Testosterone Rex
Page 4
Resources and rank matter for females. (Indeed, now might be a good moment to remind ourselves that the expression “pecking order” comes to us courtesy of hens.) Dominant female mammals have been found to get more and higher-quality food, better access to water or nest sites, and to enjoy reduced predation risks—thus, “improved reproductive success among dominant females appears to be widespread in a variety of mammal species.”34 Given everything it takes to gestate, lactate, and successfully see off one’s young into the world—food, protection, maybe a nice little nest or privileged use of a feeding ground—this makes sense. Those better able to compete for material and social resources will be more likely to successfully pass on their genes to the next generation, and even—via the quality of those offspring, or inheritance of rank35—to the generation after that.36
In short, neither promiscuity nor competition are necessarily the preserve of male reproductive success.
And a third challenge to the intuitive force of Bateman’s principles is that males can be choosy too. This, of course, makes no sense if you start from the assumption that, for them, mating comes at the rock-bottom price of a single sperm from a limitless supply. But this turns out to be a profoundly misleading way of thinking about the situation. Take, for instance, the presumed dizzying abundance and trivial cost of male sperm. As a number of scientists have pointed out, both observation and personal experience attest to the fact that males do not offer up a single sperm in exchange for an egg.37 They instead produce millions of sperm at a time (in humans, on the order of two hundred million)38 that luxuriate in the gland secretions that make up semen. While the situation varies from species to species, biologists have concluded that, in general, “the antiquated notion that males can produce virtually unlimited numbers of sperm at little cost is demonstrably incorrect.”39 Indeed, in one spider species, males run out of sperm after mating just once.40 Nor may one ejaculation be enough to ensure fertilization, further running up the biological bill.41 There are other costs to mating, too, beyond sperm. The males of many species provide “nuptial gifts,” such as nutrient-rich sperm packages, captured prey, or even parts of their own body. And for any species in which coitus is any more elaborate than a brutally efficient collision of gametes, there will be costs of time and energy for courtship.
All in all, there are good reproductive reasons for the males of some species to be discriminating. Reviews on the topic provide the amateur animal behaviourist with many fascinating case studies that indirectly illustrate the principle that mating comes with a nontrivial biological price tag for males.42 The males of some species (like the stinkbug and the bucktooth parrot fish) address the problem of sperm expenses in a Scrooge-like manner, grudgingly “tailor[ing] the size of their ejaculates”43 to the reproductive quality of the receiving female.44 Others, like the marsupial mouse Antechinus, take the opposite approach of splurging abandon, essentially mating to death during a brief breeding frenzy.45 The price of sex for the male St. Andrew’s Cross spider is so high that he only mates once. As the University of Melbourne evolutionary biologist Mark Elgar explained to me, this is because during this very special occasion “he foolishly breaks his copulatory apparatus and the female puts him out of his embarrassment by eating him.”46 (No wonder they’re cross.) Other species keep costs down with self-imposed chastity. In Elgar’s lab, male stick insects (Macleay’s Spectre) are offered a mating opportunity every week. Despite apparently having nothing more demanding to do all day than resemble a stick, they only rouse themselves to take up this mating opportunity 30–40 per cent of the time.47 Male mealworm beetles, Mormon crickets, and European starlings are similarly indifferent to female charms on a regular basis.48 Indeed, it turns out that even male Drosophila, the original poster boys for the benefits of a philandering lifestyle, sometimes refuse the advances of willing females, presumably on the grounds that they’re saving their sperm for the right partner.49
Given all the complications of the original Bateman story, it’s unsurprising that there turns out to be no straightforward relation between parental investment and parental care either. For many years, people were so carried away by the dizzying reproductive possibilities of males that they forgot to ask where all the females-tobe-fertilized were to come from.50 Overlooked was the fact that most of the females might already be busy with existing offspring. On average, male reproductive success can’t outstrip that of females, due to the simple fact that every offspring has both a father and a mother. As evolutionary biologists Hanna Kokko and Michael Jennions point out, the theoretical possibility that a male could produce dozens of offspring if he mated with dozens of females is of little consequence if, in reality, there are few females available to fertilize, and competition for them is intense. As they put it, Trivers’s parental investment theory
implicitly assumes that the best response for males, who face more mating competitors than females, is to invest more heavily in weaponry, ornaments or other traits that increase their access to mates. There is, however, a valid counterargument: when the going gets tough, the smart do something else.51
A wonderful example is the horned dung beetle.52 In this species, larger males grow long horns with which they belligerently guard entrances to the tunnels females use to mate and tend to their eggs. But while horned males wrestle at the tunnel entrances, smaller males take an easier approach that requires neither horns nor the exertions of battle. They simply sneak into a tunnel via a side entrance, find the female, and mate. (The females, incidentally, show no particular preference for their more traditionally masculine suitors.) In this case, males have one of two possible reproductive strategies, the smart “something else” approach being the one that sidesteps costly aggression and armoury. But in other species, males may evolve a more general pattern of doing “something else”: paternal care. Whether or not paternal care evolves in a species seems to depend on the interaction of many different factors not yet fully understood. But certainly, it is much more common in birds and fish than in mammals, where gestation and lactation impose such huge biological start-up costs on the mother. Yet an exception to this are the primates, among some of which, at least, paternal care is common: “many males routinely protect, rescue, patrol, baby-sit, adopt, carry, shelter, feed, play with and groom infants.”53
To be clear, the moral of all of this is not to try to argue that humans are really like buff-breasted sandpipers, stick insects, or chimpanzees. It’s not to imply that senior female managers are suppressing the ovulation of their female interns, or to caution that, at some primal level, the women who work at the child care centre want to kill your toddler, and maybe eat him too. And the suggestion is certainly not that sex differences in reproductive roles are of no consequence. Rather, the point is the incredible diversity of sex roles across the animal kingdom: across species, biological sex is defined by gamete size but this, in turn, doesn’t determine arrangements for mating or parental care.54 This means that to question the popular Bateman-inspired view of human sexual relations isn’t special pleading for humans to be exempted from foundational principles that apply to every other animal.
