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 50% genetic investment in their offspring.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 "hope" to mate with 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 mate put Children are brought up.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 the frogs into sex 1 and sex 2 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.One of their eggs 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, called 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 (50%) 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" that are evolving.One is a heavy investment or "honest" strategy.This strategy naturally opens the way for small investments, exploitative or "cunning" strategies.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 "prefers" to fuse with another honest gamete.But the natural selection pressure to repel crafty gametes is weaker than the pressure to exploit them: because crafty gametes must win this evolutionary battle or lose a lot.So the honest gamete becomes an egg, and the cunning gamete becomes a sperm.

In this way, males are insignificant creatures, and on the basis of simple "good of the species" theory, we would expect that there would be fewer and fewer males than females.Because theoretically, one male individual produces enough sperm to satisfy the needs of a hundred female individuals, we can therefore assume that the ratio of male to female individuals in an animal population should be 100:1.In other words, male individuals are more "low-value and easy to consume", 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 4% of male elephant seals performed 88% 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 a "good of the species" point of view, this situation is a huge waste; these extra males are social parasites, so to speak.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 strategy.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?Fisher proved that under normal circumstances, the most appropriate sex ratio is 50:50.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 only in one coloration.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 That's it.What we are looking for is something equivalent to an evolutionarily stable strategy (ESS), although here strategy is more of a metaphor (in the chapter on aggressive behavior we have used this metaphor).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.Investing more food in the son, making him big and strong, the parents may give him more chances to win the highest prize of "wives and concubines".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 assumes a greater "commitment" to each young child than the father from the moment of conception.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 toddler to the same size as the dead toddler.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're a male, leave your mate earlier and find another female instead of waiting until my alleles tell you to leave." 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 have to assume that some kind of evolutionary counterpressure is at work: selfish exploitation of a mate must not only be rewarded but also punished.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 what is known as the Bruce effect: Male rats secrete a chemical that, when pregnant females smell it, is able to abort themselves.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 a longer "engagement" period.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, assuming I can be sure that my spouse won't leave too, it would make sense for me to leave now." Good. If I were to leave now, my spouse would be able to work hard in her or his genes' best interest. He or she would be forced to make a much more drastic decision than I am currently making, Because I have left. My spouse understands that if he or she leaves too, the infant will surely die. So, assuming my spouse makes a decision that will be in the best interest of his or her selfish genes, I conclude that My own course of action is that it's best for me to leave first. Because my spouse may be considering the exact same course as I am, and may come to preemptively 