WASHINGTON -- Only about 10 percent of the birds and mammals that seem to mate for life are actually faithful to their partners, according to studies that suggest infidelity may be nature's way. Blame it on biology, say the experts.
Animal parents may gain important benefits for the future of the their species by a little hanky-panky, research shows. A female may stray to pick up the best genes possible for her offspring, say the experts, while males may be driven by an impulse to father as many and as often as possible.
New studies using genetic testing techniques show that even the most apparently devoted of partners often mate 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, researchers say in reports being published Friday in the journal Science.
"True monogamy actually is rare," said Stephen T. Emlen, an expert on evolutionary behavior at Cornell University. He describes a great difference between "social monogamy," 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 among the primates, the animal order that includes humans, only two monkeys, the marmoset and the tamarin, are truly monogamous. All the rest, monkeys, apes and people, often mate outside their partnerships.
Most primates, in fact, make no pretense 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 10 percent of children were not sired by the male partner of the parental pair.
Among the birds, faithful sex partnership has been thought for years to be widespread. Some species, such as the eastern bluebird, gained reputations 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 percent to 20 percent 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 10 percent 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. "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 have shown 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 future generations 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 partnership are far more complex. Emlen cautioned against drawing "simplistic" conclusions about human biology from the studies of animals.
Researchers generally believe that monogamy originated among species whose young survived best when raised by a bonded pair. This may have been what led to the rise of monogamy among people, since human children take so long to mature.
One of the most sexually faithful of animals is the California mouse, Science reports. These golden brown rodents invariably pair up for life. Genetic testing has shown that both male and female partners ignore sexual temptations outside the nest.
David Gubernick of the University of California, Davis, reports in Science that one reason for this fidelity is that both parents are required to keep newborn pups alive through their birth winter.
The parents must take turns cuddling and warming the young, Gubernick said, and if the male leaves, the mother will kill or abandon the young.