Last year I met my four-month-old nephew, Landon, for the first time. During the weekend I spent visiting him in San Diego, my inner science nerd often got the best of me. I would find myself probing my nephew’s foot reflexes and offering unsolicited explanations for why his toes curled this way or that, only to be met by my wife’s disapproving looks and the new parents’ blank stares. Soon enough I dropped the shoptalk in favor of baby talk.

Having spent my postdoctoral career in neuroscience, I have seen how important early experiences are for a baby animal’s health. In the first few days after birth, babies’ brains are like sponges soaking up their sensory environment. What to me seemed like inconsequential sights or smells had markedly different impacts on the impressionable newborns, shaping their brains as they tried to make sense of the unfamiliar world around them. But as astonishing as a baby’s brain is, on this family visit what struck me was the redevelopment of my 26-year-old brother-in-law.

In my eyes, Jack has always been my wife’s kid brother. When I first met him, Jack was a tall, lanky, wet-behind-the-ears 19-year-old kid who enlisted in the U.S. Navy right after graduating high school. As a two-tour Iraq War veteran, he saw more of the world in six years than most of us ever will, and he frequently regaled us with his large repertoire of crazy sailor stories. But in just a few months’ time, Jack had managed to permanently ground his sea legs and become a hands-on first-time father.

Even having served in Iraq, Jack will no doubt find raising Landon the biggest challenge he has ever faced. Whether he knows it or not—and whether he likes it or not—things are about to change drastically for him. Not only will Jack be financially and legally responsible for Landon for the next couple of decades, he will form and sustain an unbreakable emotional bond with his son. In the early days after birth, changes occur in the brains of both the dad and the baby. We can now see the mark left on a baby’s brain when a father is not around. When he sticks around, a father gains a cognitive edge by virtue of tending to his children. Although many of the findings are still preliminary, scientists are beginning to sketch a neural portrait of the father-child bond.

By the end of the weekend trip, I saw glimpses that Jack was beginning to accept his new identity. After struggling for several weeks to secure Landon’s car seat in the back of his souped-up Mazda RX-8, Jack finally broke down and traded it in for a sensible sedan that will let him transport the little guy more easily. In the cellular networks inside his head, a transformation was well under way.

Figuring Out Fatherhood
To unearth the roots of fatherly feelings, scientists had first to figure out where to look. On the surface, the intangible link of fatherhood appears nothing like a mother’s connection to her child. During the nine months of a pregnancy, oxytocin and other hormones course through a woman’s body, forging a biochemical bond between her and her baby. Even their heartbeats can synchronize while the child is in the womb. Following birth, a mother’s lactation serves as a natural food source for the newborn.

What a dad offers is less obvious. Sure, men help out during conception, but afterward we are not exactly crucial to a child’s survival. Nevertheless, research shows that the father-child bond makes a major contribution. If a father leaves his children to be raised solely by their mother, they are more likely to suffer a whole host of problems later in life, including emotional troubles, aggression and addiction.

The numbers are actually quite staggering. In 2008 about one in four children lived with only their mothers, whereas only 4 percent lived with just their fathers. A third of the approximately 12 million single-parent families in the U.S. live below the poverty level. Perhaps as a product of struggling to make ends meet, single parents are at a higher risk of raising children with lower academic achievement and self-esteem, as well as difficulties forming social relationships. Until recently, large population surveys were the most effective tool for investigating a father’s contribution to the upbringing of a child. But new clues are emerging from deep inside the brain. Neurosientists are now revealing one critical part of the puzzle—the biological mechanisms that connect a father and his child.

Take the sound of a baby’s cry. In 2003 psychiatrist Erich Seifritz of the University of Basel in Switzerland and a team used functional MRI to show that just as in mothers, certain areas in the brains of dads became activated with a signature pattern unlike that of nonparents who heard the same sounds. Although the team could not pinpoint exactly what had changed, the brains of both parents appeared to have adapted to recognize the sounds critical to a baby’s comfort and survival.

