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Their hormones change during pregnancy, and even picking up a baby can cause their testosterone levels to take a nosedive.

Yes, there is a growing body of evidence that men are more than just sperm donors -- they really are biologically primed to be attentive parents .

But scientists say the role they play as dads isn't just biological; it also depends a lot on the society in which they live.

Take the Aka, diminutive hunter-gatherers in the Central African Republic. They spend almost half of their time either holding their babies or being within arm's reach of them. And they let the little ones suck their nipples for comfort.

In Western societies, men who offer their breasts to their babies are rare -- but not unheard of. There have been reports of "milkmen" who spend hours letting their babies suckle as a way of bonding.

This may strike Canadian dads as weird. But the biological changes that encourage the Aka to use their nipples as pacifiers may be common to all males who spend time with their pregnant partners and then with their newborns.

"The evidence suggests there is a biology of fatherhood," says Barry Hewlett, an American anthropologist who has spent years studying the Aka and considers them to be the most attentive of fathers.

Much is known about the biology of motherhood -- how a number of hormones may help a mother bond with her new baby -- and the biology of fatherhood is still largely uncharted territory, but a half-dozen or so studies have found that fathers-to-be and new dads experience hormonal changes and altered brain activity.

These physical manifestations of fatherhood may explain why many men fall in love in with their infants, why the warm weight of a newborn feels so good on their chests.

"She'd fall asleep on me, and I would instantly go, 'I could do this all night,' " Michel Charron says.

Mr. Charron is an Ottawa father who is taking parental leave to be with his 13-month-old daughter, Sophie.

Bonding with his baby made getting up in the middle of the night and the other demands of new fatherhood much easier, he says. "It is just the most amazing thing."

The biological changes thought to promote bonding and nurturing in men appear to begin well before the birth of a child.

Anne Storey, a researcher in the psychology department at Memorial University of Newfoundland in St. John's, found that expectant fathers' levels of hormones related to pregnancy, such as estrogen and prolactin, fluctuate in a way that seems to mirror the changes taking place in mothers-to-be.

The men's levels are lower and the shifts less dramatic -- in effect theirs is a muted version of what women go through, a kiddie roller coaster instead of Thunder Run, says Katherine Wynne-Edwards, a researcher at Queen's University in Kingston, Ont. Still, in a study of 47 couples in Newfoundland, 90 per cent of the men reported experiencing pregnancy-like symptoms, including nausea.

Called the "parenting hormone," prolactin stimulates milk production in women, and in some animals leads to such maternal behaviour as nest-building and retrieving scattered young.

Its level rises, to a lesser or greater degree, in new fathers, and Alison Fleming, a researcher at the University of Toronto, has found that men with more prolactin are more alert to a baby's cry. And experience may play a role as well; when first-time fathers hear a baby in distress, their levels don't increase as much as those of men who have been through it all before.

The other major hormones are cortisol, which is associated with stress but may help to twig a new father to the many needs of a newborn, and estrogen, which may make men even more attentive. As for a man's signature testosterone, a number of studies have found that its levels go down, which may be nature's way of tempering aggression at a stressful time. Dr. Fleming and her colleagues have found that fathers with lower levels of testosterone feel more of a need to respond to their infants' cries.

In fact, Harvard University researchers have discovered that even holding a baby causes a man's testosterone levels to drop, a finding that suggests fathers of adopted children also experience biological changes. Anecdotal evidence suggests that the adoptive parents -- who often feel great excitement and stress -- may undergo hormonal changes after their new child arrives, including an increase in cortisol.

So why are some fathers little more than sperm donors, while others are superdads, staying home to raise their children? Hormones don't cause behaviour, says Dr. Wynne-Edwards, they only facilitate it.

Culture also plays a major role. The Aka men, for example, take their babies with them when they go out to drink palm wine with the guys.

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