But no less important, even within a species, biological sex doesn’t necessarily inscribe a fixed template for how the important business of reproduction should be achieved. Female bush crickets, for instance, are fiercely competitive when food resources are low, presumably because males supply them with nutrient-rich sperm packages. However, when the environment is abundant with the pollen they feast on, they switch to a more “conventional” choosy approach.55 Who would have credited pollen with the power to flip sexual nature? Or consider the two-spotted goby fish, a species in which the ratio of available males to females changes rapidly over just a few months as males die off from the exertions of mating, parenting, and life in general. Again, this environmental change has a profound effect on mating. “Early in the season, males competed aggressively with each other for matings and were very active in courtship, whereas late in the se
ason females… took over as the courting sex.”56 Then there is the dunnock (or hedge sparrow). In a book devoted to its habits, University of Cambridge zoologist Nick Davies observes that a mid-nineteenth century reverend and amateur ornithologist “encouraged his parishioners to emulate the humble life of the dunnock.” Yet as Davies’s fieldwork painstakingly documents, the hedge sparrow boasts “bizarre sexual behaviour and an extraordinarily variable mating system.”57 Depending on factors like female territory size, and how well both females and males are matched in fighting ability, dunnocks can wind up in a bewildering variety of sexual arrangements: monogamy, one female with two males, one male with two females, or two females sharing two males.58 As Davies drolly notes, had the bird-loving Reverend’s “congregation followed suit, there would have been chaos in the parish.”59
In short, even within a species, biological sex doesn’t necessarily determine mating strategies, which can instead “vary over time and space and are flexibly expressed as functions of ecological and social influences,” as Swedish biologists Malin Ah-King and Ingrid Ahnesjö sum it up.60 Parental care, they note, seems less flexible. But even this can sometimes vary within a species. For example, in some troops of wild Japanese macaque monkeys, adult males protect, carry, and groom one- and two-year old infants. But males from different troops, elsewhere in the country, show much less paternal care, or none at all.61 Even when it comes to something as fundamental as mating, then, the effects of sex are more open-ended and flexible than we might tend to assume—a point we’ll return to in the second part of the book.
So where does all of this leave us? In evolutionary biology, sexual selection is in an exciting state of turmoil; empirical revelations are turning accepted facts on their head, while conceptual changes are sending long-held assumptions flying out the window. A man with a Maserati is a fascinating phenomenon, deserving of study, to be sure. But whether he is the human biological equivalent of the well-antlered stag, his spotless luxury car the counterpart of the shimmering, biologically extravagant tail of the peacock—that is another matter altogether.
CHAPTER 2
ONE HUNDRED BABIES?
OF THE MANY BIRTH STORIES I HAVE HEARD, MY FAVOURITE is that of a woman—we’ll call her Lily—from the mothers’ group I belonged to. Lily’s story begins in the usual way. She felt weary and nauseated in the first trimester, ate voraciously over the next three months while serious growth and consolidation took place in utero, then waddled around uncomfortably, becoming increasingly tired in the third trimester as the final touches to the baby were completed. Finally, Lily went into labour early. Not dangerously early, but inconveniently early, since her partner was overseas in the United States for work. Landing back in Melbourne in the nick of time after a sleepless twenty-hour journey worrying about Lily and the unborn child, he hurried to the hospital and was directed to the ward where her labours were finally drawing to their conclusion. He dashed to her side but, in a state of exhaustion and confronted there by the sight of a little pool of blood, he queasily lurched forwards onto the bed. Lily pushed him off her with some force. The father-to-be obligingly fell back, and cracked his head slightly on the unforgiving hospital floor. Lily’s medical attendants flew immediately from her aid to his, and around the time she was pushing out their baby son, his father was tucked in a wheelchair, feeling the rush of cool air soothe his hot cheeks as he was hurried away to have his head tenderly ministered to.
