Afterwards the "lover" covers his still-erect penis with a hand, to hide it from the cuckolded male. It could be a scene from a bawdy Restoration comedy. Or the animal equivalent of Erica Jong's "zipless f***". These anecdotes come from The Myth of Monogamy, co-written by Professor David Barash, a zoologist at The University of Washington, Seattle. "There's no question of monogamy being "natural'. It isn't," he says.
"Multiple mating is common in nature, and females are as unfaithful as males." Humans are rare among mammals in bonding to each other the way we do: of 4,000 mammal species, only a handful (a few bats, foxes, voles and marmosets) are monogamous. The rest are highly promiscuous.
Helen Fisher, an anthropologist at Rutgers University, says: "The news is not that humans are adulterous, the news is that we pair up at all. I find it remarkable that we are not more adulterous." Surveys suggest that 60-70% of humans manage to be monogamous.
Monogamy probably evolved in humans along with the growth in size of our brains. Once the full-size human brain had become too big to pass through a mother's pelvis, we started giving birth to babies before the brain was fully grown. Vulnerable prehistoric infants whose fathers stayed around to help their mothers care for them would have had the best chance of surviving and reproducing. In that sense, monogamy might have become adaptive for human beings. But in the past 10 years, DNA studies have taught biologists that even monogamous species such as birds and humans are prone to adultery. And that females are almost as likely to dally as males. The offspring of up to 60% of female birds were found not to have been fathered by their social mates, according to DNA studies. Surveys among humans reveal that between 25% to 50% of men, and an average of 30% of women, own up to at least one instance of extramarital sex.
This has come as some surprise to the biologists; the traditional Darwinian expectation was that men would be adulterous, women faithful. If men had more mates, they could have more children and pass on more genes. But this is not the case for women, who can only have one pregnancy at a time. Surely it would benefit females to nurture long-term relationships, preferably with socially dominant, attractive males, in which they could rear strong, healthy children? Why would a woman risk her security in this way?
Professor Tim Birkhead of Sheffield University points to a study of black-capped chickadees in which females were found to be leaving their territory for secret liaisons. "In almost all cases it was with a male who was socially dominant over her partner," he says. "The females were going up-market. They are more active than anyone gave them credit for."
In some species, such as chimps, it's thought that female promiscuity protects against infanticide. If a new male takes over a troop, he might well kill the children of his predecessor. But if the females have bred so much confusion that nobody knows whose children are whose, the children may be safe from the murderous instincts of the new alpha male.
Helen Fisher suggests four reasons why women have adopted adulterous survival strategies. First, receiving presents from lovers could improve her subsistence by giving her extra food and shelter. Second, if she was married to an unattractive or unpleasant man, she might try to upgrade her genetic line by taking a lover with "good genes".
Third, if a woman had offspring with an array of fathers, each child would be different and there would be increased chances of some of them surviving the rigours of the environment. And finally, for some women adultery would be an insurance policy. If her husband was to die or desert her, she would have another male to enlist in parental chores.
So, according to the Fisher hypothesis, our most successful and fertile female ancestors pursued a daring dual strategy: social monogamy on the one hand, tactical adulterous liaisons on the other. To pursue this, extreme secrecy would be paramount. According to David Barash, in the animal world females are very cagey about their sexual flings, while males are brazen. Secrecy is also vital because the adultery game is a dangerous one. Throughout history there have been hideous punishments for women who dared to stray, from whipping to having their noses chopped off. Sexual relations between men and women have been like a secret war, a battle to reproduce, in which nature has equipped both sexes with certain weapons, including physiological ones. The sperm-wars theory of the sexual biologists Robin Baker and Mark Bellis is hugely controversial. Their idea was that sperm from different men mingle in the reproductive tract of females and compete to fertilise an egg. Supporting the alpha egg-getting sperms are an army of kamikaze sperms whose job is to kill off sperms from rival men. Thus a whole ejaculate works together as a winning or losing team. The fittest team gets its man to the egg.
This type of sperm competition was first discovered in insects in 1970 and then found in virtually every animal group. But could it really be an important factor in human evolution? It would mean that a significant minority of women would be sleeping with two men or more within five days around their fertile periods. From their surveys, Baker and Bellis concluded that 8% of human children were born as a result of sperm competition. The surveys also showed that an adulterous woman tends to favour her lover's rather than her husband's sperm. She is less likely to use contraception with a lover, more likely to sleep with him on a fertile day, and retains more of a lover's sperm in her vagina.
In humans, once sex is over, blood drains from the penis, but it does not completely drain from female genitals, meaning women can climax over and over again. The female of many species has a much higher sex drive than has usually been acknowledged. Kestrels copulate nearly 700 times per clutch, and lionesses mate every 15 minutes during their four-day oestrus cycle. A high sex
drive may have helped our female ancestors keep several relationships on the go.
Fortunately, relationships are not just a matter of warring sperms and multiple matings, and the main organ involved in love is really the brain. Scientists believe there are three different brain systems for three types of love: the sex drive, romantic love, and attachment. According to Helen Fisher, whose latest book, Why We Love, anatomises the neuroscience of love, "These three brain systems are not well connected. You can feel attachment for one person while feeling romantic love for another, while experiencing a strong sex drive towards a third! This brain circuitry enables us to love more than one person." So our brains make it easy for us to be adulterous.
None of this means that you can't help whether or not you are faithful or unfaithful. Nor are the scientists saying that adultery is natural and therefore good. Indeed, if you are one of the 70% of monogamous humans, you can feel smug that you have resisted thousands of years of evolutionary pressure to cheat. You may have reached the pinnacle of enlightened animal behaviour.
