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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.

"They have their babies, but they are talking guy talk. It's just amazing to watch," says Dr. Hewlett, who has studied the Aka for 30 years since he was a graduate student and wrote a book about them called Intimate Fathers.

Even so, the Aka are less child-focused than many North American families, he says. The babies and kids are part of whatever activity their parents are doing -- but not necessarily the centre of attention. As well, the Aka never talk about their children as though they are burdens. They don't complain about sleepless nights or difficult toddlers.

Over the years, Dr. Hewlett says, his work with them has changed his own approach to being a parent. He has seven children from 15 to 25 years old, and tries to spend more with them and include them in what he is doing. "I try to take the kids to work."

Not that he believes hunter-gatherers like the Aka have all the answers for Western parents.

They don't, for example, use anything resembling diapers, and some experts have gone as far as to suggest that parents in the developed world also could learn to anticipate when their babies need to go, and hold them over an appropriate receptacle. But Dr. Hewlett says he has seen too many soiled Aka moms and dads to think much of this toilet-training tactic.

Working with men who spend so much time caring for their infants -- wet or dry -- also piqued his interest in the role hormones might play in fatherhood. He conducted a study in the United States in which he took blood samples from fathers before they held their infants, and again after they had them on their chests for 15 minutes. Their prolactin levels went up.

"There is a biology of fatherhood, but it is impacted by the environment," Dr. Hewlett says. "If you don't hold the baby, certain things don't get triggered."

It isn't clear what, exactly, triggers the physical changes in fathers, although animal research, including work on two species of primates, suggest that pheromones -- chemical signals produced by the mother -- could be the key.

But the system may not be so straightforward. Dr. Wynne-Edwards and her colleagues have lowered the levels of hormones in male hamsters, which normally act like superdads. But this hasn't changed their nurturing behaviour, which she believes may have been prompted instead by their own upbringing or by something that happens when they mate.

In humans, researchers have found that testosterone levels drop significantly after marriage, long before many men become fathers.

Ottawa's Mr. Charron doesn't know if his hormone levels have changed, but he does know that he is more emotional. "I'm a softie," he says. "The tears just come" -- often prompted by a tragedy in the news that involves children.

Spending so much time with Sophie may even have rewired his brain.

James Swain, a Canadian researcher at Yale University, used functional magnetic-resonance imaging to look at the brain activity of 25 couples after they had a baby. He and his colleagues took brain images of the new moms and dads when they heard the sound of their infant crying, or looked at a picture of their newborn.

They found that the brain activity of new parents is strikingly similar to what is seen in patients with obsessive-compulsive disorder when their symptoms have been triggered.

It is not that new parents are mentally ill, but they do tend to fuss. Dr. Swain's findings may help to explain why some moms and dads report constantly sneaking in on sleeping children to make sure they are still breathing, or adjusting infants' clothing or bedding to make sure they are not too hot or too cold.

This adaptive parental fussing seems to require some of the same brain circuits gone awry in obsessive-compulsive disorder. The effect was more pronounced in women two weeks after the baby was born, Dr. Swain says, but was clearly there for men as well.

By three months, however, the brain response of the father to the sounds of his baby crying was comparable to that of the mother.

"It may be that men are biologically primed or programmed, perhaps even by their own baby, to become attentive fathers," he explains. "This is measurable in their psychology and their brain activity in specific parenting circuits in the brain."

Findings like these intrigue Michael Lamb, a psychologist at Cambridge University who is considered the father of research on fathers.

Like many in his field, at one time he believed that mothers and fathers play distinct biological roles: Mom made sure the child was warm, fed and cuddled. The baby became attached to her through this regular care. Dad's department was rough-and-tumble play, a more boisterous and physical kind of stimulation. The baby bonded with him through play.

But he has changed his mind, in part because of the time he, too, has spent among the Aka. There are differences, he says, but they are not crucially important, and are often a reflection of who is the primary caregiver. The more involved a man is, Dr. Lamb says, the more he is the hands-on, responsible parent whose job it is to take care of the child.

Mr. Charron, for example, is the primary caregiver for Sophie, now that his wife, Pascale Groulx, has returned to work. He feeds her, changes her diapers, wipes her nose and teaches her not to eat sand. At night, when she is sick or upset, she often wants him.

Dr. Lamb says more fathers are taking on the minute-by-minute jobs of parenting young children.

"I started studying American dads in the mid-seventies, and quite a few men had never changed a nappy," he says. " Many others had done so, but only under duress or extreme conditions.

"You don't find men like that any more."

Spend a few hours in a local park and you see what he means. Fathers are wiping noses, changing diapers and teaching their kids to take turns on the slide.

In the 1970s, a Canadian father's time with children averaged about 40 per cent of the time a mother spent. By the late 1990s, it had jumped to 67 per cent, and in 2004, 9.5 per cent of fathers claimed or planned to claim employment insurance while they took time off to be with their new baby.

But they still like to fling their children up in the air or dangle them by their feet. "I toss her around a little bit more," Mr. Charron says of Sophie. And when the youngsters are a bit older, their dads will begin to play proper games with them, often focusing on sports they loved while they were growing up.

Yet, Dr. Lamb says this physical kind of play is not a universal phenomenon. You see it more in North America, Britain and Australia, he says, but not in the Middle East, Sweden and some countries in Europe.

Daniel Paquette, meanwhile, argues that mother and fathers still have distinct biological roles.

According to the researcher at the University of Montreal, mothers typically comfort children, and calm them down during times of stress. Fathers open them to world, make them braver, and help them compete.

Look at how fathers typically hold babies, he says -- against their shoulders, with their little faces looking out at the world.

The researcher is now working on experiments to test his theory, and he says that a small study in France found that children do better when their parents play different roles.

Dr. Lamb isn't so sure. "We want to be really careful about implying hard-wired differences between men and women," he says.

He says there are some things only a mother can do, like lactation. Fathers may try to let their babies suckle, but even if their breasts produce something, it wouldn't be nutritionally valuable.

Dr. Lamb doubts that males ever breastfed for real, even early in human history, because it takes so much energy to produce milk.

Nipples are just something that comes in handy for Aka dads when they have a cranky baby on their hands.

But men in the West can keep their shirts on and still have a happy baby, Dr. Lamb says. "We have pacifiers."

Anne McIlroy is The Globe and Mail's science reporter.


of the animal world

Dayak fruit bats of Malaysia:

Researchers were shocked to discover that the males of this species have mammary glands that appear to produce milk. They don't know if the bats (the first case of wild male mammals producing milk) actually feed their young.

Common marmosets and

cotton-top tamarins: On average, these tiny primates gain 10 per cent of their body weight when their partners are pregnant -- energy they will need once the babies are born. The monkeys usually have twins, relatively large infants about 20 per cent of an adult's size. It is mostly fathers who carry them around on their backs.

Djungarian dwarf hamsters:

The males of this Siberian species act as midwives, delivering their own infants using their mouths and four paws. They lick the pup's nose to clear the airways, then eat the placenta, clear off the membranes and tuck the infant hamster into bed.

Murres: These seabird dads jump off of cliffs with their chicks to teach them to swim.

Seahorses: Males brood their babies in a pouch, which is nourished with a placenta-like mechanism.

-- Anne McIlroy