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 has become very "close" 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 never says, "I've invested so much in (for example) the Concorde that it's not worth throwing it away." It is very big, but in order to reduce the loss, is it good for his future to give up this business now.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 term "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 parental individuals receive 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 caring for the child, and the risks for the child, is -20 units.The cost is expressed as a negative number, since it is an "expenditure" of the parent.The time spent in the protracted pursuit is also negative, 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.For each child raised, they each get +15 points, and share the cost (-20 points), shared equally, with each party getting -10 points.They jointly pay the price of protracted courtship (-3 penalty).Therefore, the average profit per child raised is: +15-10-3=+2. 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 delay, because it does not indulge in protracted courtship. 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 +15-10=+5.It gains 3 units more than its shy and coy counterpart.And so the debauched genes began to spread. If loose females become so successful that they become dominant in the population, things start to change in the male camp as well.截至目前为止，种群内忠诚的雄性个体占有垄断地位。但如果现在种群中出现了一个薄情的雄性个体，它的景况会比其他的忠诚的对手好些。在一个雌性个体都放荡不羁的种群内，对一个薄情的雄性个体来讲，这类货色比比皆是，唾手可得。如果能顺利地抚养一个幼儿，它净得盈利+15分，而对两种代价却分文不付。对雄性个体来说，这种不付任何代价指的主要是，它可以不受约束地离开并同其他雌性个体进行交配。它的每一个不幸的妻子都得独自和幼儿挣扎着生活下去，承担起-20分的全部代价，尽管她并没因在求爱期间浪费时间而付出代价。一个放荡的雌性个体结交一个薄情的雄性个体，其净收益为＋15－20=-5；而薄情的雄性个体的收益却是＋15。在一个雌性个体都放荡不羁的种群中，薄情的雄性基因就会象野火一样蔓延开来。 如果薄情的雄性个体得以大量地迅速增长，以致在种群的雄性成员中占了绝对优势，放荡的雌性个体将陷于可怕的困难处境。任何羞忸怩的雌性个体都会享有很大的有利条件。如果羞怯忸怩的雌性个体同薄情的雄性个体相遇，它们之间绝不会有什么结果。她坚持要把求爱的时间拉长；而他却断然拒绝并去寻找另外的雌性个体。双方都没有因浪费时间而付出代价。双方也各无所得，因为没有幼儿出生。在所有雄性个体都是薄情郎的种群中，羞怯忸怩的雌性个体的净收益是零。 零看上去微不足道，但比放荡不羁的雌性个体的平均得分-5要好得多。即使放荡的雌性个体在被薄情郎遗弃之后，决定抛弃她的幼儿，但她的一颗卵子仍旧是她所付出的一笔相当大的代价。因此，羞怯忸怩的基因开始在种群内再次散布开来。 现在让我们来谈谈这一循环性假设的最后一部分。