these actions there is a sense that it is not too late.Is there any way for a female to mitigate the loss caused by her mate exploiting her in the first place?She holds an ace in her hand.She can refuse to mate.She is the wooed, she is the seller.This is because her dowry is a large and nutritious egg.Any male who can successfully mate with it can obtain a rich food store for his offspring.Females can use this to engage in intense bargaining before mating.Once she mates, she loses her trump card. She has entrusted her eggs to the male with whom she mates.Heated haggling might be a good metaphor.But we all know that's not the case.Is there any actual form of the equivalent of a sharp bargain that could have evolved by natural selection?I think there are two main possibilities, one is the domestic-bliss strategy and the other is the he-man strategy. In the simplest form of a family happiness strategy, a female scrutinizes a male in an attempt to detect in advance signs of loyalty and attachment to family life.There must be differences in degree within populations of male individuals in their propensity to be devoted husbands.If females can pre-discern this trait, they can benefit themselves by selecting males with this quality.One of the ways females do this is by putting on airs and being coy for a long time.A male who is impatient and cannot wait for a female to finally agree to mate will probably not make a devoted husband.Females weed out dishonest suitors by insisting on long engagements, and end up mating only with males who have pre-proven fidelity and lasting qualities.Female coyness is a common phenomenon among animals, as is prolonged courtship or engagement.As we have seen, a long betrothal also benefits the male, who runs the risk of being tricked into raising the offspring of other males. Rituals of courtship often include significant investments made by males prior to mating.Females can wait until a male has built a nest for her before agreeing to mate with him.Or males have to feed females with considerable amounts of food.Of course, this is all very well from the point of view of the individual female, but it also suggests another possible form of family happiness strategy.Females force males to make an expensive investment in their