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CHAPTER 1
Mating for Life?
Ah, romance. Can any sight be as sweet as a pair of mallard ducks gliding gracefully across a pond, male by female, seemingly inseparable? Or, better yet, two trumpeter swans, the legendary symbols of eternal love, each ivory neck one half of a single heart, souls of a feather staying coupled together for life?
Coupled for life — with just a bit of adultery, cuckoldry, and gang rape on the side.
Alas for sentiment and the greeting card industry, it turns out that, in the animal kingdom, there is almost no such thing as monogamy. As a wealth of recent findings makes as clear as a crocodile tear, even creatures long assumed to have faithful tendencies and to need a strong pair bond to rear their young in fact are perfidious brutes.
Biologists traditionally believed, for example, that up to 94 percent of bird species were monogamous, with one mother and one father sharing the burden of raising their chicks. Now, using genetic techniques to determine the paternity of offspring, biologists find that, on average, 30 percent or more of the baby birds in any nest were sired by someone other than the resident male. Indeed, the great challenge these days is to identify a bird species not prone to such evident philandering.
Among creatures already known to be polygamous, researchers find their subjects to be far more craftily faithless than previously believed. Mammals, for example, have never been considered paragons of virtue, yet even here revisionism is in order, and experts are recalculating downward the already pathetic figure of 2 percent to 4 percent that represented, they thought, the number of faithful mammal species. To the astonishment, perhaps disgruntlement, of many traditional animal behaviorists, much of the debauchery is committed by females.
By tracking rabbits, elk, and ground squirrels through the fields, researchers have learned that the females of all three species will copulate with numerous males in a single day, each time expelling the bulk of any partner's semen to make room for the next mating. In so doing, the female can accrue a variety of sperm, thus assuring maximal genetic diversity in her offspring. Most efficiently energetic of all may be the queen bee, who, on her sole outing from her hive, mates with a multitude of accommodating but doomed drones.
For their part, males display marvelous ingenuity in their attempts to counteract female philandering. Among Idaho ground squirrels, a male will remain unerringly by a female's side whenever she is fertile, sometimes chasing her down a hole and sitting on top of it to prevent her from cavorting with his competitors. Other squirrels use a rodent's version of a chastity belt, topping an ejaculation with a rubberlike emission that acts as a plug. Still other strategies result in what are called "sperm wars," battles by males to give their sperm the best chance of success when the female clearly is bedding around. In numerous mammals, the last male's sperm is the sperm likeliest to inseminate the female. Hence, several males may engage in an exhausting round robin, as each tries, repeatedly, to be the female's final partner. But how better to thwart one's rivals than with the right tool for the job? The male damselfly, for instance, has a scoop at the end of his penis that can be used before copulating to deftly remove the semen of a previous mate.
These fresh bits of information about mating and the near universality of infidelity are reshaping biologists' ideas about animal behavior and the dynamics of different animal social systems. The research gives the lie to the dreary stereotype that only males are promiscuous, and that what females want above all is one good male. Instead, many animal social systems very likely developed as much to allow members to cheat selectively as they did to enable animals to divide into happy couples. Most pair bonds might thus be mere marriages of convenience, offering both partners enough stability to raise their young while leaving a bit of slack for the occasional dalliance.
Most of the misconceptions about monogamy and infidelity began in Darwin's day, when he and other naturalists made perfectly reasonable assumptions about mating based on field observations of coupled animals. Nearly all birds form pairs during the breeding season, and biologists believed that the pair bond was necessary for the survival of the young. Without the contributions of both males and females to incubate the eggs and feed and protect the babies, few offspring would make it to the fledgling stage. That demand for stability presumably included monogamy as well.
But as field researchers became more refined in their observations, they began spotting instances in which one member of a supposedly monogamous avian couple would flit off for a tête-à-tête with a paramour.
Such sightings inspired biologists to apply DNA fingerprinting and other techniques used in paternity suits to help determine the parentage of chicks. They discovered that between 10 percent and 70 percent of the offspring in a nest did not belong to the male caring for them.
Take the familiar black-capped chickadee of North America. During winter, a flock of chickadees will form a dominance hierarchy in which every bird knows its position relative to its fellows, as well as the ranking of the other birds. Come the spring breeding season, the flock breaks up into pairs, with each pair defending a territorial niche and breeding in it. On occasion, however, a female chickadee mated to a low-ranking male will leave the nest and sneak into the territory of a higher-ranking male nearby. That cheating chickadee ends up with the best of both worlds: a stable mate at home to help rear the young, and the chance to bestow on at least one or two of her offspring the superior genes of a dominant male.
