The Myth of Monogamy: Fidelity and Infidelity in Animals and People (34 page)

There is also debate about how much time can elapse between two successive copulations with different men in order for sperm competition to occur. A key question is how long after ejaculation sperm can remain viable

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and, therefore, capable oPS duking it out for the privilege of fertilizing a woman's egg. Minimum estimates are two to three days; the maximum seems to be seven to ten. A reasonable number may be five to six. What this means is that if a woman has sex with someone within five or six days of having had sex with someone else, the sperm from these two men could be in direct competition.

How does one "win"? For women, it means making the best choice; that is, having the opportunity of choosing among more than one sperm provider (hence, copulating with more than one man), as well as being able to make a "good" choice among the available sperm, perhaps by setting up a competitive situation. For men, winning means having one's sperm succeed. We turn to them next.

Among men, sperm strategies might include simply making lots of the little pollywogs, assuming that fertilization is a kind of lottery. Buy more tickets and you're more likely to hit the lottery jackpot. Make more sperm, and deliver them to the right place, and you're more likely to hit the fertilization jackpot. Even in this case, however, different tactics present themselves. For example, biologists have asked, "What is the optimum strategy for males, in terms of partitioning their sperm in IPCs versus EPCs?" Geoffrey Parker concluded from a detailed mathematical model that already-mated males should generally ejaculate more sperm during EPCs than IPCs, assuming that such males are normally able to maintain an adequate sperm level within their in-pair mate. The only exception would be when a male has determined that his female has engaged in one or more EPCs, in which case he should increase his sperm numbers during IPCs. It remains to be seen whether such adjustments actually occur.

But what if fertilization is less a lottery than a race? Then it would be important to make sperm that move quickly. Or maybe it's a war, in which sperm from different men literally do battle with each other.

Baker and Bellis have accordingly proposed their "kamikaze sperm hypothesis." It suggests that only a very small proportion of human sperm are intended to function as "egg-getters." All sperm, they claim, are not created equal, nor are they created to do the same thing; namely, to fertilize eggs. Thus, sperm are not homogeneous little packages of DNA, each pruned down to the minimum size needed to achieve fertilization. The smallest sperm in a single human ejaculate, for example, can have merely 14 percent the volume of the largest. There is more variation within a single human ejaculate than in the mean sizes of sperm from all the different primates. In short, a normal human male produces a remarkable range of different types of sperm: amorphous, pin-sized, weirdly shaped, double-headed ("bicephalous"), crook-necked, double-tailed, short-tailed,

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coiled-tailed. Previously, this sperm diversity was thought to be simple pathology: Easily 30 percent of human sperm were acknowledged to be defective in some way. (Indeed, this high rate of "bad sperm" has long been thought to be one of the reasons why men make so many of them.) But if selection had simply acted on males to produce egg-getters, why should so many be defective, slow, lame, and seemingly deformed? In most other contexts--even those that seem comparatively more trivial--natural selection does a much more efficient job.

Baker and Bellis argue that semen should be seen as another human organ, comparable to the liver, the kidneys, or--more to the point--the immune system. As such, it is composed of many different kinds of highly specialized cells, all of which together contribute to getting an important job accomplished, one of which is doing battle on behalf of the rest of the body. Among rats, sperm that form copulatory plugs are those with smaller heads, which are more likely to become decapitated. Rat sperm from one copulation thus get in the way of sperm from the next. It is not impossible-- although as yet unproved--that something similar happens among human beings, too. We don't produce copulatory plugs, but the sperm from one man could nonetheless interfere with those of another; indeed, there could be an evolutionary payoff to men whose sperm are especially nasty to anyone else's.

Baker and Bellis maintain that the majority of human sperm are, in effect, kamikazes, on a suicide mission whose goal is simply to block the sperm of other males. In addition to these "blockers"--sperm with coiled and kinky tails--there are others that go out on "search-and-destroy" missions. These seem especially prone to chemical warfare, via specialized structures, known as acrosomes, that adorn their tips.

Baker and Bellis claim that when sperm from two different men are mixed, many are disabled or killed, whereas this does not happen when ejaculate from one man is separated, then recombined. This suggests that something like an immune response--a form of chemical competition-- goes on between sperm produced by different men. If so, it is possible that older sperm become blockers, since this requires less energy and vitality, while younger ones are designated to carry the ball. .. and, if they are especially fortunate, to "score." (Indeed, whereas newly minted sperm have a low proportion of coiled tails, the older the.sperm, the greater the proportion of coiling.)

Further evidence of male adaptations to sperm competition comes from assessing the detailed makeup of the male ejaculate. Human ejaculation takes place in a series of three to nine spurts, closely linked together in time. Herculean efforts by researchers (and their subjects) have enabled exami-

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nation of so-called split ejaculates, obtained by capturing a few squirts of semen from various stages of ejaculation. The results: Early and late spurts are different. The final pulses actually contain a spermicidal substance, which might well be instrumental in ambushing those of the next male likely to ejaculate in the same female! At the same time, chemicals present in the first half of the human ejaculate contribute to some protection of sperm against those chemicals present in the second half. .. and thus, also, possibly, against any chemicals deposited by the final spurts of any preceding male.