offspring before they mate, so it doesn't pay for males to abandon each other after mating.Could this be the case?This view is quite convincing.A male pays a price for waiting for a coy female to finally mate with him: he forgoes the opportunity to mate with other females, and it takes a lot of time and energy to court her.By the time it is finally able to mate with a particular female, it is already very closely related to that female.If she knows that any other female she approaches in the future will delay in the same way before copulating, she will have little temptation to abandon her. I have pointed out in a paper that here Trivers has an error in his reasoning.He argues that investing upfront in itself obligates the individual to invest in the future.This is absurd economics.A businessman would never say: I've invested too much in (for example) the Concorde and it's not worth throwing it away now.Instead, he always asks whether it would be beneficial to his future by giving up the business now in order to limit his losses, even though he has already invested a large amount in the business.Likewise, it is futile for a female to compel a male to invest heavily in her simply to prevent a male from eventually abandoning her in the future.This form of family happiness strategy depends on an important further assumption: that a majority of females are willing to do the same.If some females in the population are dissolute, ready to welcome males who desert their wives, it will benefit the male who deserts his wife, no matter how much he already invests in her offspring. Therefore, a lot depends on the behavior of most individual females.If we could think about it in terms of the way females form cliques, there would be no problem.But groups of females are no more likely to evolve than the groups of pigeons we discussed in Chapter 5.We must look for evolutionarily stable strategies.Let's take the method Smith used to analyze aggressive play and apply it to the question of sex.The situation is a little more complicated than the hawk and dove example.Because we will have two female strategies and two male strategies. As in Smith's analysis, the word strategy refers to a blind and unconscious program of behavior.We call the two strategies of females