My point, in case it’s not already obvious, is this. When it comes to the miracle of bringing new life into the world, once a man has provided the ejaculate, even if his further contribution is to be merely useless, he is still doing better than some. This is why, at first glance, the reproductive potential for males appears to so easily surpass that of females. As psychologist Dorothy Einon points out: “In the time taken for a woman to complete the menstrual cycle that releases one ovum, a man could ejaculate… 100 times”1 (although one hopes he wouldn’t be so childish as to actually count). It’s been estimated that, in what are described as “optimal” breeding conditions, a woman could bring about fifteen children into being in her lifetime.2 Some individual women have even managed to give birth to many more than this: the anonymous first wife of a Russian peasant called Feodor Vassilyev had thirty-seven pregnancies yielding sixty-nine children. The highest recorded average rate, however, is ten to eleven children per woman, this being the impressive collective accomplishment of the women of the communal religious Hutterite group in the early twentieth century.3 And, as is so often observed, a man could potentially produce ten times as many babies in a single year. This, we are often told, must inevitably make a difference to the evolutionary murmurings within. As Bradley University psychologist David Schmitt explains:
Consider that one man can produce as many as 100 offspring by indiscriminately mating with 100 women in a given year, whereas a man who is monogamous will tend to have only one child with his partner during that same time period. In evolutionary currencies, this represents a strong selective pressure—and a potent adaptive problem—for men’s mating strategies to favor at least some desire for sexual variety.4
The debt to Bateman in the chain of reasoning is obvious, in the implication that, for males, producing offspring can demand as little as a mere tablespoon of ejaculate and some modest, pleasurable exertion. But as we saw in the previous chapter, in many species the situation is decidedly more complex, with some of the long-held assumptions at the foundation of the Testosterone Rex account routinely overturned on closer inspection. So what about humans?
Consider, Einon posits, a woman who on average has sex once a week for thirty years. Now suppose she bears a generous brood of nine children. As you can easily calculate for yourself, on average she will have sex 173 times per child. And for each of the 172 coital acts that didn’t lead to a baby, there was a partner involved, having nonreproductive sex. To explore what this means for any man trying to reach the benchmark set by Schmitt of scoring a century of infants in a year, it’s worth following Einon in breaking things down to clearly see the schedule involved.
First, the man has to find a fertile woman. For the benefit of younger readers, it may be worth pointing out that throughout most of human evolution the Tinder app was not available to facilitate this. Nor, as observed in the previous chapter, was there likely to have been a limitless supply of fertile female vessels for men to access. In historical and traditional societies, perhaps as many as 80–90 per cent of women of reproductive age at any one time would be pregnant, or temporarily infertile because they were breast-feeding, Einon suggests. Of the remaining women, some of course would already be in a relationship, making sexual relations at the very least less probable and possibly more fraught with difficulties. Let’s suppose, though, that our man manages to identify a suitable candidate from the limited supply. Next, he has to prevail in the intense competition created by all the other men who are also hoping for casual sex with a fertile woman, and successfully negotiate sex with her. Say that takes a day. In order to reach his target of one hundred women per annum, our man then has just two to three days to successfully repeat the exercise, ninety-nine more times, from an ever-decreasing pool of women. All this, mind you, while also maintaining the status and material resources he needs to remain competitive as a desirable sexual partner.
So what’s the likely reproductive return on this exhausting investment? For healthy couples, the probability of a woman becoming pregnant from a single randomly timed act of intercourse is about 3 per cent, ranging (depending on the time of the month), from a low of 0 to a high of nearly 9 per cent.5 On average, then, a year of competitive courtship would result in only about three of the one hundred women becoming pregnant.6 (Although a man could increase his chances of conception by having sex with the same woman repeatedly, this would of course disrupt his very tight schedule.)7 This estimate, by the way, assumes that the man, in contradiction with the principle of “indiscriminately mating,” excludes women under tw
enty and over forty, who have a greater number of cycles in which no egg is released. It also doesn’t take into account that some women will be chronically infertile (Einon estimates about 8 per cent), or that women who are mostly sexually abstinent have longer menstrual cycles and ovulate less frequently, making it less likely that a single coital act will result in pregnancy. We’re also kindly overlooking sperm depletion, and discreetly turning a blind eye to the possibility that another man’s sperm might reach the egg first. In these unrealistically ideal conditions, a man who sets himself the annual project of producing one hundred children from one hundred one-night stands has a chance of success of about 0.0000000000000
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One remedy for these low odds, you might think, is for men to restrict their sexual attention to ovulating women. Traditional wisdom holds that this is impossible since, unlike the females of other species, women don’t advertise when they’re in the business phase of their cycle. But with recent findings that, for instance, men find isolated characteristics of women (like the scent of their bodily secretions) more attractive during the fertile period of the menstrual cycle,9 there have been suggestions that women’s ovulation isn’t so concealed after all. Whether this translates into behaviour, though, is questionable: a large-scale study of married women failed to find any evidence that sex was more likely during ovulation.10 And while this does leave open the possibility of these subtle attractions having more of an influence on casual sex, as biological anthropologist Greg Laden points out: