Infidelity is only natural: Biologists

Females stray to gather the best possible genes for their offspring, while males are driven to father as many and as often as possible, experts say.

New studies using genetic testing techniques show that even the most apparently devoted of partners often fool around, visiting nearby nests or dens or clans to enjoy the sexual company of strangers. Birds do it, apes do it, and, of course, so do some people. “True monogamy actually is rare,” says Stephen Emlen, an expert on evolutionary behaviour at Cornell University. He says there is “a great difference between socialmonogamy,” where mating pairs bond and work together to raise their young, and genetic monogamy,” where parents are faithful sex partners.

Social monogamy is relatively common, but genetic monogamy is the exception rather than the rule, the studies report. Emlen said there are only two monkeys — the Marmoset and the Tamarin — that are truly monogamous among the primates, the animal order that includes humans. All the others, monkeys, apes and people, often mate outside their partnerships.

Most primates, in fact, make no pretence of faithfully bonding for life, and it is difficult to know for sure that males actually know which of the young in the clan are their children, he said.

That may even be true for humans. An Oregon study suggested that about ten per cent of children were not sired by the male partner of the parent couple with whom they bonded.

Among the birds, faithful sex partnership has been thought for years to be widespread. Some species, such as the Eastern Bluebird, gainedreputations as shining examples of devotion. Male and female partners work together closely to build nests, incubate eggs, then feed and raise their young.The truth is, bluebirds have a sex life that rivals a television soap opera.

Patricia Adair Gowarty, a behavioral ecologist at the University of Georgia, has found that 15 per cent to 20 per cent of chicks cared for by a bonded pair of bluebirds were not fathered by the male. Gowarty reports that of 180 socially monogamous species, only about ten per cent are sexually faithful.Emlen said female birds and mammals that seek sexual partners outside their partnership may be pushed by the biological drive to produce the best possible children.

“One of the patterns is that females seek males of high status and high quality,” he said, adding, “By doing so, they are able to produce offspring of higher quality that will be able to do better and survive better. There is a lot of research going on to see if these ideas are correct.”

Several studies haveshown that “females socially bonded with very high quality males do not have copulations outside the pair bonds,” said Emlen. In effect, such females believe they already have the best, so why look for better?

Males, some researchers suggest, are biologically driven to stray by the desire to spread their genes into as many members of the next generation as possible. Among some species, such as lions, mountain gorillas and grizzly bears, this drive to influence the future genetically leads dominant males to kill and even eat the young of competing males.

Impulses that drive humans to seek sex outside their partnerships are far more complex. Researchers generally believe that monogamy originated among species whose young survived best when raised by a bonded pair. This may have led to the rise of monogamy among people, since human children take so long to mature.