coy and fast, and the two strategies of males as faithful and philanderer. The behavioral principles of these four strategies are: Shy females do not mate with males until they have gone through a weeks-long and costly courtship phase.Slutty females do not hesitate to mate with any individual.A loyal male is prepared for a long courtship and, after mating, stays with the female and helps her raise her offspring.Fickle males quickly lose patience if a female does not immediately copulate with them: they go off and find another female; To find another new love.As in the case of the hawk and the dove, it is not to say that there are only these strategies, but it is instructive to study the fate of the execution of them. Like Smith, we will use some arbitrarily assumed values ​​for various losses and gains.In order to be more general, it can also be expressed in algebraic notation, but numbers are easier to understand.We assume that the parental individuals get a genetic profit of +15 units for each successfully raised child.And the cost of raising a child, including all the food, all the time spent in caring for the child, and the risks borne for the child, is - twenty units.The cost is expressed as a negative number because it is the expenditure of the parents.The time spent in the protracted pursuit is also a negative number, and this cost is represented by -3 units. Now imagine a population in which all the females are shy and all the males are loyal.This is an ideal society where one-female-one-male pairings exist.In each pair, males and females earn equal payoffs on average.Every time a child is raised, they each get +15 points, and share the cost (-20 points), shared equally, and each party gets |10 points.They jointly pay the price of protracted courtship (penalty points - three).Thus, the average profit per child raised is: + fifteen - ten - three = + two. Now let's assume that a wanton female slips into the population.It does a great job.It does not have to pay the price of its delay, for it does not indulge in protracted courtships.Since all males in the population are loyal, he can find a good father for his offspring no matter which one he mates with.Therefore, its profit per child raised is + fifteen - ten = + five.It earns three units more than its shy and coy counterpart.And so the debauched genes began to spread. 