当羞怯忸怩的雌性个体大量增加并占据统治地位时，那些和放荡的雌性个体本来过着纵欲生活的薄情雄性个体，开始感到处境艰难。一个个的雌性个体都坚持求爱时间要长，要长期考验对方的忠诚。薄情的雄性个体时而找这个雌性个体，时而又找那个雌性个体，但结果总是到处碰壁。因此，在一切雌性个体都忸怩作态的情况下，薄情雄性个体的净收益是零。如果一旦有一个忠诚的雄性个体出现，它就会成为同羞怯忸怩的雌性个体交配的唯一雄性个体。那么他的净收益是＋2，比薄情的雄性个体要好。 所以，忠诚的基因就开始增长，至此，我们就完成了这一周而复始的循环。 象分析进犯行为时的情况一样，按我的讲法，这似乎是一种无止境的摇摆现象。 但实际上，象那种情况一样，不存在任何摇摆现象，这是能够加以证明的。整个体系能够归到一种稳定状态上。如果你运算一下，就可证明，凡是羞怯忸怩的雌性个体占全部雌性个体的5/6，忠诚的雄性个体占全部雄性个体的5/8的种群在遗传上是稳定的。当然，这仅仅是根据我们开始时任意假定的那些特定数值计算出来的，但对其他任何随意假定的数值，我们同样可以轻而易举地算出新的稳定比率。 同史密斯所进行的分析一样，我们没有必要认为存在两种不同种类的雄性个体以及两种不同种类的雌性个体。如果每一雄性个体能在5/8的时间里保持忠诚，其余的时间去寻花问柳；而每一雌性个体有5/6的时间羞怯忸怩，1/6的时间纵情放荡，那同样可以实现进化上的稳定状态。不管你怎样看待ESS，它的含义是：凡一种性别的成员偏离其适中的稳定比率时，这种倾向必然受到另一种性别在策略比率方面相应变化的惩罚，这种变化对原来的偏离行为发生不利的影响。进化上的稳定策略（ESS）因之得以保持。 我们可以得出这样的结论，主要由羞怯忸怩的雌性个体和忠诚的雄性个体组成的种群，能够进化是肯定无疑的。在这样的情况下，家庭幸福策略对于雌性个体来说，实际上看来是行之有效的。我们就不必再考虑什么由羞怯忸怩的雌性个体组成的集团了，其实羞怯忸怩对雌性个体的自私基因是有利的。 雌性个体能够以各种各样的方式将这种形式的策略付诸实践。我已经提到过，雌性个体可能拒绝同还没有为它筑好巢、至少还没有帮助它筑造一个巢的雄性个体交配。在许多单配制的鸟类中，情况的确如此，巢不筑好不交配。这样做的效果是，在受孕的时刻，雄性个体对幼儿已经付出的投资要远较其一些廉价的精子为多。 未来的配偶必须为它筑造一个巢，这种要求是雌性个体约束雄性个体的一种有效手段。我们不妨说，只要能够使雄性个体付出昂贵的代价，不论是什么，在理论上几乎都能奏效，即使付出的这种代价，对尚未出生的幼儿并没有直接的益处。 如果一个种群的所有雌性个体都强迫雄性个体去完成某种艰难而代价昂贵的任务，如杀死一条龙或爬过一座山然后才同意交配，在理论上讲，它们能够降低雄性个体在交配后不辞而别的可能性。企图遗弃自己的配偶，并要和另外的雌性个体交配以更多地散布自己基因的任何雄性个体，一想到必须还要杀死一条龙，就会打消这种念头。然而事实上雌性个体是不会将杀死一条龙或寻求圣杯这样专横的任务硬派给它们的求婚者的。因为如果有一个雌性个体对手，它指派的任务尽管困难程度相同，但对它以及它的子女却有更大的实用价值，那么它肯定会优越于那些充满浪漫情调、要求对方为爱情付出毫无意义的劳动的雌性个体。杀死一条龙或在Hellespont中游泳也许比筑造一个巢穴更具浪漫色彩，但却远远没有后者实用。 我提到过的雄性个体做出的具有求爱性质的喂食行动对于雌性个体也是有用的。 鸟类的这种行为通常被认为是雌性个体的某种退化现象，它们恢复了雏鸟时代的幼稚行为。雌鸟向雄鸟要食物，讨食的姿态象雏鸟一样。有人认为这种行为对雄鸟具有自然的诱惑力，这时雌鸟不管能得到什么额外的食物，它都需要，因为雌鸟正在建立储存，以便致力于制造很大的卵子。雄鸟的这种具有求爱性质的喂食行为，也许是一种对卵子本身的直接投资。因此，这种行为能够缩小双亲在对幼儿的初期投资方面存在的悬殊程度。 有几种昆虫和蜘蛛也存在这种求爱性质的喂食现象。很显然，有时这种现象完全可以作另外的解释。如我们提到过的螳螂的例子，由于雄螳螂有被较大的雌螳螂吃掉的危险，因此只要能够减少雌螳螂的食欲，随便干什么对它可能都是有利的。我们可以说，不幸的雄螳螂是在这样一种令人毛骨悚然的意义上对其子女进行投资的。雄螳螂被作为食物吃掉，以便帮助制造卵子，而且储存在雄螳螂尸体内的精子随之使吃掉它的雌螳螂的卵子受精。 采取家庭幸福策略的雌性个体如果仅仅是从表面上观察雄性个体，试图辨认它的忠诚的品质，这样的雌性个体容易受骗。雄性个体只要能够冒充成忠诚的爱好家庭生活的类型，而事实上是把遗弃和不忠诚的强烈倾向掩盖起来，它就具有一种很大的有利条件。只要过去被它遗弃的那些妻子能有机会将一些幼儿抚养大，这个薄情的雄性个体比起一个既是忠诚的丈夫又是忠诚的父亲的雄性对手，能把更多的基因传给后代。使雄性个体进行有效欺骗的基因在基因库中往往处于有利地位。 相反，自然选择却往往有利于善于识破这种欺骗行为的雌性个体。要做到这一点，雌性个体在有新的雄性个体追求时，要显得特别可望而难及，但在以后的一些繁殖季节中，一旦去年的配偶有所表示，就要毫不犹豫，立刻接受。