Infidelity may be nature's way, according to studies that show about 10 percent of birds and mammals that mate for life are actually faithful to their partners. The urges of biology, say the experts, promotes some forms of hanky-panky.

Studies published Friday in the journal Science suggest that animal parents can gain important benefits for their species by mating with those outside a bonded partnership.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," said Stephen T. Emlen, an expert on evolutionary behavior at Cornell University. He says there is 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 there are only two monkeys - the marmoset and the tamarin - that are truly monogamous among the primates, the animal order that includes man. All the others, 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 parent couple with whom they bonded.