如果放蕩的雌性個體竟獲得很大成功，致使它們在種群內佔據了統治地位，那麼，雄性個體的營壘中，情況也會隨之開始發生變化。截至目前為止，種群內忠誠的雄性個體佔有壟斷地位。但如果現在種群中出現了一個薄情的雄性個體，它的景況會比其他的忠誠的對手好些。在一個雌性個體都放蕩不羈的種群內，對一個薄情的雄性個體來講，這類貨色比比皆是，唾手可得。如果能順利地撫養一個幼兒，它淨得盈利＋十五分，而對兩種代價卻分文不付。對雄性個體來說，這種不付任何代價指的主要是，它可以不受約束地離開並同其他雌性個體進行交配。它的每一個不幸的妻子都得獨自和幼兒掙扎著生活下去，承擔起－二十分的全部代價，儘管她並沒因在求愛期間浪費時間而付出代價。一個放蕩的雌性個體結交一個薄情的雄性個體，其淨收益為＋十五－二十＝－五；而薄情的雄性個體的收益卻是＋十五。在一個雌性個體都放蕩不羈的種群中，薄情的雄性基因就會像野火一樣蔓延開來。 如果薄情的雄性個體得以大量地迅速增長，以致在種群的雄性成員中佔了絕對優勢，放蕩的雌性個體將陷於可怕的困難處境。任何羞忸怩的雌性個體都會享有很大的有利條件。如果羞怯忸怩的雌性個體同薄情的雄性個體相遇，它們之間絕不會有什麼結果。她堅持要把求愛的時間拉長；而他卻斷然拒絕並去尋找另外的雌性個體。雙方都沒有因浪費時間而付出代價。雙方也各無所得，因為沒有幼兒出生。在所有雄性個體都是薄情郎的種群中，羞怯忸怩的雌性個體的淨收益是零。零看上去微不足道，但比放蕩不羈的雌性個體的平均得分－五要好得多。即使放蕩的雌性個體在被薄情郎遺棄之後，決定拋棄她的幼兒，但她的一顆卵子仍舊是她所付出的一筆相當大的代價。因此，羞怯忸怩的基因開始在種群內再次散佈開來。 現在讓我們來談談這一循環性假設的最後一部分。當羞怯忸怩的雌性個體大量增加並佔據統治地位時，那些和放蕩的雌性個體本來過著縱慾生活的薄情雄性個體，開始感到處境艱難。一個個的雌性個體都堅持求愛時間要長，要長期考驗對方的忠誠。薄情的雄性個體時而找這個雌性個體，時而又找那個雌性個體，但結果總是到處碰壁。因此，在一切雌性個體都忸怩作態的情況下，薄情雄性個體的淨收益是零。如果一旦有一個忠誠的雄性個體出現，它就會成為同羞怯忸怩的雌性個體交配的唯一雄性個體。那麼他的淨收益是＋二，比薄情的雄性個體要好。所以，忠誠的基因就開始增長，至此，我們就完成了這一週而復始的循環。 像分析進犯行為時的情況一樣，按我的講法，這似乎是一種無止境的搖擺現象。但實際上，像那種情況一樣，不存在任何搖擺現象，這是能夠加以證明的。整個體系能夠歸到一種穩定狀態上。如果你運算一下，就可證明，凡是羞怯忸怩的雌性個體佔全部雌性個體的五／六，忠誠的雄性個體佔全部雄性個體的五／八的種群在遺傳上是穩定的。當然，這僅僅是根據我們開始時任意假定的那些特定數值計算出來的，但對其他任何隨意假定的數值，我們同樣可以輕而易舉地算出新的穩定比率。 同史密斯所進行的分析一樣，我們沒有必要認為存在兩種不同種類的雄性個體以及兩種不同種類的雌性個體。如果每一雄性個體能在五／八的時間裡保持忠誠，其餘的時間去尋花問柳；而每一雌性個體有五／六的時間羞怯忸怩，一／六的時間縱情放蕩，那同樣可以實現進化上的穩定狀態。不管你怎樣看待ESS，它的含義是：凡一種性別的成員偏離其適中的穩定比率時，這種傾向必然受到另一種性別在策略比率方面相應變化的懲罰，這種變化對原來的偏離行為發生不利的影響。進化上的穩定策略（ESS）因之得以保持。 我們可以得出這樣的結論，主要由羞怯忸怩的雌性個體和忠誠的雄性個體組成的種群，能夠進化是肯定無疑的。在這樣的情況下，家庭幸福策略對於雌性個體來說，實際上看來是行之有效的。我們就不必再考慮什麼由羞怯忸怩的雌性個體組成的集團了，其實羞怯忸怩對雌性個體的自私基因是有利的。 雌性個體能夠以各種各樣的方式將這種形式的策略付諸實踐。我已經提到過，雌性個體可能拒絕同還沒有為它築好巢、至少還沒有幫助它築造一個巢的雄性個體交配。在許多單配製的鳥類中，情況的確如此，巢不築好不交配。這樣做的效果是，在受孕的時刻，雄性個體對幼兒已經付出的投資要遠較其一些廉價的精子為多。 未來的配偶必須為它築造一個巢，這種要求是雌性個體約束雄性個體的一種有效手段。我們不妨說，只要能夠使雄性個體付出昂貴的代價，不論是什麼，在理論上幾乎都能奏效，即使付出的這種代價，對尚未出生的幼兒並沒有直接的益處。如果一個種群的所有雌性個體都強迫雄性個體去完成某種艱難而代價昂貴的任務，如殺死一條龍或爬過一座山然後才同意交配，在理論上講，它們能夠降低雄性個體在交配後不辭而別的可能性。企圖遺棄自己的配偶，並要和另外的雌性個體交配以更多地散佈自己基因的任何雄性個體，一想到必須還要殺死一條龍，就會打消這種念頭。然而事實上雌性個體是不會將殺死一條龍或尋求聖盃這樣專橫的任務硬派給它們的求婚者的。因為如果有一個雌性個體對手，它指派的任務儘管困難程度相同，但對它以及它的子女卻有更大的實用價值，那麼它肯定會優越於那些充滿浪漫情調、要求對方為愛情付出毫無意義的勞動的雌性個體。殺死一條龍或在Hellespont中游泳也許比築造一個巢穴更具浪漫色彩，但卻遠遠沒有後者實用。 我提到過的雄性個體做出的具有求愛性質的餵食行動對於雌性個體也是有用的。鳥類的這種行為通常被認為是雌性個體的某種退化現象，它們恢復了雛鳥時代的幼稚行為。