这样对那些刚开始第一个繁殖季节的年轻的雄性个体来说，不论它们是骗子与否，都会自动受到惩罚。天真无知的雌性个体在第一年所生的一窝小动物中，体内往往有比例相当高的来自不忠诚的父亲的基因，但忠诚的父亲在第二以及以后的几年中却具有优势，因为它有了一个可靠的配偶，不必每年都要重复那种浪费时间、消耗精力、旷日持久的求爱仪式。在一个种群中，如果大部分的个体都是经验丰富而不是天真幼稚的母亲的子女--在任何生存时间长的物种中，这是一个合乎情理的假设--忠诚而具模范父亲性格的基因在基因库中将会取得优势。 为简便起见，我把雄性个体的性格讲得似乎不是纯粹的忠诚就是彻头彻尾的欺诈。事实上，更有可能的是，所有的雄性个体--其实是所有的个体--多少都有点不老实，它们的程序编制就是为了利用机会去占它们配偶的便宜。由于自然选择增强了每一个配偶发现对方不忠诚的行为的能力，因此使重大的欺骗行为降到了相当低的水平。雄性个体比雌性个体更能从不忠诚的行为中得到好处。即使在一些物种中，雄性个体表现出很大程度的亲代利他主义行为，但我们必须看到，它们付出的劳动往往比雌性个体要少些，而且随时潜逃的可能性更大些。鸟类和哺乳类动物中，通常存在这种情况是肯定无疑的。 但是也有一些物种，其雄性个体在抚养幼儿方面付出的劳动实际上比雌性个体多。鸟类和哺乳类动物中，这种父方的献身精神是极少有的，但在鱼类中却很常见。What is the reason?这种现象是对自私基因理论的挑战，为此我长时间以来感到迷惑不解。最近卡利斯勒（TRCarlisle）小姐在一个研究班上提出了一个很有独创性的解释，由此，我深受启发。她以上面我们提及的特里弗斯的"无情的约束"概念去阐明下面这种现象。 许多种类的鱼是不交尾的，它们只是把性细胞射到水里。受精就在广阔的水域里进行，而不是在一方配偶的体内。有性生殖也许就是这样开始的。另一方面，生活在陆地上的动物如鸟类、哺乳动物和爬虫等却无法进行这种体外受精，因为它们的性细胞容易干燥致死。一种性别的配子--雄性个体的，因为其精子是可以流动的--被引入另一种性别个体--雌性个体--的湿润的内部。上面所说的只是事实，而下面讲的却是概念的东西。居住在陆地上的雌性动物交配后就承受胎儿的实体。胎儿存在于它体内。即使它把已受精的卵子立即生下来，做父亲的还是有充裕的时间不辞而别，从而把特里弗斯所谓的"无情的约束"强加在这个雌性个体身上。不营怎样，雄性个体总是有机会事先决定遗弃配偶，从而迫使做母亲的作出抉择，要么抛弃这个新生幼儿，让它死去；要么把它带在身边并抚养它。因此，在陆地上的动物当中，照料后代的大多数是母亲。 但对鱼类及生活在水中的其他动物而言，情况有很大的差别。如果雄性动物并不直接把精子送进雌性体内，我们就不一定可以说，做母亲的受骗上当，被迫"照管幼儿"。配偶的任何一方部可以有机会逃之夭夭，让对方照管刚受精的卵子。 说起来还存在这样一个可能性：倒是雄性个体常常更易于被遗弃。对谁先排出性细胞的问题，看来可能展开一场进化上的争斗。首先排出性细胞的一方享有这样一个有利条件--它能把照管新生胎儿的责任推给对方。另一方面，首先放精或排卵的一方必然要冒一定的风险，因为它的未来的配偶不一定跟着就排卵或放精。在这种情况下，雄性个体处于不利地位，因为精子较轻，比卵子更易散失。 如果雌性个体排卵过早，就是说，在雄性个体还未准备好放精就产卵子，这关系不大。因为卵子体积较大，也比较重，很可能集结成一团，一时不易散失。所以说，雌性鱼可以冒首先排卵的"风险"。雄性鱼就不敢冒这样的风险，因为它过早放精，精子可能在雌性鱼准备排卵之前就散失殆尽，那时雌性鱼即使再排卵也没有实际意义。鉴于精子易于散失，雄性鱼必须等待到雌性鱼排卵后才在卵子上放精。但这样，雌性鱼就有了难得的几秒钟时间可以趁机溜走，把受精卵丢给雄性鱼照管，使之陷入特里弗斯所说的进退两难的境地。这个理论很好地说明，为什么水中的雄性动物照料后代的这种现象很普遍而在陆上的动物中却很少见。 我现在谈谈鱼类以外的另一种雌性动物采取的策略，即大丈夫策略。在采取这种策略的物种中，事实上，雌性动物对得不到孩子们的爸爸的帮助已不再计较，而把全部精力用于培育优质基因。于是它们再次把拒绝交配作为武器。它们不轻易和任何雄性个体交配，总是慎之又慎，精心挑选，然后才同意和选中的雄性个体交配。某些雄性个体确实比其他个体拥有更多的优质基因，这些基因有利于提高子女的生育机会。如果雌性动物能够根据各种外在的迹象判断哪些雄性动物拥有优质基因，它就能够使自己的基因和它们的优质基因相结合而从中获益。以赛艇奖手的例子来类比，一个雌性个体可以最大限度地减少它的基因由于与蹩脚的桨手搭档而受到连累的可能性。它可以为自己的基因精心挑选优秀的桨手作为合作者。 一般他说，大多数雌性动物对哪些才是最理想的雄性动物不会发生意见分歧，因为它们用以作为判断的依据都是一样的。结果，和雌性个体的大多数交配是由少数这几个幸运的雄性个体进行的。它们是能够愉快胜任的，因为它们给予每一雌性个体的仅仅是一些廉价的精子而已。