As Lance Somerfeld learned, babies are excellent teachers.

His son provided round-the-clock on-the-job training, free of charge.

Within days of becoming a father, the 36-year-old New York City resident learned how to soothe a fussy baby. How to burp him, feed him and swaddle him.

Yet in some ways, Somerfeld's son began shaping him into a father even before delivery.

Although men may not be aware of it, they actually undergo hormonal changes as they prepare for fatherhood, says neuropsychiatrist Louann Brizendine, author of The Male Brain (Broadway Books, $24.99), released in March.

At first, those hormones tell them to panic, or at least pay attention.

Levels of a stress hormone called cortisol — the same ancient chemical that instructs men to fight or take flight — tend to spike about four to six weeks after men learn they're going to be fathers, subsiding as the mother's pregnancy progresses, Brizendine says.

"It is a cortisol surge that wakes our brains up every morning," Brizendine says. "So this surge may put the father-to-be's brain on alert and in a sense wake him up to the impending reality of a new baby's coming, and alert him that he'd better get things ready."

Philip Andrew, who organizes a fathers' group in Lincoln, Neb., says he tried to channel his pre-baby anxiety into educating himself about infants. Concerned that he was unprepared to handle a baby, he and his wife signed up for childbirth and parenting classes at the hospital. Learning about what to expect helped ease his mind and allowed him to remain calm on the big day, Andrew says.

About three weeks before the baby arrives, levels of testosterone — sometimes called the "male hormone," associated with competitiveness, aggression and sex drive — fall by roughly a third, Brizendine says.

That may have helped the species to survive, she says. A human baby needs two full-time caregivers — maybe more. So a baby is more likely to survive if Dad is at home to help, rather than out looking for new romantic conquests, she says.

Bill Stratbucker, 40, was surprised at the things he did after his twins were born — such as going out to buy nipple shields for his wife, which help babies nurse. "Some part of me felt like I wanted to hold on to the way I was before kids," says Stratbucker, a pediatrician in Grand Rapids, Mich., whose twins are now 6 years old. "You just change and mature a lot."

Reacting to Mom

Even the male brain changes, Brizendine says.

Tests show that men actually get better at hearing a baby's cry — zeroing in on the sound and responding to it — as the due date of their own child approaches.

Other hormonal changes are a head-scratcher, she says.

At the same time that testosterone is falling, a man's supply of prolactin — a hormone that helps mothers make milk — rises more than 20%, Brizendine says.

"We still don't know what prolactin is doing in dads," she says.

Male hormones begin to readjust when the baby is 6 weeks old, returning to pre-fatherhood levels by about the time the baby is walking, Brizendine says.

Scientists can't completely explain why men's hormones fluctuate, Brizendine says. It's possible that men are reacting to women's pheromones, airborne chemical messengers secreted by their skin and sweat glands, she says. Scientists think these chemicals may trigger a man's brain to "begin making the hormonal changes necessary for paternal behavior," she says.

YOUR HEALTH: New dads can be a cautious lot

These pheromones appear to work both ways, at least in animals. In a study of mice, the males released their own pheromones. That triggered the female mice to make more prolactin, which also stimulates the growth of maternal brain circuits, Brizendine says.

Through these unconscious chemical signals, she says, mothers and fathers may be spurring more loving, nurturing behavior in each other, increasing the odds their babies will survive.

It's possible, Brizendine says, that "the baby is actually in control from before birth in just this very way: The baby is controlling the hormones of the mother, and she is controlling the hormones of the father-to-be."

Babies exert long-term effects on their fathers. Studies show that dads who are more involved in child care have lower testosterone levels than uninvolved fathers. But do lower testosterone levels really make men more nurturing? Or does nurturing change a man's hormones? Scientists don't yet know.

Scientists have more solid answers when it comes to the ways that babies affect the brain.

Looking at a baby's face — with its pudgy cheeks and big eyes — causes the brain's "parental-instinct" area to light up within one-seventh of a second.

A baby's smile activates the same brain circuits involved with falling in love, Brizendine says.

As if practice makes perfect, Brizendine says, "the more a man holds and cares for his child, the more connections his brain makes for paternal behavior."

Bigger picture

Parents say they need that fierce devotion to power through the tough times, waking every two hours to feed a hungry newborn or pacing the halls with a colicky 3-month-old.

Rahul Parikh says even his training as a pediatrician didn't adequately prepare him for those first few months of fatherhood. Unlike medical interns, who take turns working 24-hour shifts, parents never get a night off.

"I was a ghost of myself for a little while," says Parikh, 38, of Walnut Creek, Calif., who has a 2½-year-old daughter and another baby due in August. "Even starting medical school and becoming a doctor wasn't like that. At home, when the baby screams, it's all on you."

Parikh says he's not surprised by new research suggesting that 14% of new dads develop postpartum depression — about the same rate seen in new moms. Yet even the most involved fathers often notice that their wives experience parenthood differently.

Many mothers find themselves consumed by the demands of their infants — not to mention their own health needs, as they recover from childbirth. In spite of their willingness to pitch in, Andrew says some dads tell him that they feel a bit peripheral — or even ignored.

"I wasn't the be-all and end-all for my wife anymore," says Josh Hornick, 51, a father of two from Amherst, Mass., who trains life coaches. "And that's as it should be. If I could choose whether to have my wife be more attentive to me or to our children, I would want her to be more attentive to my children. But it's still a shock."

By maintaining a little emotional distance — or a least a broader perspective — Andrew says fathers can help keep a household running. While moms focus on essentials such as breast-feeding, dads can keep the rest of the household from falling apart by doing housework, driving older kids to school or making sure that bills get paid.

"The dad has a bigger picture on things, instead of that laserlike focus on the baby," says Andrew, 39, whose children are 6, 8 and 13. "It works as a balance."

Matt Schneider, whose sons are 5 and almost 2, decided to stay home full time after his first son was born. He's enjoying fatherhood more than he ever imagined.

"It's fun to see to see them developing into real people and see their personalities grow and develop, and to see yourself in them," says Schneider, 35, a former schoolteacher who organizes the NYC Dads Group for active fathers with Somerfeld. "I was planning to stay out of the workforce a year. Now it's been almost five years."