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.