雌鳥向雄鳥要食物，討食的姿態像雛鳥一樣。有人認為這種行為對雄鳥具有自然的誘惑力，這時雌鳥不管能得到什麼額外的食物，它都需要，因為雌鳥正在建立儲存，以便致力於製造很大的卵子。雄鳥的這種具有求愛性質的餵食行為，也許是一種對卵子本身的直接投資。因此，這種行為能夠縮小雙親在對幼兒的初期投資方面存在的懸殊程度。 有幾種昆蟲和蜘蛛也存在這種求愛性質的餵食現象。很顯然，有時這種現象完全可以作另外的解釋。如我們提到過的螳螂的例子，由於雄螳螂有被較大的雌螳螂吃掉的危險，因此只要能夠減少雌螳螂的食慾，隨便幹什麼對它可能都是有利的。我們可以說，不幸的雄螳螂是在這樣一種令人毛骨悚然的意義上對其子女進行投資的。雄螳螂被作為食物吃掉，以便幫助製造卵子，而且儲存在雄螳螂屍體內的精子隨之使吃掉它的雌螳螂的卵子受精。 採取家庭幸福策略的雌性個體如果僅僅是從表面上觀察雄性個體，試圖辨認它的忠誠的品質，這樣的雌性個體容易受騙。雄性個體只要能夠冒充成忠誠的愛好家庭生活的類型，而事實上是把遺棄和不忠誠的強烈傾向掩蓋起來，它就具有一種很大的有利條件。只要過去被它遺棄的那些妻子能有機會將一些幼兒撫養大，這個薄情的雄性個體比起一個既是忠誠的丈夫又是忠誠的父親的雄性對手，能把更多的基因傳給後代。使雄性個體進行有效欺騙的基因在基因庫中往往處於有利地位。 相反，自然選擇卻往往有利於善於識破這種欺騙行為的雌性個體。要做到這一點，雌性個體在有新的雄性個體追求時，要顯得特別可望而難及，但在以後的一些繁殖季節中，一旦去年的配偶有所表示，就要毫不猶豫，立刻接受。這樣對那些剛開始第一個繁殖季節的年輕的雄性個體來說，不論它們是騙子與否，都會自動受到懲罰。天真無知的雌性個體在第一年所生的一窩小動物中，體內往往有比例相當高的來自不忠誠的父親的基因，但忠誠的父親在第二以及以後的幾年中卻具有優勢，因為它有了一個可靠的配偶，不必每年都要重複那種浪費時間、消耗精力、曠日持久的求愛儀式。在一個種群中，如果大部分的個體都是經驗豐富而不是天真幼稚的母親的子女在任何生存時間長的物種中，這是一個合乎情理的假設忠誠而具模範父親性格的基因在基因庫中將會取得優勢。 為簡便起見，我把雄性個體的性格講得似乎不是純粹的忠誠就是徹頭徹尾的欺詐。事實上，更有可能的是，所有的雄性個體其實是所有的個體多少都有點不老實，它們的程序編製就是為了利用機會去占它們配偶的便宜。由於自然選擇增強了每一個配偶發現對方不忠誠的行為的能力，因此使重大的欺騙行為降到了相當低的水平。雄性個體比雌性個體更能從不忠誠的行為中得到好處。即使在一些物種中，雄性個體表現出很大程度的親代利他主義行為，但我們必須看到，它們付出的勞動往往比雌性個體要少些，而且隨時潛逃的可能性更大些。鳥類和哺乳類動物中，通常存在這種情況是肯定無疑的。 但是也有一些物種，其雄性個體在撫養幼兒方面付出的勞動實際上比雌性個體多。鳥類和哺乳類動物中，這種父方的獻身精神是極少有的，但在魚類中卻很常見。What is the reason?這種現象是對自私基因理論的挑戰，為此我長時間以來感到迷惑不解。最近卡利斯勒（T．R．Carlisle）小姐在一個研究班上提出了一個很有獨創性的解釋，由此，我深受啟發。她以上面我們提及的特里弗斯的無情的約束概念去闡明下面這種現象。 許多種類的魚是不交尾的，它們只是把性細胞射到水裡。受精就在廣闊的水域裡進行，而不是在一方配偶的體內。有性生殖也許就是這樣開始的。另一方面，生活在陸地上的動物如鳥類、哺乳動物和爬蟲等卻無法進行這種體外受精，因為它們的性細胞容易乾燥致死。一種性別的配子雄性個體的，因為其精子是可以流動的被引入另一種性別個體雌性個體的濕潤的內部。上面所說的只是事實，而下面講的卻是概念的東西。居住在陸地上的雌性動物交配後就承受胎兒的實體。胎兒存在於它體內。即使它把已受精的卵子立即生下來，做父親的還是有充裕的時間不辭而別，從而把特里弗斯所謂的無情的約束強加在這個雌性個體身上。不管怎樣，雄性個體總是有機會事先決定遺棄配偶，從而迫使做母親的作出抉擇，要麼拋棄這個新生幼兒，讓它死去；要麼把它帶在身邊並撫養它。因此，在陸地上的動物當中，照料後代的大多數是母親。 但對魚類及生活在水中的其他動物而言，情況有很大的差別。如果雄性動物並不直接把精子送進雌性體內，我們就不一定可以說，做母親的受騙上當，被迫照管幼兒。配偶的任何一方部可以有機會逃之夭夭，讓對方照管剛受精的卵子。說起來還存在這樣一個可能性：倒是雄性個體常常更易於被遺棄。對誰先排出性細胞的問題，看來可能展開一場進化上的爭鬥。首先排出性細胞的一方享有這樣一個有利條件它能把照管新生胎兒的責任推給對方。另一方面，首先放精或排卵的一方必然要冒一定的風險，因為它的未來的配偶不一定跟著就排卵或放精。在這種情況下，雄性個體處於不利地位，因為精子較輕，比卵子更易散失。如果雌性個體排卵過早，就是說，在雄性個體還未準備好放精就產卵子，這關係不大。因為卵子體積較大，也比較重，很可能集結成一團，一時不易散失。所以說，雌性魚可以冒首先排卵的風險。雄性魚就不敢冒這樣的風險，因為它過早放精，精子可能在雌性魚準備排卵之前就散失殆盡，那時雌性魚即使再排卵也沒有實際意義。鑒於精子易於散失，雄性魚必須等待到雌性魚排卵後才在卵子上放精。但這樣，雌性魚就有了難得的幾秒鐘時間可以趁機溜走，把受精卵丟給雄性魚照管，使之陷入特里弗斯所說的進退兩難的境地。這個理論很好地說明，為什麼水中的雄性動物照料後代的這種現象很普遍而在陸上的動物中卻很少見。 