Dr. Kelly Lambert investigated the brain differences between two male mice species to study the biology of nurturing fathers. (Lambert Behavioral Neuroscience Laboratory/The Washington Post)

Unlike the male pundits, politicians and even financiers who’ve opined freely recently about what they consider “natural” roles for mothers and fathers, with mom at home and dad at work, behavioral neuroscientist Kelly Lambert’s methodical approach has led her to a much more complicated conclusion.

From her perch at Randolph-Macon College in rural Ashland, Va., Lambert has spent years designing elaborate experiments to test nurturing in both male and female rodents. She anesthetizes the animals, carefully removes their brains, firms the brains up with formalin, freezes them, then shaves them into slices thinner than a strand of human hair to study under a microscope.

What Lambert’s rodent brain slices are revealing is nothing short of revolutionary, challenging the loud pundits and long-held cultural views that only mothers are wired for nurture.

Lambert, one of a small but growing number of scientists who study the biology of father behavior, is finding that not just mothers experience surges of hormones associated with bonding and nurturing. The same hormones increase, though not to the same degree, in fathers.

Rat mothers are not the only ones whose brains become sharper, making them more efficient foragers and more courageous and level-headed than females without offspring. Lambert has found that the same is true of fathers’ brains. Fatherhood makes the male California deer mouse smarter, too.

Perhaps her most astonishing finding is that the mere presence of a pup is so powerful that it restructures even the brain of a virgin male common deer mouse, a member of what she calls a “sperm donor” species.

Unlike the monogamous, naturally nurturing male California deer mouse, the male common deer mouse typically goes his merry way after copulation, leaving all the caregiving to the female. But the longer the “sperm donor” common deer mouse is exposed to a pup, Lambert’s most recent studies show, the more his brain becomes wired to nurture.

“It’s time that matters: face time, pup time,” Lambert said. “That’s what’s so fascinating. We can take a male animal not predisposed to nurture and, with more time with pups, start seeing changes to the landscape of the brain.”

Could those brain changes turn a “naturally deadbeat dad,” as she calls them, into a nurturing, caregiving father?

“It’s certainly a possibility,” Lambert said.

What’s a father for?

Lambert’s work on paternal brains and behavior is shaking up some basic assumptions about parenting. So is other emerging research on fatherhood, including studies showing that expectant human dads produce higher levels of the hormone responsible for breastfeeding and that fathers’ testosterone levels drop after a baby is born.

“Her results are really quite stunning,” said Patricia Churchland, a professor at the University of California at San Diego who writes frequently about the intersection of brain science and philosophy. “What she’s finding about exposure to pups and the effect on the paternal brain is really very new and very, very important.”

For years, biological and social scientists largely ignored fathers in their research. Even Lambert had to be nudged by a student to consider them.

It wasn’t hard to justify paying less attention to fathers, Lambert said. In only 5 percent of mammal species do males become involved fathers. For the remaining 95 percent, males leave that entirely to the mother.

Scientific opinion, in turn, has shaped cultural attitudes that nature intended mothers not to be breadwinners, out in the world, but nurturers caring for children at home.

But if nature teaches anything, Lambert said, it’s that there is no one way to parent. Reptiles are what she calls “drive-by parents,” laying eggs and leaving offspring to fend for themselves.

But in 90 percent of bird species, both mothers and fathers care for their young, a product of monogamy and pair bonding for life, Lambert said.

Fathers play a critical nurturing role in other species, too. Giant water bug fathers carry more than 100 eggs on their backs for several weeks until they hatch. Father seahorses carry fertilized eggs in a pouch, as if pregnant. “There’s even a species of bat where the father lactates,” Erica Glasper, Lambert’s former student and now a neuroscientist at the University of Maryland, said of the Dayak fruit bat in Southeast Asia.