象形海豹和极乐鸟大概也是这种情况。雌性动物只允许少数几只雄性动物坐享所有雄性动物都梦寐以求的特权--一种追求私利的策略所产生的特权，但雌性个体总是毫不含糊，成竹在胸，只允许最够格的雄性个体享有这种特权。 雌性动物试图挑选优质基因并使之和自己的基因相结合，按照它的观点，它孜孜以求的是哪些条件呢？其中之一是具有生存能力的迹象。任何向它求爱的个体已经证明，它至少有能力活到成年，但它不一定就能够证明，它能够活得更久些。 凡选择年老雄性个体的雌性个体，同挑选在其他方面表明拥有优质基因的年轻个体的雌性个体相比，前者生的后代并不见得就多些。 其他方面指的是什么？可能性很多。也许是表明能够捕获食物的强韧的肌肉，也许是表明能够逃避捕食者的长腿。雌性个体如能将其基因和这些特性结合起来可能是有好处的，因为这些特性在它的儿女身上或许能发挥很好的作用。因此，我们首先必须设想存在这样的雌性动物，它们选择雄性个体是根据表明拥有优质基因的万无一失的可靠迹象，不过，这里牵涉到达尔文曾发现的一个非常有趣的问题，费希尔对之也进行过有条理的阐述。在雄性个体相互竞争，希望成为雌性个体心目中的大丈夫的社会里，一个做母亲的能为其基因所做的最大一件好事是，生一个日后会成为一个令人刮目相看的大丈夫的儿子。如果做母亲的能保证它的儿子将成为少数几个走运的雄性个体中的一个，在它长大之后能赢得社会里的大多数交配机会，那么，这个做母亲的将会有许多孙子孙女。这样说来，一个雄性个体所能拥有的最可贵的特性之一，在雌性个体看来只不过是性感而已。一个雌性个体和一个诱人非凡并具有大丈夫气概的雄性个体支配，很可能养育出对第二代雌性个体具有吸引力的儿子。这些儿子将为其母亲生育许多孙子孙女。这样，我们原来认为雌性个体选择雄性个体是着眼于如发达的肌肉那种显然是有实用价值的特性，但是这种特性一旦在某一物种的雌性个体中普遍被认为是一种具有吸引力的东西时，自然选择就会仅仅因为它具有吸引力而继续有利于这种特性。 雄极乐鸟的尾巴作为一种过分奢侈的装饰，因此可能是通过某种不稳定的、失去控制的过程进化而来的。在开始的时候，雌性个体选中尾巴稍许长一些的雄性个体，在它心目中这是雄性个体的一种可取的特性，也许因为它象征着健壮的体魄。雄性个体身上的短尾巴很可能是缺乏某种维生素的征象--说明该个体觅食能力差。或许短尾巴的雄性动物不善于逃避捕食者，因此尾巴被咬掉一截。请注意，我们不必假定短尾巴本身是能够遗传的，我们只需假定短尾巴可以说明某种遗传上的缺陷。不管怎样，我们可以假定，早期的极乐鸟物种中，雌鸟偏爱尾巴稍微长一些的雄鸟。只要存在某种促进雄鸟尾巴长度发生自然变化的遗传因素，随着时间的推移，这个因素就会促使种群中雄鸟尾巴的平均长度增加。雌鸟遵循的一条简单的准则是：把所有的雄鸟都打量一番，并挑选尾巴最长的一只，如此而已。背离这条准则的雌鸟准会受到惩罚，即使尾巴已经变得如此之长，实际上成了雄鸟的累赘。因为一只雌鸟如果生出的儿子尾巴不长，它的儿子就不可能被认为是有吸引力的。只有在尾巴确实已长到可笑的程度，因而它们明显的缺点开始抵消性感这方面的优点时，这个趋向才得以终止。 这是个令人难以接受的论点；自达尔文初次提出这个论点并把这个现象称为性选择以来，已有不少人对之表示怀疑。扎哈维（A.Zahavi）就是其中之一，他的"狐狸，狐狸"论点我们已经看到过。作为一个对立面，他提出截然相反的"累赘原理"（handicap principle）。他指出，正是因为雌性个体着眼于选择雄性个体的优质基因，才使雄性弄虚作假有了市场。雌性个体所选择的发达的肌肉可能真的是一个优点，但有什么东西阻止雄性个体卖弄假肌肉呢？这些假肌肉并不比我们人类的棉花垫肩更具实质内容。如果雄性个体卖弄假肌肉反而比长出真肌肉省事，性选择应有利于促使个体长出假肌肉的基因。可是，要不了多久，逆选择（counter-selection）将促使能够看穿这种欺骗的雌性个体的进化。扎哈维的基本前提是，雌性个体终将识破虚假的性卖弄。因此他得出的结论是，真正能够成功的是那些从不故弄玄虚的雄性个体。它们掷地有声地表明它们是老老实实的。如果我们讲的是肌肉，那么，装出肌肉丰满的样子的雄性个体很快就要为雌性个体所识破。反之，以相当于举重等动作显示其肌肉真正发达的雄性个体是能够获得雌性信赖的。换句话说，扎哈维认为，一个大丈夫不仅看上去要象一个健全的雄性个体，而且要真的是一个健全的雄性个体，否则不轻信的雌性个体是会嗤之以鼻的。所以，只有是货真价实的大丈夫，它的炫耀行为才能进化。 到现在为止，扎哈维的理论还没有什么问题。下面我们要谈的是他的理论中使人难以接受的那一部分。他认为，尽管极乐鸟和孔雀的长尾巴,鹿的巨角以及其他的性选择的特性，看起来是这些个体的累赘，因而始终是不合理的现象，但这些特征之得以进化正是因为它们构成累赘。一只雄鸟长了一条长长的、笨重的尾巴，为的是要向雌性个体夸耀，说明尽管它有这样一条长尾巴，象他这样一个健壮的大丈夫还是能够活下去。 这个理论很难使我信服，尽管我所持的怀疑态度已不象我当初听到这个论点时那么坚决。