我現在談談魚類以外的另一種雌性動物採取的策略，即大丈夫策略。在採取這種策略的物種中，事實上，雌性動物對得不到孩子們的爸爸的幫助已不再計較，而把全部精力用於培育優質基因。於是它們再次把拒絕交配作為武器。它們不輕易和任何雄性個體交配，總是慎之又慎，精心挑選，然後才同意和選中的雄性個體交配。某些雄性個體確實比其他個體擁有更多的優質基因，這些基因有利於提高子女的生育機會。如果雌性動物能夠根據各種外在的跡象判斷哪些雄性動物擁有優質基因，它就能夠使自己的基因和它們的優質基因相結合而從中獲益。以賽艇獎手的例子來類比，一個雌性個體可以最大限度地減少它的基因由於與蹩腳的槳手搭檔而受到連累的可能性。它可以為自己的基因精心挑選優秀的槳手作為合作者。 一般地說，大多數雌性動物對哪些才是最理想的雄性動物不會發生意見分歧，因為它們用以作為判斷的依據都是一樣的。結果，和雌性個體的大多數交配是由少數這幾個幸運的雄性個體進行的。它們是能夠愉快勝任的，因為它們給予每一雌性個體的僅僅是一些廉價的精子而已。象形海豹和極樂鳥大概也是這種情況。雌性動物只允許少數幾隻雄性動物坐享所有雄性動物都夢寐以求的特權一種追求私利的策略所產生的特權，但雌性個體總是毫不含糊，成竹在胸，只允許最夠格的雄性個體享有這種特權。 雌性動物試圖挑選優質基因並使之和自己的基因相結合，按照它的觀點，它孜孜以求的是哪些條件呢？其中之一是具有生存能力的跡象。任何向它求愛的個體已經證明，它至少有能力活到成年，但它不一定就能夠證明，它能夠活得更久些。凡選擇年老雄性個體的雌性個體，同挑選在其他方面表明擁有優質基因的年輕個體的雌性個體相比，前者生的後代並不見得就多些。 其他方面指的是什麼？可能性很多。也許是表明能夠捕獲食物的強韌的肌肉，也許是表明能夠逃避捕食者的長腿。雌性個體如能將其基因和這些特性結合起來可能是有好處的，因為這些特性在它的兒女身上或許能發揮很好的作用。因此，我們首先必須設想存在這樣的雌性動物，它們選擇雄性個體是根據表明擁有優質基因的萬無一失的可靠跡象，不過，這裡牽涉到達爾文曾發現的一個非常有趣的問題，費希爾對之也進行過有條理的闡述。在雄性個體相互競爭，希望成為雌性個體心目中的大丈夫的社會裡，一個做母親的能為其基因所做的最大一件好事是，生一個日後會成為一個令人刮目相看的大丈夫的兒子。如果做母親的能保證它的兒子將成為少數幾個走運的雄性個體中的一個，在它長大之後能贏得社會裡的大多數交配機會，那麼，這個做母親的將會有許多孫子孫女。這樣說來，一個雄性個體所能擁有的最可貴的特性之一，在雌性個體看來只不過是性感而已。一個雌性個體和一個誘人非凡並具有大丈夫氣概的雄性個體支配，很可能養育出對第二代雌性個體具有吸引力的兒子。這些兒子將為其母親生育許多孫子孫女。這樣，我們原來認為雌性個體選擇雄性個體是著眼於如發達的肌肉那種顯然是有實用價值的特性，但是這種特性一旦在某一物種的雌性個體中普遍被認為是一種具有吸引力的東西時，自然選擇就會僅僅因為它具有吸引力而繼續有利於這種特性。 雄極樂鳥的尾巴作為一種過分奢侈的裝飾，因此可能是通過某種不穩定的、失去控制的過程進化而來的。在開始的時候，雌性個體選中尾巴稍許長一些的雄性個體，在它心目中這是雄性個體的一種可取的特性，也許因為它象徵著健壯的體魄。雄性個體身上的短尾巴很可能是缺乏某種維生素的徵象說明該個體覓食能力差。或許短尾巴的雄性動物不善於逃避捕食者，因此尾巴被咬掉一截。請注意，我們不必假定短尾巴本身是能夠遺傳的，我們只需假定短尾巴可以說明某種遺傳上的缺陷。不管怎樣，我們可以假定，早期的極樂鳥物種中，雌鳥偏愛尾巴稍微長一些的雄鳥。只要存在某種促進雄鳥尾巴長度發生自然變化的遺傳因素，隨著時間的推移，這個因素就會促使種群中雄鳥尾巴的平均長度增加。雌鳥遵循的一條簡單的準則是：把所有的雄鳥都打量一番，並挑選尾巴最長的一隻，如此而已。背離這條準則的雌鳥準會受到懲罰，即使尾巴已經變得如此之長，實際上成了雄鳥的累贅。因為一隻雌鳥如果生出的兒子尾巴不長，它的兒子就不可能被認為是有吸引力的。只有在尾巴確實已長到可笑的程度，因而它們明顯的缺點開始抵消性感這方面的優點時，這個趨向才得以終止。 這是個令人難以接受的論點；自達爾文初次提出這個論點並把這個現象稱為性選擇以來，已有不少人對之表示懷疑。扎哈維（A．zahavi）就是其中之一，他的狐狸，狐狸論點我們已經看到過。作為一個對立面，他提出截然相反的累贅原理（handicap principle）。他指出，正是因為雌性個體著眼於選擇雄性個體的優質基因，才使雄性弄虛作假有了市場。雌性個體所選擇的發達的肌肉可能真的是一個優點，但有什麼東西阻止雄性個體賣弄假肌肉呢？這些假肌肉並不比我們人類的棉花墊肩更具實質內容。如果雄性個體賣弄假肌肉反而比長出真肌肉省事，性選擇應有利於促使個體長出假肌肉的基因。可是，要不了多久，逆選擇（counter-selection）將促使能夠看穿這種欺騙的雌性個體的進化。扎哈維的基本前提是，雌性個體終將識破虛假的性賣弄。因此他得出的結論是，真正能夠成功的是那些從不故弄玄虛的雄性個體。它們擲地有聲地表明它們是老老實實的。如果我們講的是肌肉，那麼，裝出肌肉豐滿的樣子的雄性個體很快就要為雌性個體所識破。反之，以相當於舉重等動作顯示其肌肉真正發達的雄性個體是能夠獲得雌性信賴的。換句話說，扎哈維認為，一個大丈夫不僅看上去要像一個健全的雄性個體，而且要真的是一個健全的雄性個體，否則不輕信的雌性個體是會嗤之以鼻的。