Brains, after all, are not static. Neurons constantly rewire themselves in response to new experiences and changes in our surroundings. Additional neurons can also materialize, a process called neurogenesis. The mechanisms of neurogenesis are not fully understood, but scientists have connected extra brain-cell growth with learning new things.

Brainpower Boost
Building off these observations, Gloria K. Mak and Samuel Weiss, two neuroscientists at the University of Calgary in Alberta, designed a series of experiments to figure out how offspring might reshape a father’s brain. In results published in 2010 Mak and Weiss showed that the brains of mouse dads do not simply rewire, they also sprout additional neurons. The cells form brand-new connection pathways, or circuits, in the days following the birth of the pups. In the olfactory bulb, new neurons developed that responded specifically to the smells of his pups. Another set of neurons grew in the father’s hippocampus, a crucial memory center in the brain, which presumably helped to consolidate the smell of his pups into long-term memory.

The mouse father only gained the extra brain cells if he stayed in the nest, though. If he was removed on the day of his pups’ birth, his brain remained the same. As Weiss sees it, this study demonstrates that the experience “is not just changing what exists [in the brain] but developing something brand-new to serve the relationship.”

In mammals, neurons located in the nose use special odor receptors to detect scents and shuttle the information to the olfactory bulb, which is the integration center for our sense of smell. Simply sniffing his pups, though, was not enough to cause new neurons to spring into existence. When Mak and Weiss placed a mesh screen across a cage to separate a dad from his pups, they saw no additional brain cells appear. This test and other similar ones indicate that neither the birth of the new offspring nor their smells alone change a dad’s brain, Weiss says. Rather the hands-on experience of being a father brings about the extra dose of brain cells. Physical contact with the pups, coupled with the experience of their smells, is what makes the neurons grow, the researchers suggest.

But are pups different from pals? A few weeks’ separation is usually enough for adult mice to forget all about their former cage mates. Mak and Weiss demonstrated that the parent-child bond indeed stands out. These new neurons formed their own brain circuits, thus helping to form long-term memories and therefore a lasting bond. With distinct memory pathways forged, the mouse fathers easily recognized their offspring by smell even after they had been separated for three weeks. “We still struggle to understand why new neurons are born in the brains of all mammals, including humans,” Weiss says. “It certainly appears as though one of the main functions may be to adapt to change, form new circuits, and, in this case, [create] what we call a ‘social memory’ between the father and his offspring.”

Like Mother, Like Father
To solidify social memories, the brain relies on hormones to control the connection of those newly forged neurons. Mak and Weiss found that the father’s ability to form new brain cells is at the mercy of a hormone called prolactin—the same hormone responsible for milk production in new mothers. When they disrupted the brain’s ability to produce prolactin, they discovered that fathers did not form any offspring-specific brain cells.

Also parallel to how babies and mothers bond, many studies have shown that human fathers with higher levels of oxytocin (the “love hormone”) exhibit stronger paternal instincts and motivation in the first months of their child’s life. In findings published last December, Atsuko Saito of the University of Tokyo and Katsuki Nakamura of Kyoto University pushed that observation further by studying the food-sharing habits of marmoset father monkeys. Marmoset dads readily feed their youngsters during their first four months. But after six months, the fathers begin to ignore their now adolescent offspring and keep their food for themselves. To test what drives the change in behavior, the team infused oxytocin into the brains of marmoset fathers. Regardless of the dose Saito and Nakamura administered—and with no change in the fathers’ appetites—the male marmosets were more likely to indulge their offspring’s clamoring for food.

Because prolactin and oxytocin are both heavily tied to social interaction, their involvement in the father-child bond may not be surprising. Nevertheless, new data are providing a broader prospective. As psychologist Elizabeth Gould of Princeton University and her colleagues pointed out in an October 2010 review article, hormones relating to sex and stress have now also been linked to paternal behaviors.