当时我就指出，根据这种理论可以得出这样的逻辑结论：进化的结果应该使雄性个体只有一条腿和一只眼睛。扎哈维是以色列人，他立即反驳我说，"我们最好的将军中有些是独眼的！"不过问题还是存在的。累赘的论点似乎带有根本性的矛盾。如果累赘是真实的--这种论点的实质要求累赘必须是真实的--累赘本身正如它可能吸引雌性个体一样，同样对该个体的后代肯定是一种惩罚。因此不管怎样，至关重要的是这个累赘不能传给女儿。 如果我们以基因语言来表达累赘理论，我们大概可以这样说：使雄性个体长出如长尾巴之类的累赘物的基因在基因库里变得多起来，因为雌性个体选择身负累赘物的雄性个体。这种情况的产生是因为，使雌性个体作出这种选择的基因在基因库里也变得多起来的缘故。这是因为对身负累赘物的雄性个体有特殊感情的雌性个体，往往会自动地选择在其他方面拥有优质基因的雄性个体。理由是，尽管身负这种累赘物，这些雄性个体已活到成年。这些拥有"其他"方面优点的基因将使后代具有健壮的体格。而这些具有健壮体格的后代因此得以存活并繁殖使个体生长累赘物的基因，以及使雌性个体选择身负累赘物的雄性个体的基因。倘若促使生长累赘物的基因仅仅在儿子身上发挥作用，就象促使对累赘物产生性偏爱的基因仅仅影响女儿那样，这个理论也许可以成立。如果我们只是以文字对这个理论去进行论证，我们就无从知道这个理论是否正确。如果我们能以数学模式来再现这种理论，我们就能更清楚地看到它的正确程度。到目前为止，那些试图以模型来表现累赘原理的数学遗传学家都失败了。这可能是因为这个原理本身不能成立，也可能是因为这些数学遗传学家不够水平。其中有一位便是史密斯。但我总感觉到前者的可能性较大。 如果一只雄性动物能以某种方式证明它比其他雄性动物优越，而这种方式又无需故意使自己身负累赘，那么它无疑会以这种方式增加它在遗传方面取得成功的可能性。因此，象形海豹赢得并确保它们的"妻妾"，靠的不是它对雌性个体具有吸引力的堂堂仪表，而是简单地靠暴力把妄图接近其"妻妾"的任何雄性象形海豹撵走。"妻妾"的主人大都能击败这种可能的掠夺者，它们之所以拥有"妻妾"显然是因为它们有这样的能力。掠夺者很少能取胜，因为它们如能取胜，它们早该成为"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壮的雄性象形海豹相结合，而这只雄性象形海豹有足够的能力击退一大群过剩的、不顾死活的单身雄性象形海豹所发动的一次又一次的挑衅。这只雌性象形海豹的儿子如果走运的话，它就能继承其父的能力，也拥有一群"妻妾"。事实上，一只雌性象形海豹没有很大的选择余地，因为如果它有外遇，它就要遭到"妻妾"主人的痛打。不过，跟能在搏斗中取胜的雄性个体结合的雌性个体，能为其基因带来好处，这条原理是站得住脚的。我们已经看到这样一些例子，即一些雌性个体宁愿和拥有地盘的雄性个体交配，另外一些宁愿和在统治集团里地位高的雄性个体交配。 至此本章的内容可以归结为：我们看到，在动物界中各种不同的繁殖制度--一雌一雄、雌雄乱交、"妻妾"等等--都可以理解为雌雄两性间利害冲突所造成的现象。雌雄两性的个体都"想要"在其一生中最大限度地增加它们的全部繁殖成果。由于精子和卵子之间在大小和数量方面存在根本上的差别，雄性个体一般地说大多倾向于雌雄乱交，而缺乏对后代的关注。雌性个体有两种可供利用的对抗策略。我在上面曾称之为大丈夫策略和家庭幸福策略。一个物种的生态环境将决定其雌性个体倾向于采取其中的哪一种策略，同时也决定雄性个体如何作出反应。事实上，在大丈夫策略和家庭幸福策略之间还有许多居间策略。我们已经看到，有时候，做父亲的甚至比做母亲的更关心孩子们的生活。本书不打算描述某些具体动物物种的生活细节。因此我不准备讨论是什么促使一个物种倾向于某种繁殖制度而不倾向于另一种繁殖制度。我要探讨的是普遍地存在于雌雄两性之间的差异，并说明如何解释这些差异。我因此不想强调两性间差异不大的那些物种；一般地说，这些物种的雌性个体喜欢采取家庭幸福策略。 首先，雄性个体往往是追求鲜艳的色彩以吸引异性，而雌性个体往往满足于单调的色彩。两性个体都力图避免被捕食者吃掉，因此两性个体都会经受某种进化上的压力，使它们的色彩单调化。鲜艳的色彩吸引捕食者，犹如吸引异性伴侣一样。用基因语言来说，这意味着使个体色彩变得鲜艳的基因，比使个体色彩单调的基因，更可能被捕食者吃掉而结束生命。另一方面，促使个体具有单调色彩的基因不象促使个体具有鲜艳色彩的基因那么容易进入下一代的体内，因为色彩单调的个体不吸引异性配偶。这样就存在两种相互矛盾的选择压力：捕食者倾向于消灭基因库里色彩鲜艳的基因，而性配偶则倾向于消灭色彩单调的基因。和其他许多情况一样，有效的生存机器可以认为是两种相互矛盾的选择压力之间的折衷物。眼下使我们感到兴趣的是，雄性个体的最适折衷形式似乎不同于雌性个体的最适折衷形式。这种情况当然和我们把雄性个体视为下大赌注以博取巨额赢款的赌徒完全一致，因为雌性个体每生产一个卵子，雄性个体就可以生产数以百万计的精子，因此种群中的精子在数量上远远超过卵子。