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It 's a brave person who dares to tackle a concept quite so deeply charged as motherhood. But Sarah Blaffer Hrdy, emeritus professor of anthropology at the University of California, Davis, found herself with no choice. The old, weak, vision of mothers as passive and dependent just didn 't fit with what she was seeing on the savannah and in the forests. There she witnessed a deadly battle between male and female primates, each armed with complex strategies and very different agendas. Re-examining the way that males and females interact led Hrdy to challenge the way we understand mothers, as she explained in her book Mother Nature.

How have our views of primate behaviour changed over the past few decades?

Right from the beginning, many people made the implicit patriarchal assumption that the world ran in accordance with Victorian ideals, with sexually ardent males in pursuit of coy females. So once scientists began to look at primate behaviour from female as well as male perspectives, this led to a real transformation. Some have read feminist politics into the transformation, but as I see it, if selection pressures on half the species were ignored, and then subsequently taken into account, this simply means better science being done.

For once we realised how much individual variation exists, we looked for sources of variation in female as well as male reproductive success, and that led fieldworkers to theorise about the strategies females engage in to increase their reproductive success and inclusive fitness. This sets off a chain reaction once we give females strategies, then in order to understand males we also need to know how males respond to female strategies, and then in turn to consider female counter-strategies to male counter-strategies.

Was it your work on infanticide in langurs that set you thinking this way?

Infanticide is a classic paradigm for evolutionary interactions between the sexes. The langur monkeys I studied in India live in breeding groups with a single male, and every 27 months on average you would have males from an all-male band come in from outside and take over the troop. When that happens, one of the usurpers would attempt to eliminate offspring sired by his predecessor.

Essentially, they were eliminating the last mate choice the mother made, and also increasing their own probability of breeding with her because a bereaved mother will resume ovulating sooner than she would have done if she had continued to lactate.

You see this happening, and you think, how are females responding?

Females in that situation have a range of options, none of them terribly good. Among them is mating with a range of males to confuse paternity. Males then have to counter their counter-strategies. Today, evolutionary theorists like William Rice at the University of California, Santa Cruz, use the term "sexually antagonistic co-evolution" to describe this kind of dynamic process.

The idea of those counter-strategies will be a real shock to many people, especially the notion that females might benefit from having quite a lot of partners who feel that they may or may not be the father.

When I published The Woman That Never Evolved, I expanded the argument that females might try to mate with a number of males to confuse paternity, and that this might apply in humans as well. The idea was not well received. For example, Don Symons, who wrote the key work The Evolution of Human Sexuality, said in an essay called "Another woman that never existed" that this simply could not have been part of our evolutionary heritage. His reasoning was that a human female needs investment from a husband to provide for her and help her raise her extremely costly infant.

Of course I agree that human females need a lot of help, but I envision mothers as resourceful opportunists who elicit help from a range of different parties. Many aspects of women 's personalities are about just this eliciting support. But Symons was assuming that that kind of help was only going to come from a husband certain of his paternity, otherwise, he believed, no male would invest. As far as I was concerned, this was just another patriarchal presumption. For by then I already knew that so long as care is not too costly and exclusive, males help rear even infants who are not their own. In baboons, for example, females mate with a number of males, and males they have mated with will look out for that female 's offspring. New data from Robert Seyfarth and Dorothy Cheney 's field site at Mikumi in South Africa indicates that 41 per cent of all infants born there are killed by males, but these killers are males the mother did not mate with. Ryne Palombit 's "play-back" experiments from that population reveal that when a strange male is harassing infants, some resident males respond to maternal calls for help if the plea comes from a female they have mated with. Certainty of paternity then is not the sine qua non for "paternal" assistance.

What about humans?