It is well known that men produce larger ejaculations when their sex lives have been interrupted, and then resumed. By itself, this appears to be a simple physical consequence of whether or not seminal fluid has had a chance to accumulate over time; with regular emptying, the volume of any one ejaculation is necessarily less. It is not surprising, therefore, that Baker and Bellis found that when a man spends time away from his female partner, he produces more sperm per ejaculation once sexual relations are resumed. More remarkable is their discovery--by analyzing the condom contents of some extraordinarily cooperative subjects--that sperm concentration is higher when such men actually engage in sexual intercourse than when they masturbate. (A sperm-competition perspective, incidentally, also suggests that masturbation may be a way of ensuring that the sperm available to be ejaculated in sexual intercourse have a relatively long "shelf, life"--by getting rid of older sperm, thus ensuring that what's left is fresh. The likelihood is that younger sperm are better able to compete, especially more able to penetrate the cervical mucus. Baker and Bellis also suggest that older sperm may serve a secondary role, as blockers, guarders, kamikazes, troublemakers, troubleshooters, and so forth.)

In addition, during copulation, the amount of sperm transferred seems to be further adjusted according to the risk of sperm competition, especially how long it has been since the last copulation with the same woman, and even how much time the two have spent together during the previous few days. As Baker and Bellis put it, "Males may not look very sophisticated in the moments leading up to and during ejaculation but... some very sophisticated adjustments are taking place."

There is yet more.

Consistent with their courageous--or foolhardy--willingness to probe some of
Homo sapiens'
most intimate secrets, Baker and Bellis have also looked at the human penis. Although not remarkable by mammalian standards generally, the human penis is the largest among all primates, and Baker and Bellis suggest that its size and shape may also have been sculpted by sperm competition. They point out that in less than a minute

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after ejaculation, semen forms a soft, spongy coagulate, a mass that is vulnerable to being removed by a subsequent sexual partner, assuming that he encounters the woman quickly enough and that he is adequately equipped to do the job.

There is nothing unique, by the way, about penises being designed to compete with other males. In Chapter 2, we encountered the remarkable, competition-oriented penises of damselflies and sharks. Human beings clearly have nothing directly comparable; indeed, the penis of
Homo sapiens
is notably unadorned as such organs go. Nonetheless, the long-standing male obsession with penis size and shape may in itself suggest an accurate, if unconscious, recognition that this organ may actually be about as consequential as the most ardent Freudian or locker-room comparator has generally assumed.

For Baker and Bellis, the length of the penis as well as its enlarged bulbous tip suggest its role as a "suction piston" for sperm removal, a kind of natural "plumber's helper," designed to break up and possibly even remove coagulated sperm deposited by a previous male. This effect would be enhanced by the often vigorous thrusting that characterizes sexual intercourse and ejaculation among humans ... and which is surprisingly difficult to explain or justify otherwise.

Let's further develop this admittedly homey metaphor: A plumber's helper is used to push blockage farther down the system, which, in our context, would seem counterproductive, actually facilitating fertilization by the
earlier
male's sperm. Except that the female reproductive tract is essentially a dead end, so the "suction piston" hypothesis might have some validity after all. Another potential problem: If sperm competition has been so important in producing men's anatomy, especially the seemingly oversized penis, why do men have proportionately smaller testicles than chimps? Possible answer: Maybe sperm competition is even more important for chimps, since however important EPCs may have been for humans, they are even more prominent among
Pan troglodytes.
But then, why don't chimps also have large, competitive penises? No one knows. (Nor, to our knowledge, has anyone asked.)

For obvious reasons, vagina and penis are closely matched in all species, not unlike a lock and key. So human penis size may have been largely determined by the size of the vagina, rather than by the dictates of sperm competition among males. Vagina size, in turn, seems to have been determined by the size of the infant--specifically, its head diameter--that, must pass through in childbirth. As human head size increased during human evolution to favor large brains, so, presumably, did the size of the vagina. And this may in turn have generated pressure for the evolution of larger, more competitive penises as well.

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Robin Baker and Mark Bellis are fervent proponents of the role of sperm competition among human beings. And they may well be onto something. But as the great nineteenth-century statesman Talleyrand suggested for his diplomats: "Pas trop de zele" ("Not too much zeal"). In science, too, zeal can be troublesome, especially when it leads to the temptation to sensationalize, to overgeneralize, to ignore contrary evidence. Even though Baker and Bellis's arguments are so convivial to the thrust of the present book that we have presented them at length, it must be noted that nearly all of their conjectures are still that, far from proven. The dirty little secret of science is that it is not done by omniscient deities or computer-driven robots but by scientists, who are fallible human beings. Although we seek ultimately to unravel genuine external truths about the natural world, not simply to validate our own preconceptions, one of those truths is that we are readily seduced by our own ideas and just as reluctant to give up on them--even in the face of contrary evidence--as anyone else.

Thus, it may be that sperm competition itself is less important than this book, or Baker and Bellis, has suggested. A study of chimpanzees in which 1,137 copulations were observed found that more than 70 percent involved multiple matings; that is, females mating with more than one male. Only 2 percent took place during isolated one male-one female consortships. So far, so "good," at least insofar as the case for sperm competition is concerned. Yet most conceptions occur during these essentially monogamous consortships (which take place during maximum tumescence and, thus, peak fertility). Such findings urge caution: Even behavior such as promiscuous chimpanzee sexuality, which screams "sperm competition," might actually involve less than meets the ear, no matter how favorably attuned.

Human beings, moreover, are less prone to EPCs than are chimps. As evidence, human sperm concentration diminishes more rapidly with repeated copulations, suggesting that men are less adapted than male chimps to competing with the sperm of other men. As with other species, human sperm are cheap, but not free. In one intriguing experiment, men engaged in a "10-day depletion experience," averaging 2.4 ejaculations per day. Afterward, their sperm output remained below their earlier, predepletion levels for more than five months! By contrast, male chimps--who have to deal with polyandrous, EPC-inclined females--can ejaculate every hour for five hours, after which their sperm count is only halved, with very rapid recovery. So whatever the importance of sperm competition for
Homo sapiens,
it is likely not as pronounced as it could be.