所以任何一个卵子比任何一个精子实现性融合（sexual fusion）的机会要大得多。相对而言，卵子是有价值的资源。因此，雌性个体不必象雄性个体那样具有性吸引力就能保证它的卵子有受精的机会。一个雄性个体的生殖能力完全可以使一大群雌性个体受孕，生育出一大批子女。即使一只雄性个体因为有了美丽的长尾巴而引来了捕食者或缠结在丛林中而过早死亡，但它在死以前可能已经繁殖了一大群子女。一只没有吸引力或色彩单调的雄性个体，甚至可能和一只雌性个体同样长寿，但它子女却很少，因而它的基因不能世代相传。一个雄性个体如果失去了它的不朽的基因，那它即使占有了整个世界又将怎么样呢？另一个带有普遍性的性区别是，雌性个体在和谁交配的问题上比雄性个体更爱挑剔。不管是雌性个体还是雄性个体，为了避免和不同物种的成员交配，这种挑剔还是必要的。从各个方面来看，杂交行为是不好的。有时，象人和羊交配一样，这种行为并不产生胚胎，因此损失不大。然而，当比较接近的物种如马和驴杂交时，这种损失，至少对雌性配偶来说，可能是相当大的。一个骡子胚胎可能由此形成，并在它的子宫里呆上十一个月。骡子消耗母体全部亲代投资的很大一部分，不仅包括通过胎盘摄取的食物，以及后来吃掉的母乳，而且最重要的是时间，这些时间本来可用于抚养其他子女的。骡子成年以后，它却是没有繁殖力的。这可能是因为，尽管马和驴的染色体很相象，使它们能合作建造一个健壮的骡子躯体，但它们又不尽相象，以致不能在减数分裂方面进行适当的合作。不管确切的原因是什么，从母体基因的观点来看，母体为抚育这只骡子而花掉的非常大量的资源全部浪费了。雌驴子应当十分谨慎，和它交配的必须是一只驴子，不是一匹马。任何一个驴子基因如果说，"喂，如果你是雌驴，那就不管它是马还是驴，只要它是年老的雄性个体，你都可以和它交配"，这个基因下次就可能跑到骡子的体内，结果将是死路一条。母体花在这只幼骡身上的亲代投资将大大降低它养育有生殖力的驴子的能力。另一方面，如果雄性个体和其他不同物种的成员交配，它的损失不会太大，尽管它从中也得不到什么好处。但我们却可以认为，在选择配偶的问题上，雄性个体不致过分苛求。凡是对这种情况进行过研究的人都会发现情况确实是如此。 即使在同一物种中，挑剔的情况还是会有的。同一血族之间的交配，和杂交一样，可能产生不利的遗传后果，因为在这种情况下，致命的或半致命的隐性基因获得公然活动的机会。这种情况再次使雌性个体的损失比雄性个体大，因为母体花在某一幼儿身上的资源总是要大些。凡是存在乱伦禁忌的地方，我们可以认为雌性个体会比雄性个体更严格地遵守这种禁忌。如果我们假定在乱伦关系中，年龄较大的一方相对来说更有可能是主动者的话，那么我们应该看到，雄性个体年龄比雌性个体年龄大的乱伦行为一定较雌性个体年龄比雄性个体年龄大的乱伦行为普遍，譬如说，父／女乱伦应该比母／子乱伦更普遍。兄弟／姐妹乱伦行为的普遍性介乎两者之间。 一般地说，雄性个体比雌性个体往往有更大的乱交倾向。雌性个体只能以比较慢的速度生产有限的卵子，因此，它和不同的雄性个体进行频繁的交配不会有什么好处。另一方面，雄性个体每天能够生产数以百万计的精子，如果它利用一切机会和尽量多的雌性个体交配，它只会从中得到好处而不会有任何损失。过于频繁的交配行为事实上对雌性个体的害处并不很大，但好处肯定也是没有的。另一方面，雄性个体却能乐此不疲，不管它和多少个不同的雌性个体交配。过分这个字眼对雄性个体来说没有实际意义。 我没有明确地提到人类，但当我们思考如本章涉及的一些有关进化的论点时，我们不可避免地要联想到我们自己的物种和我们自己的经验。雌性个体只有在对方在一定程度上表明能够长期忠贞不渝时才肯与之交配，这种做法对我们来说并不陌生。这可能说明，人类的妇女采取的是家庭幸福策略，而不是大丈夫策略。人类社会事实上大多数实行一夫一妻制。在我们自己的社会里，父母双方对子女的亲代投资都是巨额的，而且没有明显的不平衡现象。母亲直接为孩子们操劳，所做的工作比父亲多。但父亲常常以比较间接的方式辛勤工作，为孩子们提供源源不断的物质资源。另一方面，有些人类社会有杂交习俗，而有些则实行妻妾制度。这种令人惊讶的多样性说明，人的生活方式在很大程度上取决于文化而不是基因。然而，更大的可能性是，男人大多倾向于杂交，女人大多倾向于一夫一妻。根据进化的理论，我们也可以预见到这两种倾向。在一些具体的社会里，哪一种倾向占上风取决于具体的文化环境，正如在不同的动物物种中，要取决于具体的生态环境一样。 我们自己的社会有一个肯定与众不同的特点，这就是性的炫耀行为。我们已经看到，根据进化的理论，凡有不同性别个体存在的地方，喜欢炫耀的应该是男人，而女人则喜欢朴实无华。在这一点上，现代的西方男人无疑是个例外。当然，有些男人衣饰鲜艳，有些女人衣饰朴素，这也是事实。但就大多数的情况而言，在我们的社会里，象孔雀展示尾巴一样炫耀自己的毫无疑问是妇女，而不是男人。 面对这些事实，生物学家不得不感到疑惑，他观察到的社会是一个女人争夺男人而不是男人争夺女人的社会。在极乐鸟的例子里，我们认为雌鸟的色彩之所以朴素是因为它们不需要争夺雄鸟。雄鸟色彩鲜艳华丽，因为雌鸟供