Studies of hunter-gatherers suggest that meat brought in by male hunters is shared among the group rather than channelled to their mates or offspring. Kristen Hawkes at the University of Utah has proposed that men share in order to enhance prestige which in turn by appealing to women improves their love life. Along similar lines, Polly Wiessner also of the University of Utah, who studied the !Kung of Botswana and foragers in New Guinea, thinks that hunters are sharing meat in order to influence the political composition of the group, since kin and others helpful for rearing their offspring tend to cluster around successful hunters.

I agree, and believe that such sharing is integral to a species that evolved (as I believe humans did) as a cooperative breeder. The motives of these hunters are very different from those assumed by Irv DeVore of Harvard University and the late Sherwood Washburn, when they laid out their "man the hunter" model. Like Darwin, they assumed that fathers hunted to provide for their highly dependent offspring, and that the men who were the smartest and the most capable with tools were the best hunters who provided for their infants better. And they thought that 's how humans evolved really big brains and so forth. It all depended on a one-man woman waiting for her mate to bring food back. It 's an inherently appealing paradigm that just will not go away, even as the evidence for it is dismantled.

So are humans also busily confusing paternity?

Some are, some aren 't. I often see claims on the back of books about evolutionary psychology that 10 per cent of human offspring are sired by someone other than the husband. But that kind of global statistic is misleading. That high proportion applies in some populations, say inner cities where mothers are pursuing a strategy of mating with a number of males because job opportunities for men are awful and no one man in that community can reliably support her offspring. High rates of misattributed paternity are also found in Amazonian societies like the Canela or the Ache, where men who go off hunting or fishing may or may not come back with anything to eat. Or else, a woman 's husband might die, or decamp. Under such circumstances, mothers cannot afford to have just one mate.

Fortunately for children in those societies, mothers often have more than one, and use strategies of what I call "polyandrous motherhood". Here mothers line up multiple "possible fathers" and sequential "fathers", facilitated by a widely accepted folk belief in "partible paternity". Every man a woman had sex with in the 10 or so months preceding a birth is considered a progenitor, who is expected to provide both the pregnant women and her progeny with protein-rich gifts.

What exactly is this partible paternity?

The Ache, Canela, Bari, Yanamamo people from different language groups from a vast Amazonian area believe that fetuses are built up over time, like the lustre on pearls, by repeated applications of semen. According to anthropologist Kim Hill of the University of New Mexico, the Ache have different words to refer to "the man who put it in", "the one who mixed it" and so forth, and men who provide either sex or meat to the mother are considered to have contributed to the creation of a child.

All these "godfathers" are expected to help provide for the child. And as Penn State anthropologist Steve Beckerman has shown for the Bari of Venezuela, a husband plus one secondary father correlate with the best chances of offspring survival. This is why a Canela woman who suspects she is pregnant will try to seduce the group 's best hunters and fishermen. I like to imagine some white-haired grandmother dreaming up this story. It 's so obviously beneficial to the survival of her grandchildren.

How common is this?

More common than I had realised before starting to research Mother Nature. It was really eye-opening, and I am now starting to catalogue cases of polyandrous motherhood The pattern crops up not just in much of lowland South America, but if we are talking about informal polyandry (rather than institutionalised polyandrous marriage), it is very widespread.(...)

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Given all this dubious fatherhood, our ancestresses were going to need a wide range of helpers to help rear their young. Who fits the bill?

I am increasingly convinced that in contrast to other apes, humans evolved as co-operative breeders. It takes about 13 million kilocalories to rear a human infant from birth through to nutritional independence, and hunter-gatherer women don 't peak in their skill at gathering food until they are quite old. Mothers needed help from allomothers group members other than the mother. I agree with Kristen Hawkes that grandmothers were important for raising children. After all, post-reproductive females are an especially dedicated, skilled, motivated type of allomother. But I also think other group members aunts, older siblings, brothers and former lovers helped at various points in the child 's long period of dependency. In the last few years, empirical support has accumulated to the point that we can say that when group members other than the mother and father are on hand to help provide for offspring, survival rates zoom up.

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