Gould has published numerous papers detailing the connection between the human stress hormone cortisol (corticosterone in rodents) and structural changes in the brain. Although stress usually has a negative connotation, Gould and her colleagues have used experiments with rodents to show that it can be both good and bad for the brain, depending largely on context. For example, bad stressors, such as when animals are briefly submersed in cold water or exposed to a natural predator, have negative effects on the brain, reducing the brain’s ability to generate new neurons and rewire itself. But as Gould and her colleagues published last July, stressors such as exercise and sex, which also boost corticosterone levels, actually stimulate new brain cells to grow. The challenges of fatherhood may well fall into the category of good stress.

Although male sex hormones seem to be deeply intertwined with the birth of offspring, other species show that the hormones have inconsistent effects. In certain rodents and fish, fathers produce excess testosterone. They take good care of their young and simultaneously maintain aggressive tendencies that help them to, say, defend the nest against predators. In tropical birds and primates, however, elevated testosterone levels get in the way of good parenting. Human fathers with excess amounts of testosterone may exhibit less sympathy for and desire to respond to a crying baby.

These studies make a strong case for hormones as the brokers of certain paternal behaviors. As Weiss points out, this line of research is “adding a new dimension to the impact that hormones can have on adult brain-cell production.”

A Critical Link
Whereas an arsenal of hormones cultivates a father’s brain in the presence of a baby, a child may actually be born ready to bond. To test this idea, a research team led by neurobiologist Katharina Braun of Otto von Guericke University of Magdeburg in Germany turned to a rodent with a remarkably familiar nest structure. Degu rat mothers and fathers split the parenting duties. Similar to human fathers, degu dads spend the early days of their pups’ lives helping with basic care, huddling over them to keep them warm and bathing them with gentle licks when needed. As the pups get older, the fathers begin to play with their toddler offspring by chasing them, romping and roughhousing around the cage.

Braun and her team reasoned that degu nests lacking fathers would create a social and emotional void for the offspring, just as a missing dad would affect the dynamics of a human family. Indeed, they found that if a rodent father remained in the nest with his pups, his babies’ brains developed normally. But if the father was removed from the nest shortly after the birth of his pups, they observed in two regions of the brain that the newborns developed fewer synapses, the short chemical junctions that allow brain cells to communicate with one another.

At a stage of development when most of the brain should be burgeoning with new connections, the pups raised without a father had deficits in the orbitofrontal cortex and the somatosensory cortex. The orbitofrontal cortex is part of the prefrontal cortex, which regulates decision making, reward and emotion. And although it is difficult to extrapolate from rodent studies to effects in humans, it is worth noting that faulty synapses and processing problems in this locale might well explain why we see some kids who grow up without a dad wrestle with occasionally serious behavioral problems.

Taken together, these rat studies suggest a model for why fathers matter. A newborn emerges into the world having spent weeks afloat in amniotic fluid, its senses somewhat deprived and its somatosensory cortex ripe for change. But instead of flourishing in the early postnatal days, the synapses of the somatosensory cortex wither away when degus are raised without a father. As a result, the newborns may not process touch as well as they should, which could lead to a number of other developmental problems, such as metabolism issues and irregular hormone production.

A father’s brain, it seems, is significantly and beautifully intertwined with his offspring’s. “Having two parents is one thing,” Weiss points out, “but having effective relationships between parents and offspring is yet something else. It’s actually the effectiveness of the relationships [that matters].”

Perhaps my nephew, bolstered by a healthy set of brain connections that formed in response to the simple fact of Jack’s touch, has already collected the tools he will need to fend off behavioral and emotional challenges as he grows older. And while I can’t exactly probe Jack’s brain to see if he is sprouting neurons, I noticed an undeniable change in his focus as his new bond took hold. Small movements and sounds from Landon that went unnoticed by most people mysteriously captured Jack’s attention. It is comforting to think that a small set of neurons might be tucked away in Jack’s head solely dedicated to his son.