所以，只有是貨真價實的大丈夫，它的炫耀行為才能進化。 到現在為止，扎哈維的理論還沒有什麼問題。下面我們要談的是他的理論中使人難以接受的那一部分。他認為，儘管極樂鳥和孔雀的長尾巴，鹿的巨角以及其他的性選擇的特性，看起來是這些個體的累贅，因而始終是不合理的現象，但這些特徵之得以進化正是因為它們構成累贅。一隻雄鳥長了一條長長的、笨重的尾巴，為的是要向雌性個體誇耀，說明儘管它有這樣一條長尾巴，像他這樣一個健壯的大丈夫還是能夠活下去。 這個理論很難使我信服，儘管我所持的懷疑態度已不像我當初聽到這個論點時那麼堅決。當時我就指出，根據這種理論可以得出這樣的邏輯結論：進化的結果應該使雄性個體只有一條腿和一隻眼睛。扎哈維是以色列人，他立即反駁我說，我們最好的將軍中有些是獨眼的！不過問題還是存在的。累贅的論點似乎帶有根本性的矛盾。如果累贅是真實的這種論點的實質要求累贅必須是真實的累贅本身正如它可能吸引雌性個體一樣，同樣對該個體的後代肯定是一種懲罰。因此不管怎樣，至關重要的是這個累贅不能傳給女兒。 如果我們以基因語言來表達累贅理論，我們大概可以這樣說：使雄性個體長出如長尾巴之類的累贅物的基因在基因庫裡變得多起來，因為雌性個體選擇身負累贅物的雄性個體。這種情況的產生是因為，使雌性個體作出這種選擇的基因在基因庫裡也變得多起來的緣故。這是因為對身負累贅物的雄性個體有特殊感情的雌性個體，往往會自動地選擇在其他方面擁有優質基因的雄性個體。理由是，儘管身負這種累贅物，這些雄性個體已活到成年。這些擁有其他方面優點的基因將使後代具有健壯的體格。而這些具有健壯體格的後代因此得以存活並繁殖使個體生長累贅物的基因，以及使雌性個體選擇身負累贅物的雄性個體的基因。倘若促使生長累贅物的基因僅僅在兒子身上發揮作用，就像促使對累贅物產生性偏愛的基因僅僅影響女兒那樣，這個理論也許可以成立。如果我們只是以文字對這個理論去進行論證，我們就無從知道這個理論是否正確。如果我們能以數學模式來再現這種理論，我們就能更清楚地看到它的正確程度。到目前為止，那些試圖以模型來表現累贅原理的數學遺傳學家都失敗了。這可能是因為這個原理本身不能成立，也可能是因為這些數學遺傳學家不夠水平。其中有一位便是史密斯。但我總感覺到前者的可能性較大。 如果一隻雄性動物能以某種方式證明它比其他雄性動物優越，而這種方式又無需故意使自己身負累贅，那麼它無疑會以這種方式增加它在遺傳方面取得成功的可能性。因此，象形海豹贏得並確保它們的妻妾，靠的不是它對雌性個體具有吸引力的堂堂儀表，而是簡單地靠暴力把妄圖接近其妻妾的任何雄性象形海豹攆走。妻妾的主人大都能擊敗這種可能的掠奪者，它們之所以擁有妻妾顯然是因為它們有這樣的能力。掠奪者很少能取勝，因為它們如能取勝，它們早該成為妻妾的主人了！因此，凡是只同妻妾的主人交配的雌性象形海豹，就能使它的基因和健壯的雄性象形海豹相結合，而這只雄性象形海豹有足夠的能力擊退一大群過剩的、不顧死活的單身雄性象形海豹所發動的一次又一次的挑釁。這只雌性象形海豹的兒子如果走運的話，它就能繼承其父的能力，也擁有一群妻妾。事實上，一隻雌性象形海豹沒有很大的選擇餘地，因為如果它有外遇，它就要遭到妻妾主人的痛打。不過，跟能在搏鬥中取勝的雄性個體結合的雌性個體，能為其基因帶來好處，這條原理是站得住腳的。我們已經看到這樣一些例子，即一些雌性個體寧願和擁有地盤的雄性個體交配，另外一些寧願和在統治集團裡地位高的雄性個體交配。 至此本章的內容可以歸結為：我們看到，在動物界中各種不同的繁殖制度雌一雄、雌雄亂交、妻妾等等都可以理解為雌雄兩性間利害衝突所造成的現象。雌雄兩性的個體都想要在其一生中最大限度地增加它們的全部繁殖成果。由於精子和卵子之間在大小和數量方面存在根本上的差別，雄性個體一般地說大多傾向於雌雄亂交，而缺乏對後代的關注。雌性個體有兩種可供利用的對抗策略。我在上面曾稱之為大丈夫策略和家庭幸福策略。一個物種的生態環境將決定其雌性個體傾向於採取其中的哪一種策略，同時也決定雄性個體如何作出反應。事實上，在大丈夫策略和家庭幸福策略之間還有許多居間策略。我們已經看到，有時候，做父親的甚至比做母親的更關心孩子們的生活。本書不打算描述某些具體動物物種的生活細節。因此我不準備討論是什麼促使一個物種傾向於某種繁殖制度而不傾向於另一種繁殖制度。我要探討的是普遍地存在於雌雄兩性之間的差異，並說明如何解釋這些差異。我因此不想強調兩性間差異不大的那些物種；一般地說，這些物種的雌性個體喜歡採取家庭幸福策略。 首先，雄性個體往往是追求鮮艷的色彩以吸引異性，而雌性個體往往滿足於單調的色彩。兩性個體都力圖避免被捕食者吃掉，因此兩性個體都會經受某種進化上的壓力，使它們的色彩單調化。鮮艷的色彩吸引捕食者，猶如吸引異性伴侶一樣。用基因語言來說，這意味著使個體色彩變得鮮艷的基因，比使個體色彩單調的基因，更可能被捕食者吃掉而結束生命。另一方面，促使個體具有單調色彩的基因不像促使個體具有鮮艷色彩的基因那麼容易進入下一代的體內，因為色彩單調的個體不吸引異性配偶。這樣就存在兩種相互矛盾的選擇壓力：捕食者傾向於消滅基因庫裡色彩鮮艷的基因，而性配偶則傾向於消滅色彩單調的基因。和其他許多情況一樣，有效的生存機器可以認為是兩種相互矛盾的選擇壓力之間的折衷物。眼下使我們感到興趣的是，雄性個體的最適折衷形式似乎不同於雌性個體的最適折衷形式。