How Men's Bodies Change in Fatherhood


Here at Kulala, we talk to a lot of mothers. The majority of our followers are mothers, as are a majority of our app users.

It makes sense. Mothers are typically the ones biologically tied to their children in the beginning: we’re the ones who are usually pregnant, and therefore the ones who give birth.

If we’re breastfeeding, that extends our physical ties to our children for several months or even years. Birth mothers’ bodies are irrevocably altered by childbirth and its aftermath. So it makes sense we’re the ones more attuned to all our newborns’ needs, including their sleep needs and sleep patterns.

But did you know that fathers’ bodies also change when they become parents?

And no, we’re not talking about “dad bod.” At least, not in the Instagram-meme sense.

There’s a common myth that women are more “suited” for parenthood because of this biological connection to their children. That a mother’s instincts are stronger than that of a father’s.

But the truth is, a new parent is a new parent. Parenting is not necessarily instinctual for anyone. It’s something we learn to do fast, because we have to.

The changes women undergo to become mothers are obvious, when they’re the ones giving birth: their bodies change drastically to accommodate a growing baby and prepare them for childbirth. Mothers’ bodies then change again once that baby is born and her body prepares to feed and nurture the baby.

Changes to men’s bodies and hormones are not that drastic. But that doesn’t mean women are the only ones to undergo biological changes. Science is just beginning to find that men, too, experience both hormonal and brain changes when they become parents.

Let’s delve into the ways men’s bodies change to become fathers.

new father

A Dip in Testosterone

Testosterone is considered a stereotypically male hormone. It plays a key role in male fetal development and puberty. It’s one of the things that drive men to find partners; in fact, there are studies that suggest that men with higher levels of testosterone tend to be more attractive to women.

Yet once one has a partner and starts a family, men no longer need that drive. Some experts believe men have evolved to lose some of that testosterone they no longer need.

The New York Times documents how a 2011 study followed a group of 624 single, childless men in the Philippines from age 21 to 26. The anthropologist who pioneered the study, Dr. Lee Gettler, found that of those men, the segment of them who became fathers during that time—465 of them—experienced more significant drops in testosterone than those who remained childless.

This is not the only study of its kind. There have been several more who’ve uncovered similar results, discovering that this reduction in testosterone can happen just before or after the birth of the man’s first child. There’s no clear answer as to what exactly prompts the drop. But Dr. Gettler said that in his own research, he discovered that the more dramatic the drop, the bigger effect it seems to have on a man’s caregiving abilities.

“We found that if brand-new fathers had lower testosterone the day after their babies were born,” said Dr. Gettler, “they did more caregiving and baby-related household tasks months later.”

There are other studies that suggest that the lower a man’s testosterone, the more likely he is to release oxytocin and dopamine when interacting with his child. These are important reward and bonding hormones. This means that caring and spending time with one’s child releases a neurochemical reward—feelings of happiness, contentment, and warmth.


A Change in Brain Chemistry 

Testosterone isn’t the only thing to change for new fathers. Studies suggest men’s brains also undergo structural changes to prepare them for the skills of parenting.

In a study in 2014, developmental neuroscientist at the University of Denver named Pilyoung Kim put 16 new fathers into an M.R.I. machine at two different intervals: once between the first 2 to 4 weeks of their babies’ lives, and once between 12 and 16 weeks. Dr. Kim found brain changes that previously had only been seen in new moms. Namely, that the areas of the brain linked to attachment, nurturing, empathy and the ability to interpret and react to a baby’s behavior were more elevated between 12 and 16 weeks than they were between 2 and 4 weeks.

Dr. Kim surmises that since men don’t experience the hormonal surges that women do during pregnancy and childbirth, these brain changes might be even more significant. “The anatomical changes in the brain may support fathers’ gradual learning experience over many months,” Dr. Kim suggests.

This isn’t to say that mothers’ and fathers’ brains behave exactly the same. Another study, done in 2012 by neuroscientists at Bar-Ilan University in Israel, show that different areas of the brain light up depending on whether or not the participant was a mother or father. For mothers, areas which enable them to care, nurture, and detect risk were most active. For fathers, the most active regions were where thought, goal orientation, and planning and problem solving reside.

This all suggests that fathers adapt to parenthood in similar but different ways than mothers do. This means that while there can be a difference in role for each parent, these changes can create equally strong attachments.

Another study to support this, conducted in 2010, found that for women, peaks in oxytocin and dopamine occurred when they spent time nurturing their children. Men had those same peaks in oxytocin and dopamine as well—but not when performing nurturing duties. For men, these peaks happened when they took part in rough-and-tumble play with their children. 

Children need both—nurturing and playtime—for their development. Isn’t it cool that each parent is biologically designed to provide a different kind?

clown fish

Gender Roles and Evolution  

Have men always shown these changes to their brains and bodies when becoming fathers, or is it something that’s relatively new, evolution-wise?

We don’t know.

But one interesting example of doting male caregivers can be found in an unexpected place: under the sea.

The New York Times documented how, in much of the animal world, from mammals to birds to reptiles, the females are the primary caregivers over their young. Yet there are several species of fish for which the opposite is true.

For some species, the female fish dumps her eggs in a location then swims away. And then the male fish fertilizes them—and keeps watch over them until they’re hatched.

Why do male fish do this, instead of female fish?

Researchers aren’t sure. But there are some things they have discovered.

An important distinction in males who exhibit this behavior is that in all species who do this, the fish utilize external fertilization. This means sperm and egg meet in the open. This makes sense, researchers say, because male fish can then be certain they are the ones to fertilize the eggs, and ensure that the eggs they are watching over are their own spawn.

(Not that this method is foolproof; other studies have tested paternity in fish, and found male fish keeping watch over nests where only half the young shared their paternity!)

While scientists still aren’t sure why some fish are the caring fathers and absent mothers they are, the fact that this pattern exists is significant. It shows that “there’s no evolutionary rule that moms have to be the main providers—a lesson we can apply to our own species,” says Dr. Benun Sutton, an evolutionary biologist at the City University of New York.

pregnant seahorse

So what does this mean for humans?

It’s 2021; hopefully, by now, most of us have jettisoned the cliché ideas of the pregnant, barefoot domesticated mom and the absent working father.  

Of course, there are some things cisgender male fathers still can’t do: become pregnant, give birth, and breastfeed, namely (but wouldn’t it be nice if we could pass at least one of those things on to them?).

However, as science shows, this doesn’t mean women are more “suited” to the role of parent than are men.

Being a new parent is a steep learning curve for everyone—men, women, and everyone on the gender spectrum alike! From feeding to sleeping to development, new parents have a lot to learn.

That’s one of the reasons Kulala exists—newborn babies’ sleep patterns can feel like chaos, and there’s so much conflicting information out there on what the best thing is to do. Moms don’t instinctually know this stuff any better than dads do. That’s why we rely on science and research to give us the facts on baby sleep and health—and pass those along to you.

So Dads, don’t worry that your female partner will become a “naturally” better parent than you—there’s simply no basis in science for this to be true.

After all gender roles have evolved—and evolutionary roles can, too.

And who knows—maybe someday we’ll evolve to be more like fish, or even seahorses and their close relatives, sea dragons. They are the only species throughout the animal kingdom where the males carry and give birth to their young.

Until then, we can only dream!

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