One of my first memories growing up was wishing that my father would be home more" recalls Andrew Hudnut M.D, a family doctor in Sacramento, California. "I was 8, and we had just returned from a canoe trip. I remember thinking, 'I don't want a bigger house or more money. I just want my dad around.'"

When his wife gave birth, Hudnut arranged his practice so he could be home to take care of his son, Seamus, two days a week; he sees patients on the other three workdays. "It was a very natural transition," he reports. "I'm grateful to have the opportunity my father never had."

Part of a new generation of men who are redefining fatherhood and masculinity, Hudnut, who is 33, is unwilling to accept the role of absentee provider that his father's generation assumed. With mothers often being the breadwinners of the family, many young fathers are deciding that a man's place can also be in the home—part-time or even full-time.

According to census figures, one in four dads takes care of his preschooler during the time the mother is working. The number of children who are raised by a primary-care father is now more than 2 million and counting. By all measures, fathers, even those who work full-time, are more involved in their children's lives than ever before. According to the Families and Work Institute in New York City, fathers now provide three-fourths of the child care mothers do, up from one-half 30 years ago.

Many men and women wonder if all of this father care is really natural. According to popular perceptions, men are supposedly driven by their hormones (primarily testosterone) to compete for status, to seek out sex and even to be violent—conditions hardly conducive to raising kids. A recent article in Reader's Digest, "Why Men Act As They Do," is subtitled "It's the Testosterone, Stupid." Calling the hormone "a metaphor for masculinity," the article concludes, "...testosterone correlates with risk: physical, criminal, and personal." Don't men's testosterone-induced chest-beating and risk-taking limit their ability to cradle and comfort their children?

Two Canadian studies suggest that there is much more to masculinity than testosterone. While testosterone is certainly important in driving men to conceive a child, it takes an array of other hormones to turn men into fathers. And among the best fathers, it turns out, testosterone levels actually drop significantly after the birth of a child. If manhood includes fatherhood, which it does for a majority of men, then testosterone is hardly the ultimate measure of masculinity.

In fact, the second of the two studies, which was recently published in the Mayo Clinic Proceedings, suggests that fathers have higher levels of estrogen the well-known female sex hormone—than other men. The research shows that men go through significant hormonal changes alongside their pregnant partners changes most likely initiated by their partner's pregnancy and ones that even cause some men to experience pregnancy-like symptoms such as nausea and weight gain. It seems increasingly clear that just as nature prepares women to be committed moms, it prepares men to be devoted dads.

"I have always suspected that fatherhood has biological effects in some, perhaps all, men," says biologist Sue Carter, distinguished professor at the University of Maryland. "Now here is the first hard evidence that men are biologically prepared for fatherhood."

The studies have the potential to profoundly change our understanding of families, of fatherhood and of masculinity itself. Being a devoted parent is not only important but also natural for men. Indeed, there is evidence that men are biologically involved in their children's lives from the beginning.

It's well known that hormonal changes caused by pregnancy encourage a mother to love and nurture her child. But it has long been assumed that a father's attachment to his child is the result of a more uncertain process, a purely optional emotional bonding that develops over time, often years. Male animals in some species undergo hormonal changes that prime them for parenting. But do human dads? The two studies, conducted at Memorial University and Queens University in Canada, suggest that human dads do.

In the original study, published in Evolution and Human Behavior, psychologist Anne Storey, and her colleagues took blood samples from 34 couples at different times during pregnancy and shortly after birth. The researchers chose to monitor three specific hormones because of their links to nurturing behavior in human mothers and in animal fathers.

The first hormone, prolactin, gets its name from the role it plays in promoting lactation in women, but it also instigates parental behavior in a number of birds and mammals. Male doves who are given prolactin start brooding and feeding their young, Storey found that in human fathers, prolactin levels rise by approximately 20 percent during the three weeks before their partners give birth.