Female barn swallows likewise are finicky about their adulterous encounters. When cheating, a female invariably copulates with a male endowed with a slightly longer and more symmetrical tail than that of her mate; the more sumptuous tail appears to be evidence that the male is resistant to parasites, a characteristic of broad appeal to the female. Not only may she help her young to gain the resistant trait, but, by avoiding infested partners, she limits her own exposure to bloodsucking parasites.
Some females that mate promiscuously gain not so much the best genes as enough genetic diversity to ensure that at least some of their offspring will thrive. A honeybee queen leaves her hive only once, but during that single outing she mates with as many as twenty-five drones patrolling nearby. Tabulating her wantonness is easy: to complete intercourse, the poor drone must explode his genitals onto the queen's body, perishing in the exercise but leaving behind irrefutable evidence of an encounter. While the queen bee does have considerable reproductive demands — that is, sufficient sperm to fertilize about four million eggs — any one of the drones could provide enough sperm to accommodate her, so her profligate behavior seems intended to guarantee genetic diversity in her brood.
But there are evolutionary counterbalances that can keep female cheating in check. Females that actively seek outside affairs risk losing the devotion of their mates. If a male barn swallow observes his mate copulating with a neighbor, he retaliates by reducing his attention to her babies. That is why most adulterous encounters are done quickly and covertly, and why biologists call the events "sneakers."
Of course, males themselves hardly merit any haloes. In aneffort to spread their seed as widely as possible, some go to exquisitely complicated lengths. The older males of the purple martin, the world's biggest swallow, will happily betray their younger counterparts. An older martin will establish his nest, attract a mate, and then quickly reproduce. His straightforward business tended to, he will start singing songs designed to lure a younger male to his neighborhood. That inexperienced yearling moves in and croons a song to entice his own consort, who soon after her arrival is ravished by the elder martin. As a result, a yearling male rarely manages to fertilize more than 30 percent of his mate's eggs, although he is the one who ends up caring for the brood.
In a theme to warm the hearts of aging Hollywood turks, older males often manage to upstage, cuckold, or otherwise humiliate the youthful males around them. Male mallards consistently attempt to force sex on females paired to other males. The female struggles mightily to avoid this copulation by flying away, diving under water, or fighting back — and if her partner is around, he joins in the defense. The only males that manage to overcome all resistance and ravish the female are the most mature and experienced ducks on the block.
But rape is not common in the animal kingdom, and males more often rely on physiology for success. In many cases, natural selection favors males with the most generous ejaculation, resulting in the development of some formidable testicles. The more likely a female is to mate with more than one male, the bigger the species' sperm-producing organs will be. Biologists comparing the dimensions of testes among the higher primates have found that chimpanzees display the largest set relative to their body size. Chimps are the ones that live in coed troops with considerable mating and cross-mating by all. The male with the largest quantity of sperm is likely to swamp the semen of his contenders.
Gorillas are bigger in body but smaller in scrotum, an indication that the great apes' social system does not facilitate sperm wars. Through a combination of genuine ferocity and chest-beating bluster, a dominant silverback male manages to control a harem of females with little interference, or competing sperm, from other males. It does nobody any good to carry around large testicles if one male can monopolize most of the females. Rather than waste energy on growing bigger genitals, subordinate males contrive to overthrow silverbacks so that they have a shot at harem privileges.
Human beings have midsized testicles, good evidence, biologists say, that our species is basically monogamous, but there are no guarantees.
Oh, yes. Humans. It takes an intrepid biologist to apply the new findings about infidelity among animals to the study of people — who obstreperously continue to insist on concepts like free will, consciousness, and unpredictability. As it happens, there are plenty of brave theorists around, and many propose that the human urge to cheat has an evolutionary basis.
Babies need long-term care, which probably led to pair-bonding among humans early in our evolution. But even a happily married man could well be driven to stray from his partner by the urge to slip a few more of his genes into the pool. For her part, a woman might philander to mate with a man who looks to have hardier genes, or at the very least offers her a shank of zebra meat for her favors. Many evolutionary biologists suggest that lapses in monogamy helped spawn male sexual jealousy, bringing about such unpleasant cultural manifestations as female genital disfiguration, foot-binding in China, and other mechanisms by which males have controlled female wandering. And though a woman may have good reason to feel similarly enraged by her spouse's infidelities, she is usually smaller and thus hard put to tie down a man long enough to sheathe him in an iron jockstrap.
But women are not entirely helpless. Evolution has provided them with ways to avoid being too closely monitored by men — most notably through the gift of cryptic ovulation. Unlike that of a rhesus macaque, a woman's butt does not turn bright red when she is fertile. Unlike a moth, she doesn't send a pheromonal broadcast of her status onto the airwaves. Nor does she stand by the window yowling like a cat. So the male can't as easily guard her during the dangerous days of ovulation.
