when plan A does not work out so well. Call this the "Emma Bovary " strategy:
I am going to argue in this chapter that adultery may have played a big part in shaping human society because there have often been advantages to both sexes from within a monogamous marriage in seeking alternative sexual partners. This conclusion is based on studies of human society, both modern and tribal, and on comparisons with apes and birds. By describing adultery as a force that shaped our mating system, I am not "justifying" it. Nothing is more " natural" than people evolving the tendency to object to being cuckolded or cheated on, so if my analysis were to be interpreted as justifying adultery, it would be even more obviously interpreted as justifying the social and legal mechanisms for discouraging adultery: What I am claiming is that adultery and its disapproval are both "naturaclass="underline" "
In the 1970s, Roger Short, a British biologist who later moved to Australia, noticed something peculiar about ape anatomy.
Chimpanzees have gigantic testicles; gorillas have minuscule ones.
Although gorillas are four times the weight of chimps, chimps' testicles weigh four times as much as gorillas'. Short wondered why that was and suggested that it might have something to do with the mating system. According to Short, the bigger the testicles, the more polygamous the females. 1z
The reason is easy to see. If a female animal mates with several males, then the sperm from each male competes to reach her eggs first; the best way for a male to bias the race in his favor is CO
produce more sperm and swamp the competition. (There are other ways. Some male damsel flies use their penis to scoop out sperm that was there first; male dogs and Australian hopping mice both
" lock " their penis into the female after copulation and cannot free
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it for some time, thus preventing others from having a go; male human beings seem to produce large numbers of defective
" kamikaze" sperm that form a sort of plug that closes the vaginal door to later entrants.)" As we have seen, chimpanzees live in groups where several males may share a female, and therefore there is a premium on the ability to ejaculate often and voluminously—
he who does so has the best chance of being the father. This conjecture holds up across all the monkeys and across all rodents. The more they can be sure of sexual monopoly, as the gorilla can, the smaller their testes; the more they live in multimale promiscuous groups, the larger their testes."
It began to look as if Short had stumbled on an anatomical clue to a species' mating system: Big testicles equals polygamous females: Could it be used to predict the mating system of species that had not been studied? For example, very little is known about the societies of dolphins and whales, but a good deal is known of their anatomy, thanks to whaling: They all have enormous testicles, even allowing for their size: The testicles of a right whale weigh more than a ton and account for 2 percent of its body weight. So, given the monkey pattern, it is reasonable to predict that female whales and dolphins are mostly not monogamous but will mate with several males: As far as is known, this is the case. The mating system of the bottle-nosed dolphin seems to consist of forcible
"herding" of fertile females by shifting coalitions of males and sometimes even the simultaneous impregnation of such a female by two males at the same time—a case of sperm competition more severe than anything in the chimpanzee world." Sperm whales, which live in harems like gorillas, have comparatively smaller testicles; one male has a monopoly over his harem and has no sperm competitors.
Let us now apply this prediction to man. For an ape, man 's testicles are medium-sized—considerably bigger than a gorilla 's.
Like a chimpanzee 's, human testicles are housed in a scrotum that hangs outside the body where it keeps the sperm that have already been produced cool, therefore increasing their shelf life, as it were.'
This is all evidence of sperm competition in man: MONOGAMY AND THE NATURE OF WOMEN
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But human testicles are not nearly as large as those of chimps, and there is some tentative evidence that they are not operating on full power (that is, they might once have been bigger in our ancestors): Sperm production per gram of tissue is unusually low in man. All in all, it seems fair to conclude that women are not highly promiscuous, which is what we expected to find:"
It is not just monkeys, apes, and dolphins that have large testicles when faced with sperm competition. Birds do, too. And it is from birds that the clinching clue comes about the human mating system. Zoologists have long known that most mammals are polygamous and most birds are monogamous. They put this down to the fact that the laying of eggs gives male birds a much earlier opportunity to help rear his children than a male mammal ever has: A male bird can busy himself with building the nest, with sharing the duties of incubation, with bringing food for the young; the only thing he cannot do is lay the eggs: This opportunity allows junior male birds to offer females a more paternal alternative than merely inseminating them, an offer that is accepted in species that have to feed their young, such as sparrows, and rejected in those that do not feed their young, such as pheasants.
Indeed, in some birds, as we have seen, the male does all these things alone, leaving his mate with the single duty of egg laying for her many husbands. In a mammal, by contrast, there is not much he can do to help even if wants to: He can feed his wife while she is pregnant and thereby contribute to the growth of the fetus, and he can carry the baby about when it is born or bring it food when it is weaned, but he cannot carry a fetus in his belly or feed it milk when it is born. The female mammal is left literally holding the baby, and with few opportunities to help her, the male is often better off expending his energy on an attempt to be a polygamist.
Only when opportunities for further mating are few and his presence increases the baby 's safety—as in gibbons—will he stay.
This kind of game-theory argument was commonplace by the mid-1970s, but in the 1980s when it became possible for the first time to do genetic blood testing of birds, an enormous surprise was in store for zoologists: They discovered that many of the baby
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birds in the average nest were not their ostensible father 's offspring.
Male birds were cuckolding one another at a tremendous rate. In the indigo bunting, a pretty little blue bird from North America that seemed to be faithfully monogamous, about 40 percent of the babies the average male feeds in his nest are bastards. 1e The zoologists had entirely underestimated an important part of the life of birds. They knew it happened, but not on such a scale. It goes under the abbreviation EPC, for extra-pair copulation, but I will call it adultery, for that is what it is. Most birds are indeed monogamous, but they are not by any means faithful.
Anders Moller is a Danish zoologist of legendary energy whom we have already met in the context of sexual selection. He and Tim Birkhead from Sheffield University have written a book that summarizes what is now known about avian adultery, and it reveals a pattern of great relevance to human beings. The first thing they proved is that the size of a bird 's testicles varies according to the bird 's mating system. They are largest in polyandrous birds, where several males fertilize one female, and it is not hard to see why. The male who ejaculates the most sperm will presumably fertilize the most eggs.
That came as no surprise. But the testicles of lekking birds, such as sage grouse, where each male may have to inseminate fifty females in a few weeks, are unusually smalclass="underline" This puzzle is resolved by the fact that a female sage grouse will mate only once or twice and usually with only one male. That, remember, is the whole point of female choosiness at leks. So although the master cock may need to mate with many hens, he need not waste much sperm on each because those sperm will have no competitors. It is not how often a male bird copulates that determines the size of his testicles but how many other males he competes with.