這種情況當然和我們把雄性個體視為下大賭注以博取巨額贏款的賭徒完全一致，因為雌性個體每生產一個卵子，雄性個體就可以生產數以百萬計的精子，因此種群中的精子在數量上遠遠超過卵子。所以任何一個卵子比任何一個精子實現性融合（sexual fusion）的機會要大得多。相對而言，卵子是有價值的資源。因此，雌性個體不必像雄性個體那樣具有性吸引力就能保證它的卵子有受精的機會。一個雄性個體的生殖能力完全可以使一大群雌性個體受孕，生育出一大批子女。即使一隻雄性個體因為有了美麗的長尾巴而引來了捕食者或纏結在叢林中而過早死亡，但它在死以前可能已經繁殖了一大群子女。一隻沒有吸引力或色彩單調的雄性個體，甚至可能和一隻雌性個體同樣長壽，但它子女卻很少，因而它的基因不能世代相傳。一個雄性個體如果失去了它的不朽的基因，那它即使佔有了整個世界又將怎麼樣呢？ 另一個帶有普遍性的性區別是，雌性個體在和誰交配的問題上比雄性個體更愛挑剔。不管是雌性個體還是雄性個體，為了避免和不同物種的成員交配，這種挑剔還是必要的。從各個方面來看，雜交行為是不好的。有時，像人和羊交配一樣，這種行為並不產生胚胎，因此損失不大。然而，當比較接近的物種如馬和驢雜交時，這種損失，至少對雌性配偶來說，可能是相當大的。一個騾子胚胎可能由此形成，並在它的子宮裡呆上十一個月。騾子消耗母體全部親代投資的很大一部分，不僅包括通過胎盤攝取的食物，以及後來吃掉的母乳，而且最重要的是時間，這些時間本來可用於撫養其他子女的。騾子成年以後，它卻是沒有繁殖力的。這可能是因為，儘管馬和驢的染色體很相像，使它們能合作建造一個健壯的騾子軀體，但它們又不盡相像，以致不能在減數分裂方面進行適當的合作。不管確切的原因是什麼，從母體基因的觀點來看，母體為撫育這隻騾子而花掉的非常大量的資源全部浪費了。雌驢子應當十分謹慎，和它交配的必須是一隻驢子，不是一匹馬。任何一個驢子基因如果說，喂，如果你是雌驢，那就不管它是馬還是驢，只要它是年老的雄性個體，你都可以和它交配，這個基因下次就可能跑到騾子的體內，結果將是死路一條。母體花在這只幼騾身上的親代投資將大大降低它養育有生殖力的驢子的能力。另一方面，如果雄性個體和其他不同物種的成員交配，它的損失不會太大，儘管它從中也得不到什麼好處。但我們卻可以認為，在選擇配偶的問題上，雄性個體不致過分苛求。凡是對這種情況進行過研究的人都會發現情況確實是如此。 即使在同一物種中，挑剔的情況還是會有的。同一血族之間的交配，和雜交一樣，可能產生不利的遺傳後果，因為在這種情況下，致命的或半致命的隱性基因獲得公然活動的機會。這種情況再次使雌性個體的損失比雄性個體大，因為母體花在某一幼兒身上的資源總是要大些。凡是存在亂倫禁忌的地方，我們可以認為雌性個體會比雄性個體更嚴格地遵守這種禁忌。如果我們假定在亂倫關係中，年齡較大的一方相對來說更有可能是主動者的話，那麼我們應該看到，雄性個體年齡比雌性個體年齡大的亂倫行為一定較雌性個體年齡比雄性個體年齡大的亂倫行為普遍，譬如說，父／女亂倫應該比母／子亂倫更普遍。兄弟／姐妹亂倫行為的普遍性介乎兩者之間。 一般地說，雄性個體比雌性個體往往有更大的亂交傾向。雌性個體只能以比較慢的速度生產有限的卵子，因此，它和不同的雄性個體進行頻繁的交配不會有什麼好處。另一方面，雄性個體每天能夠生產數以百萬計的精子，如果它利用一切機會和盡量多的雌性個體交配，它只會從中得到好處而不會有任何損失。過於頻繁的交配行為事實上對雌性個體的害處並不很大，但好處肯定也是沒有的。另一方面，雄性個體卻能樂此不疲，不管它和多少個不同的雌性個體交配。過分這個字眼對雄性個體來說沒有實際意義。 我沒有明確地提到人類，但當我們思考如本章涉及的一些有關進化的論點時，我們不可避免地要聯想到我們自己的物種和我們自己的經驗。雌性個體只有在對方在一定程度上表明能夠長期忠貞不渝時才肯與之交配，這種做法對我們來說並不陌生。這可能說明，人類的婦女採取的是家庭幸福策略，而不是大丈夫策略。人類社會事實上大多數實行一夫一妻制。在我們自己的社會裡，父母雙方對子女的親代投資都是巨額的，而且沒有明顯的不平衡現象。母親直接為孩子們操勞，所做的工作比父親多。但父親常常以比較間接的方式辛勤工作，為孩子們提供源源不斷的物質資源。另一方面，有些人類社會有雜交習俗，而有些則實行妻妾制度。這種令人驚訝的多樣性說明，人的生活方式在很大程度上取決於文化而不是基因。然而，更大的可能性是，男人大多傾向於雜交，女人大多傾向於一夫一妻。根據進化的理論，我們也可以預見到這兩種傾向。在一些具體的社會裡，哪一種傾向佔上風取決於具體的文化環境，正如在不同的動物物種中，要取決於具體的生態環境一樣。 我們自己的社會有一個肯定與眾不同的特點，這就是性的炫耀行為。我們已經看到，根據進化的理論，凡有不同性別個體存在的地方，喜歡炫耀的應該是男人，而女人則喜歡樸實無華。在這一點上，現代的西方男人無疑是個例外。當然，有些男人衣飾鮮艷，有些女人衣飾樸素，這也是事實。但就大多數的情況而言，在我們的社會裡，像孔雀展示尾巴一樣炫耀自己的毫無疑問是婦女，而不是男人。 面對這些事實，生物學家不得不感到疑惑，他觀察到的社會是一個女人爭奪男人而不是男人爭奪女人的社會。在極樂鳥的例子裡，我們認為雌鳥的色彩之所以樸素是因為它們不需要爭奪雄鳥。雄鳥色彩鮮艷華麗，因為雌鳥供不應求，對雄鳥可以百般挑剔。雌極樂鳥之所以供不應求是因為卵子這種資源比精子稀少。現代的西方男性到底發生了什麼變化？男人果真成了被追求的性對象了嗎？他們真的因供不應求而能百般挑剔嗎？如果情況果真如此，那又是為什麼呢？