To further confuse men, women have breasts. In great apes, conical breasts are a signal that a female is lactating and thus has low reproductive value. But in humans, a woman's perpetually swollen chest makes it ambiguous to males when she's fertile and when she's lactating, again confounding a man's effort to restrain her reproductive activities. Perhaps this explains why men are so fixated on breasts: they're looking for clues. Too bad they're looking in all the wrong places.
CHAPTER 2
The Urge to Cuddle
It is just the potion for a bellicose world. In some cases, it works as an aphrodisiac, inspiring males to seek females more ardently and females to invite their overtures more passionately. It causes the body to swell with arousal and touches off the luscious waves of climax. Afterward, it serves as the classic postcoital cigarette, fostering a feeling of relaxed satisfaction. At other times, it is a promoter of family values, making new mothers more likely to nurture their young and new fathers happier to help out around the nest. Even among those who are neither sex partners nor parents, the compound can induce an overwhelming urge to cuddle.
The magic chemical is oxytocin, a small, potent peptide hormone secreted by the almond-sized pituitary gland at the base of the brain. Oxytocin has long been known as the chemical that stimulates uterine contractions during childbirth and helps the mother's breasts begin secreting milk for her newborn. Now it seems the hormone does far more than serve as a muscle contractor. Scientists find that it is active in both sexes and helps to orchestrate many of life's more pleasurable social and sexual interactions — between male and female, parent and offspring, neighbor and neighbor. It is the satisfactional hormone, nature's way of ushering in joy.
As is so often the case with the most interesting science, much of the work to date has been done on nonhuman species, and nobody can say yet how readily the findings will apply to us. Oxytocin almost surely is a component of human sexuality and happiness, but it is equally likely that the hormonal pathway, as it wends through our bodies and brains, will prove irritatingly complex. Don't expect to see oxytocin tablets supplanting Prozac any time soon.
Nevertheless, oxytocin mania clearly is upon us. Studies of rats, rabbits, field mice, sheep, and other animals show that the hormone acts on many regions of the brain known to participate in sexual and affectionate behavior. The experiments strongly suggest that when the body has been primed for reproduction by sex hormones like estrogen and testosterone, oxytocin is the signal that makes the creature act on the message and scout out a partner. In one study, researchers at Rockefeller University, in New York, discovered that a female mouse given an extra dose of oxytocin during ovulation is 60 to 80 percent more industrious in ensuring that males mount her than is a female not given the shot. The randy female arches her back and displays her hindquarters more fetchingly.
Under other circumstances, oxytocin helps strengthen the bond between parent and offspring. Mother rats treated with oxytocin will pick up and nuzzle their pups more frequently than will female rats without the extra oxytocin; father rats given oxytocin are more likely to build a nest for the pups and guard them zealously. But when male rats are injected with a drug that blocks the activity of oxytocin, they not only neglect to nurture their offspring — they may go so far as to view the newborns as a handy source of food.
Beyond its sexual and reproductive value, oxytocin seems to be a fraternizing compound. Field mice, which tend to crave the companionship of other mice, are naturally responsive to oxytocin. When they are given supplemental doses, the rodents will spend even more time in physical contact, striving to get so close that they are practically crawling beneath one another's fur. But in a closely related mouse that is by nature a solitary creature and shuns its peers except for quickie sexual encounters, the brain has a very different architecture, one that is far less sensitive to oxytocin. For these mice, shots of the hormone fail to work their affiliative magic.
The difference in the two responses may show that what is most important in oxytocin's effect is not so much the total amount of the hormone circulating through the blood as it is the brain's receptivity to the substance. And that sensitivity is dictated by a complex set of factors, like sex hormones and other proteins in the body.
Thus, oxytocin may elicit any number of responses or no response at all, depending on the body's general biochemical balance of the moment. Scientists have yet to figure out which hormonal signals indicate that oxytocin will spark a given response. They believe that, while unsocial species are not likely to possess the necessary equipment to respond to a hormonal call to friendship, in naturally social species it is oxytocin that brings everyone together.
Among humans, the link between oxytocin and behavior is considerably more elusive but nonetheless provocative. In two experiments examining the role of the hormone in men, researchers determined that in the moments preceding orgasm and during ejaculation itself, the level of oxytocin in the blood is three to five times higher than it is otherwise. It could be that oxytocin is needed to start the contractions of orgasm; it could be that the hormone is crucial to the intense feeling of pleasure associated with sexual crescendo; most likely, it is both of the above.
(Continues…)
Excerpted from "The Beauty of the Beastly"
by .
Copyright © 1995 Natalie Angier.
Excerpted by permission of Houghton Mifflin Harcourt Publishing Company.
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