And among the 5 percent of male mammals who become active caretakers — which include humans — the level of paternal involvement varies. Father owl monkeys and titi monkeys, or marmosets, actually spend more time carrying and feeding babies than the mothers do. So do male prairie voles.

Sue Carter, a behavioral neurobiologist at the University of North Carolina at Chapel Hill, has spent her career studying pairie voles: “Sometimes they midwife the birth. They grab the baby and start licking it before it’s even out of the membrane it’s born in.”

Carter’s studies, like Lambert’s, have found that virgin male prairie voles, when exposed to pups, experience a surge of the hormones oxytocin and vasopressin, the so-called “love” hormones that encourage social bonding, much as mothers do.

“We weren’t expecting that,” Carter said. “But we found that if you put a male prairie vole with a baby, he’s going to start taking care of it immediately 80 percent of the time, even when it’s not his and he hasn’t seen it before. That’s true in humans as well. A baby has very powerful physiological properties.”

The daddy brain

Lambert’s lab is small, with two closet-size rooms for rat and mouse cages, a long table and a small circular maze for her experiments, a counter with microscopes, and shelves stacked with carefully preserved slices of rat and mouse brains. A poster for the movie “Ratatouille” decorates one wall.

An enormous freezer stands at the far end of the lab, stocked with almond-size rodent brains. It is next to the cryostat machine that makes slices no thicker than 40 microns, the diameter of about eight red blood cells. “Sort of like an Arby’s slicer,” Lambert said.

Lambert is fascinated by what she calls the brain’s “neuroplasticity.” Until recently, she said, scientists thought the brain never changed, that the neurons you were born with were the only neurons you’d ever have. “So you had to take care of them,” she said, laughing.

But new work by neuroscientists is showing that environment and experience can physically shape the brain, she said, which in turn can influence behavior in unexpected ways.

Lambert and her research partner Craig Kinsey, a neuroscientist at the University of Richmond, were the first to report that motherhood made rats smarter, more emotionally resilient and physically agile. That groundbreaking finding was not only published in scientific journals but also became fodder for jokes on late-night talk shows.

Then one day, a student asked Lambert, “But what about the dads?”

Lambert was embarrassed that she hadn’t thought to ask the question herself. She, her students and colleagues soon began experimenting with the California deer mouse and the common deer mouse.

She pulled out photomicrographs of the brains, comparing the two species. The brains of the more paternal California deer mice have much thicker bundles of vasopressin and oxytocin cell bodies and fibers that function “like an interstate highway” to more easily move the bonding “love” hormones through the hypothalamus, the reward and motivation center of the brain.

The brains of the uninterested common deer mouse fathers, in contrast, are wired to fear pups, she said.

But when Lambert began exposing virgin males of both species to pups, that’s when things got really interesting.

The hippocampus — the area of the brain involved in learning, deciding what to pay attention to and memory — began producing new cells when virgin males were exposed to pups. The vasopressin fibers that move love hormones began to grow thicker even in the virgin non-nurturing common deer mice.

“That’s what’s so exciting,” Lambert said. “We know moms’ brains are wired from the start for nurture. But this shows that dads just may need more time to come on line and be paternal.”

Adapt or die

As Lambert packed up the brain slices, she contemplated the ongoing controversy about what’s “natural” for mothers and fathers.

Lambert’s own mother stayed home, but, with only a high school education, she felt trapped in an unhappy marriage. Lambert worried about what was best for her two daughters when they were young, but having loving babysitters, a husband who shared caregiving responsibilities and a flexible job made her decision easier.

“I worried what I would be preparing them for if I were home full time — I’m not the best homemaker — and I wanted them to learn to be self-sufficient,” she said. “So I took them to conferences with me, to my lab. I use what I do to prepare them for the world.”

If nature teaches anything, Lambert said, it’s that those species flexible enough to adapt to changing environments are the ones that survive.

“Life is messy. Every role is not carved in stone,” she said. “If we say every family should be one particular way, if anything changes you run the risk of becoming extinct.”