The second hormone, cortisol, is well known as a stress hormone, but it is also a good indicator of a mother's attachment to her baby. New mothers who have high cortisol levels can detect their own infant by odor more easily than mothers with lower cortisol levels. The mothers also respond more sympathetically to their baby's cries and describe their relationship with their baby in more positive terms. Storey and her colleagues found that for expectant fathers, cortisol was twice as high in the three weeks before birth than earlier in the pregnancy.

Biologist Katherine Wynne-Edwards, who conducted the research with Storey, explains that while cortisol is seen as the "fight or flight" hormone, it might more accurately be described as the "heads-up-eyes-forward-something-really-important-is-happening" hormone. It may help prepare parents for approaching birth. Cortisol levels normally increase in women as pregnancy advances; indeed, a cumulative rise in stress-hormone levels sets off labor and delivery.

The third hormone, testosterone, is abundant in male animals during mating but decreases during nurturing. If bird fathers are given testosterone, they spend more time defending their territory and mating than taking care of existing offspring. Research has shown that human males experience a surge in testosterone when they win sporting events and other competitions.

In Storey's study, testosterone levels plunged 33 percent in fathers during the first three weeks after birth. Levels then returned to normal by the time the babies were four to seven weeks old. However brief the dip in testosterone, it may have effects that endure for the life of the child. According to University of California at Riverside psychologist Ross Parke, it may "let the nurturing side of men come to center stage." The dip may set in motion the more-cooperative, less-competitive enterprise of parenting. By encouraging fathers to interact with their kids, this brief hormonal change might actually induce the bonding process.

Wynne-Edwards and graduate student Sandra Berg designed another study to test Storey and Wynne-Edwards' earlier findings. They measured the hormone levels of the fathers over a longer period of time and incorporated into the study a control group of men who had never had children. The control group was matched by age, season and time of day tested—all of which can affect hormone levels. Finally, by using saliva samples instead of blood draws, they were able to test the fathers and the men in the control group much more frequently.

In addition to confirming the earlier findings for testosterone reduction and cortisol change, the researchers also found that the fathers had elevated levels of estrogen. The increase started 30 days before birth and continued during all 12 weeks of testing after birth. Although estrogen is best known as a female sex hormone, it exists in small quantities in men, too. Animal studies show that estrogen can induce nurturing behavior in males.

Acting in the brain as well as in other parts of the body, estrogen in men, and testosterone in women, makes humans extremely versatile behaviorally. "We spend an awful lot of time looking for differences between the sexes and trumpeting them when we find them," observes Wynne-Edwards, "but our brains are remarkably similar, built from the same DNA."

In fact, going into the study, Wynne-Edwards predicted that the "daddy brain" would use the same nerve circuits, triggered by many of the same hormones, as the "mommy brain." "If Mother Nature wanted to turn on parental behavior in a male," she reasoned, "the easiest thing would be to turn on pathways already there for maternal behavior."

The studies also found that a father's hormonal changes closely paralleled those of his pregnant partner.

The researchers believe that intimate contact and communication between partners may induce the hormonal changes that encourage a father to nurture his children. Storey explains, "My best guess is that women's hormone levels are timed to the birth—and men's hormone levels are tied to their partners."

Exactly how this occurs is unknown. There may be actual physiological signals exchanged between partners in close contact, such as the transmission of pheromones. Similar to odors, pheromones are volatile chemical substances that animals constantly give off through their skin or sweat but that are undetectable. Pheromones can stimulate specific reactions—especially mating—in other animals. Think of a female dog in heat attracting all those barking mate dogs in the neighborhood.

Classic studies show that menstruation is communicated, and synchronized, through pheromones among dorm mates in college. If women in dorms respond to one another's pheromones, then a man and a woman who share intimate space could certainly communicate chemical messages. These pheromones could biologically cue a man that his partner is pregnant and kick off the hormonal changes that prompt him to be a dad in deed as well as in seed. Pregnancy certainly could, in fact, be signaled.

The level of intimacy within a couple seems to be a factor in how a mother's body chemically signals approaching birth to a father. All of the men tested were living with their pregnant partners. Emotional closeness may also generate hormonal changes, although this possibility was not examined in detail. Still, couples reported feeling closer to their partner if they were taking about the baby and sharing details about the pregnancy.

