HE RARELY gets into fights, remains faithful to his partner for life, and enjoys staying at home and looking after the kids. Is this just romantic nonsense, or can the perfect male truly exist?

Well, he has been discovered - but he's also 9in long, covered in fur and lives in underground burrows in America. The animal in question is the prairie vole - a plump, cuddly version of a mouse - and his exemplary behaviour has led scientists to try to find out what makes him tick. The answer is so absurdly simple that it seems too good to be true: a hormone in the brain that turns bachelors into protective mates and devoted fathers. Although it is exclusively in prairie voles that it weaves its magic, this chemical is present in other animals too. There is some evidence that humans can also come under the hormone spell, and this raises some profound questions about our own behaviour - from falling in love to parenthood.

The male prairie vole, it has to be said, is not a complete wimp. A shy creature most of the time, he is rarely given to violence - but one thing that makes him turn really nasty is another male sticking his snout into the nest. If this happens, the prairie vole will fight the intruder tooth and claw to protect his mate.

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Most of the time, however, there is nothing he enjoys more than cuddling up to her in the warmth of their underground nest, the prelude to a honeymoon involving a very steamy bout of mating - once an hour for a whole day is about par for the course. But the male prairie vole is no fly-by-night. When the pups are born he becomes a devoted father, sharing grooming and caring for them with his mate. There is also a strict incest taboo: sex among family members is almost unheard of.

What makes the prairie vole more extraordinary still is that few male mammals of other species have such a strong inclination towards monogamy or fatherhood. Not even his close cousins, the meadow vole and the montane vole, exhibit anything other than stereotypical male behaviour such as brawling, mating with as many females as possible and running off at the very first sign of parenthood.

The search for a clue to the prairie vole's behaviour began with Thomas Insel and colleagues at the National Institute of Mental Health in Maryland, USA. They took a close look at the male vole's brain and found that there was hardly anything out of the ordinary compared with the brain of a meadow or montane vole - except for the way it responds to a hormone called vasopressin.

Until then vasopressin had led a fairly humdrum life in physiology, largely looking after the volumes of water that the kidneys absorb. Its only other function is to send messages between nerve cells in the brain, and it is probably this role that helps explain the vole's behaviour.

Insel's group found proteins in the vole's brain cells that lock on to vasopressin. A hormone can only work if it locks on to special proteins on a cell, in much the same way as two pieces of Lego. Once the locking has occurred, the cell becomes excited, triggering a series of chemical changes which eventually alter behaviour, growth or development.

Finding these vasopressin receptors led the team to test what would happen to the male's behaviour if he were treated with vasopressin. They started off by confronting a virgin male - timid and nave of any sexual encounter - with another male. Immediately the virgin male backed off without a fight. But if the vole was given a shot of vasopressin (the hormone produced naturally in the male vole's brain when its mate is threatened), a Jekyll-and-Hyde change occurred; the shy little creature turned into a raging monster, exactly as he would if protecting his mate, and savaged the intruder. The violence could be turned off just as easily with a drug that stopped vasopressin from working. The vasopressin seemed to hold the key to triggering aggression in males.

Vasopressin also appears to control the male's monogamy. Virgin males were introduced to a pair of females which had been spayed to stop them giving the come-on to a mate. Though they received no encouragement, the male voles immediately tried seducing both females in turn - just as most other male mammals would. But a shot of vasopressin changed all that: the males stayed faithful to one female and ignored her friend. This is probably the nearest anyone has ever come to discovering a chemical for faithfulness.

But the list of achievements for vasopressin doesn't stop there. Geert De Vries at the University of Massachusetts at Amherst thinks it may also be responsible for making the prairie voles into caring fathers. Like most typical bachelors, virgin males usually show no interest in pups. But when they are given vasopressin artificially and presented with a clutch of pups, they become exactly like doting fathers and start grooming them. However, if a vasopressin-blocking drug is given, the voles turn into typical bachelors again, completely uninterested in the pups.

De Vries and his colleagues also found evidence that, soon after mating, vasopressin prepares the voles for fatherhood. Certain brain cells make vasopressin become hyperactive, and these could churn out quantities of the hormone into the part of the brain responsible for primitive emotions, such as preparing the male for his forthcoming parental duties.

This may all sound uncannily relevant to human males - but only up to a point. No scientist is ready to put his or her neck on the block and claim that a hormone could turn a licentious Lothario into a model

father. 'Making the jump from vole to human is a dangerous one,' cautions James Winslow, a member of Insel's research team. He notes that voles are slaves to their hormones and humans are supposed to be different. We are more than just a bag of nerves and chemicals, he argues, being influenced by culture, experience and a multitude of environmental factors that don't enter into a vole's way of life.

Yet beneath their complicated exteriors, humans have strong primeval impulses like any other mammal - and Winslow should know. He became involved in this work because of the arrival of his first child. 'Something remarkable happened to me and my feeling about children which I was pretty sure I didn't learn from my personal experience.' But was this a male fatherhood hormone at work? Winslow doesn't admit anything readily: 'My guess is that there is something there in the brain waiting to express itself when a child arrives, and it has a big effect on both mothers and fathers.'

Scientists are probably right to be ultra-cautious. The monogamous lifestyle of humans and voles is similar, but the way the two establish that relationship is not. We have a different neural system which regulates social behaviour. Removing ovaries doesn't stop sexual behaviour in women, for example, although it does in voles. The behaviour of humans and voles may appear similar, but we have the ability to override some of our deeper instincts.

Yet there are some intriguing parallels. A 1987 study of masturbating male students by Stafford Lightman, now at Bristol Royal Infirmary, and his colleagues at Westminster Hospital and the Institute of Psychiatry showed that at the point of sexual arousal vasopressin is released into the peripheral circulation. The thing that determines behaviour, it seems, is not the sex hormone itself but the way certain brain proteins lock on to it and activate it.

Another sex hormone, oxytocin, is released during ejaculation. It is nicknamed 'the hormone of mother love' because some scientists, like De Vries, believe it makes vole and human mothers care for offspring, as well as stimulating milk secretion.

Human behaviour is so complicated that scientists are hoping a simpler mechanism such as the vole's can give new clues to our behaviour, particularly when things go wrong. It might well point to new treatments for human mental disorders, particularly those affecting the way we relate to other people. For example, diseases such as schizophrenia and depression involve feelings of isolation which make relations with other people especially difficult.

Perhaps the most promising hormone breakthrough in this field is with the obsessive compulsive disorders - returning time and time again to check that the cooker is turned off, for example, or the eating disorder bulimia. James Leckman at Yale University has found unusually high levels of oxytocin in the spinal fluid of patients with these disorders. If oxytocin is involved, then artificial hormones could be used to remedy the brain's defective chemistry.

Yet these disorders raise uncomfortable moral issues. What, for example, constitutes an abnormal obsession? Leckman has a profound insight into one of the most powerful of human obsessive behaviours: falling in love. 'It's intriguing to think that during courtship there is a certain amount of obsessive behaviour over a potential mate,' he says. The driving passion may be nothing more romantic than those two love chemicals, vasopressin and oxytocin.

And so the virtuous life of the prairie vole leads us into some thorny moral dilemmas. Maybe the most contentious question of all is this: if vasopressin and oxytocin do control men's behaviour, should we ever use drugs that effect these hormones? After all, would it be right to turn on chivalry, fidelity and willing fatherhood by administering a pill? -

(Photograph omitted)