'So many men, so little time!' The actress and temptress Mae West jested about it, but scientists - male ones anyway - are convinced they have proved it. Women - far from being naturally monogamous - are, like men, naturally promiscuous. Biologists believe that women are genetically programmed to have sex with several different men in order to increase the chances of healthy children with the greatest likelihood of survival.

The theory helps to explain the high incidence of mistaken paternity. Evidence suggests that one in seven people is not the biological child of the man he or she believes is the father. The American television comedy Sex in the City, criticised for being a male fantasy depicting sexually aggressive females, is apparently only showing women doing what comes naturally.

Conventional theory has suggested that only men are naturally promiscuous, since it increases their likelihood of having many offspring by different women.

However, two new reports in scientific journals have added to a growing body of evidence that females from across the animal kingdom - including birds, bees, fish, scorpions, crabs, reptiles and mammals - are promiscuous, and researchers say they are convinced human females are the same.

A report last week in the Proceedings of the US National Academy of Sciences showed for the first time that when a female guppy - a freshwater fish - is promiscuous, rather than monogamous, she increases the quantity and quality of her offspring.

A study, in Nature, shows that hens mate with several males, but actively choose which sperm they are fertilised by. Tim Birkhead, professor of evolutionary biology at the University of Sheffield, discovered that 'when females are mated by cocks they don't like they just chuck the sperm out - they squirt it straight back out after mating.'

Birkhead concludes: 'It reinforces the view that females aren't just passive vehicles for sperm.'

Birkhead believes that, like jungle fowl, human females show evidence of having spontaneous abortions of clusters of cells not compatible with what they want.

Evolutionary biologists say the evidence has become overwhelming and ties in with anthropological studies. 'Since humans have evolved from other forms of life, it is likely they have many of the same mechanisms. The motivation that makes us seek other partners is evolutionary. The evidence strongly suggests that human females are promiscuous, but that the level of promiscuity depends on circumstances,' said Birkhead, who recently published a book called Promiscuity.

Women are thought to get several evolutionary advantages from being promiscuous. The most obvious is 'fertility insurance'. Birkhead cites the case of a man who went for a vasectomy after his wife gave birth to her third child. However, the surgeon found the man had a congenital defect that meant he couldn't possibly be the father of any of the children.

One study followed couples waiting for NHS fertility treatment, where the men were 'azoospermic', meaning they produced no sperm and were totally infertile. The researchers found that 25 per cent of the women became pregnant before fertility treatment started.

Promiscuity is also suggested by the 'good gene' theory, as exhibited by the great reed warbler. The female warbler may nest with a male with a small song repertoire, but she will seek 'extra-pair copulation' with males with big song repertoires, which tend to live longer. This way she gets the best offspring and they are looked after.

The female pseudo-scorpion which has a brood-sack to store fertilised eggs, provides another example. If she has mated with just one male, there is a high rate of spontaneous abortion, but if the scorpion has mated with two males, the likelihood of abortion is a lot lower, because the female can choose the best sperm.

'We don't all get the exact partner we want, we all make some compromise. That's true of humans as well. A woman might find a man who is good at providing food and looking after children, but she doesn't necessarily want him to be the father of her kids,' said Birkhead.

By being promiscuous, women can also ensure greater care for their children. The Inuit people of Arctic North America have traditionally had 'co-marriage' where two couples effectively marry each other. This means that the children have two sets of parents to look after them in the harsh conditions.

However, the strongest evidence that women are promiscuous actually comes from men themselves - or rather their testicles. The concept of sperm competition suggests that males have an incentive to produce as much sperm as possible in order to flood out those of a rival mate. This implies the more naturally promiscuous the female, the larger the male testicles will evolve.

Female chimpanzees are wildly promiscuous, and male chimpanzees have large testicles - there is a reason why males chimps are dressed up in circuses; in contrast, female gorillas are extremely faithful, and so male gorillas have tiny testicles. Human males have mid-size testicles (40g or 0.08 per cent of body weight), suggesting moderate promiscuity among human females.

The only comfort that men can take from the animal world is that females also have an incentive not to have all their offspring from adulterous liaisons.

'If they are totally unfaithful to their social partner, they might just be abandoned,' said Birkhead.