Whether this is the cause or the result of hormonal changes remains unknown for now. But the intimacy effect and the subsequent hormonal shifts may also be the reason many men experience pregnancylike symptoms.

When he is not taking care of Seamus, Hudnut treats both men and women in his practice. He recalls several patients who came to him complaining of such typical pregnancy symptoms as weight gain and nausea—all of whom were men. He remembers one second-time father who knew that his wife was pregnant even before she told him. He started having morning sickness, just as he had during her first pregnancy.

Pregnancy symptoms in men are actually more common than most people believe. Two studies found that approximately 90 percent of men experience at least one pregnancy-related symptom, sometimes severe enough to prompt an expectant father to seek medical help.

According to a study reported in Annals of Internal Medicine, more than 20 percent of men with pregnant wives sought care for symptoms related to pregnancy "that could not otherwise be objectively explained." Unfortunately, like pregnancy symptoms in women, there is little that can be done to make the symptoms go away—except wait.

Pregnancy symptoms in men, however well documented, are generally dismissed as being all in the father-to-be's head. Now it seems they may also be in his hormones. Storey and her colleagues found that the men who experienced more pregnancy symptoms actually had higher levels of prolactin. They also had a greater reduction in testosterone after exposure to sounds of crying and other "infant cues" that simulated the experience of being with an actual baby.

For men who feel nauseated or gain weight, no one yet knows for sure whether the changes in hormones are to blame. Surging hormones, however, have long been blamed for women's morning sickness and other pregnancy side effects. The fact that men also experience hormone changes suggests it is more than empathy that causes many of them to feel their partner's pain.

While it now seems a father may accompany his wife on her hormonal roller coaster during pregnancy, interacting with the baby may keep his hormones spinning even after the birth.

It's no secret that hormone levels can change in response to behavior. Sex, sports and work success can all send testosterone production spiraling upward. Might not nurturing a child—or conversely, the sight, sound and smell of a newborn—also change fathers' levels of testosterone?

In the original study, the researchers asked couples to hold dolls that had been wrapped in receiving blankets worn by a newborn within the preceding 24 hours. (After their wives gave birth, fathers held their actual baby.) They listened to a six-minute tape of a real newborn crying and then watched a video of a baby struggling to breast-feed. The investigators took blood from the men and women before the test and 30 minutes later.

What they found is startling: Men who expressed the greatest desire to comfort the crying baby had the highest prolactin levels and the greatest reduction in testosterone. And testosterone levels plummeted in those men who held the doll for the full half-hour.

Even though scientists have long observed changes in animal and human behavior as a result of shifting hormone levels, they do not yet understand exactly how hormones accomplish such change. The hormone-behavior link remains one of the great mysteries of the brain. Perhaps hormones stimulate more neuron connections in the part of the brain responsible for nurturing. Or perhaps hormones encourage neurons in nurturing pathways to fire more quickly.

Wynne-Edwards thinks hormones might turn a two-lane pathway in the father's brain into a four-lane superhighway. A neural road expansion might make fathers better able to recognize the smell or sound of their baby. It might even act on smell receptors in the nose to mitigate the smell of a baby's dirty diaper. Countless are the ways in which hormones could influence a father's brain to be more responsive to his baby.

Although testosterone may be the "primary" male sex hormone, research makes it clear that other hormones are also significant, especially during the transition into fatherhood Wynne-Edwards believes the research is "a validation of the experiences that men know they have had. It also goes a long way to bumping testosterone off its pedestal as the only hormone that is important to men."

Parke believes that the research suggests something even more radical: "Men are much more androgynous than we think." We have the capability to be aggressive and nurturing. The traditional view of men as predominantly aggressive really sells men short and denies their capability to experience the range of human emotions.

The research suggests that a man's hormones may play an important role in helping him experience this full range of emotions especially in becoming a loving and devoted dad. In fact, it offers the first evidence that to nurture is part of man's nature.