Friday, September 25, 1998 Published at 11:17 GMT 12:17 UK
Infidelity 'is natural'
Females 'stray to gather the best possible genes for their offspring'
Infidelity may be natural according to studies that show nine out of 10 mamals and birds that mate for life are unfaithful.
Experts found animals that fool around are only following the urges of biology.
New studies using genetic testing techniques show that even the most apparently devoted of partners often go in search of the sexual company of strangers.
Females stray to gather the best possible genes for their offspring, while males are driven to father as many and as often as possible.
"True monogamy actually is rare," said Stephen T Emlen, an expert on evolutionary behaviour at Cornell University.
According to him, there are two kinds of monogamy - social and genetic.
In the first kind partners bond and work together to raise their young. With "genetic monogamy," parents are faithful sex partners.
While social monogamy is relatively common, genetic monogamy is the exception rather than the rule.
Dr Emlen said there are only two monkeys, the marmoset and the tamarin, are truly monogamous.
All other primates, includes humans, often mate outside their partnerships.
"One of the patterns is that females seek males of high status and high quality," said Dr Emlen.
"By doing so, they are able to produce offspring of higher quality that will be able to do better and survive better."
Males are said to be biologically driven to stray by the desire to spread their genes into as many members of the next generation as possible.
But the reasons why people have sex outside a relationship 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.
Birds of a feather
Faithful sex partnership has been thought for years to be widespread among birds.
The eastern bluebird was considered a prime example, with male and female partners working together to build nests, incubate eggs, then feed and raise their young.
But researchers have found that the bluebirds have a sex life that rivals a television soap opera.
Patricia Adair Gowarty, a behavioural ecologist at the University of Georgia, has found that 15% to 20% of chicks cared for by a pair of bluebirds were not fathered by the male.
She found that only 10% of 180 socially monogamous species are sexually faithful.
The research is published in the journal Science.