Before and after Sarah Blaffer Hrdy met her infant grandson for the first time, she spit into a vial. Two weeks later, when her husband arrived to meet the newborn, she had him do the same thing.

Lab tests later revealed that Hrdy’s levels of a brain chemical called oxytocin spiked by 63 percent that evening. Her husband’s spit showed a 26 percent jump after his initial meeting, but several days later, it also increased to 63 percent.

“There was no difference in the end result between me and my husband, it just took him a little more exposure to his grandson to get there,” she says. Now a professor emerita at the University of California, Davis, the esteemed anthropologist has written extensively about the science of human maternity. (Here are seven things you may not have known about the dark history of Mother’s Day.)

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"All mammalian females have maternal responses, or ‘instincts,’ but this does not, as is often assumed, mean that every mother who gives birth is automatically [ready] to nurture her offspring," says Hrdy. "Rather, gestational hormones prime mothers to respond to stimuli from her infant, and after birth, step by step, she is responding to cues."

That’s not only the case for women who physically give birth: Hrdy and her husband are grandparents, but it’s not at all surprising to her that they both experienced similar bumps in oxytocin, a hormone associated with maternal bonding. And as far as she's concerned, mothers who give birth and mothers who adopt should both be considered "biological mothers," based on the changes that happen in their bodies when they become parents.

"Both undergo similar neuroendocrinological transformations—even in the absence of giving birth or of lactation," says Hrdy.

View Images Transgender dad Liam Johnson holds his one-year-old daughter Aspen on her birthday. Liam Johnson, 20, and Racquelle Trammell, 30, made the tough decision to halt transitioning to have a baby. Liam identifies as a man, but he is still able to get pregnant and give birth naturally. Racquelle had to stop taking estrogen to ensure her sperm could fertilise an egg. Photograph by JJ Fabre, Barcroft Media via Getty

Hrdy’s work speaks to the many shades of motherhood that are possible in humans. In many societies, who gets to be a mother—and who wants to be one—looks different today than it did just a few decades ago. More than ever, women are delaying when and how many children they have—or living happily without any offspring at all. Same-sex parents are increasingly accepted. And earlier this year, a transgender woman became the first to breastfeed her baby.

While everyone has a unique idea of what it means to be a mom, science can tell us a lot about why moms of all kinds behave the way they do.

Of Mice and Cannibal Virgins

Chemically speaking, one of the most powerful drivers of maternal behavior seems to be the famous “feel good” hormone, oxytocin. This complex neuropeptide plays a variety of roles in mammal reproduction, including pair bonding, womb contraction, and release of breast milk.

“An orgasm, eye contact, hugs, soft touch—all these things release oxytocin,” says Bianca J. Marlin, a postdoctoral researcher at Columbia University’s department of neuroscience.

In 2015, Marlin co-authored a study in the journal Nature on the effect oxytocin had on mice. When female virgin lab mice heard the cries of young mice pups, they ignored them, or in some cases, cannibalized them. Mice mothers, however, would seek out the source of the cry to retrieve and care for the pup. (Check out these fierce animal moms that go to extremes for their young.)

Then, they injected the virgin mice with oxytocin.

“When we added oxytocin to cannibalizing female virgins, they stopped cannibalizing and learned to retrieve pups the same way moms did,” Marlin says. “We changed their behavior from snacking on pups to actually taking care of pups.”

The team then looked at the auditory part of the brains in mice that heard the pups’ calls. For virgin mice without any added hormones, the auditory brain cells were firing, but in random ways that didn’t signal for a response.

“When we added oxytocin, we saw that the neurons started to change their conversation,” Marlin says. “Not only did we change the behavior, but we also changed the neural signature of the pup call in the virgins. They acted and responded like mothers.”

When Marlin’s team injected male mice with oxytocin, they found that the males took longer than virgin females to alter their behavior: “Can males take care of pups? Yes, but the timescale was a lot longer compared to the virgins. Virgins learned to retrieve in 12 hours, males learned to retrieve in three to five days.”

So, does that mean female brains are fundamentally wired to be maternal? Not quite, argues Daphna Joel, a neuroscientist from the University of Tel-Aviv. For starters, mice are not men, and it’s important to understand how the human brain specifically responds to hormonal shifts.

In a 2015 paper published in the Proceedings of the National Academy of Sciences, Joel and her colleagues examined whether science could discern any difference between the male and female brains in humans. For instance, were the parts of the brain more commonly associated with emotion and communication—qualities women are stereotypically thought to be better at—somehow different or more developed in female brains?

“We found that this is not the case,” Joel says. “Rather, the brain of most humans is comprised of a unique mosaic of features, some in the form more common in women compared to men, and some in the form more common in men compared to women.” Some mosaics are even common in both female and male brains, the study found.

View Images A mother holds her baby in her arms. Photograph by JOEL SARTORE, Nat Geo Image Collection

Back to Our Roots

Beyond pure biology, social structures have also played large roles in our modern understanding of maternity. To learn how our environment has influenced human parental care, anthropologists often look to primates, our closest evolutionary cousins, as well as to modern-day tribes of hunter-gatherers.

During the 1980s and ‘90s, University of Utah anthropologist Kristen Hawkes and her research colleagues spent time with the Hadza, a tribe of hunter-gatherers in Tanzania.

“What emerged out of our observations was how important economically the old ladies were,” she says. “Who woulda’ thought of that!”

Her team’s findings support the so-called “grandmother hypothesis,” which suggests that grandmothers played an important role in human evolution. Unlike most other primates, humans are vulnerable well after they wean from their mothers. Young kids can’t easily forage for their own food, and if a mother is caring for new children, grandmothers would have effectively subsidized child care, Hawkes says.

The effect isn’t limited to grandmothers. Sisters and daughters also pitch in with child rearing in these communities. In these instances, the women are providing valuable benefits to the community.

“The hypothesis has never been about babysitting,” Hawkes says. “It's about economic productivity.”

View Images Musician Kurt Kipapa, a father of 10, holds his smallest child. Photograph by JODI COBB, Nat Geo Image Collection

Modern Families

Today, that same kind of bonus parental care from relatives can allow some working mothers to return to their jobs. But increasingly, women in developed countries are opting to delay or even skip motherhood altogether.

“Seeking success is a biological motivation in humans,” says University of California, Santa Barbara, anthropologist Lisa McAllister. “We have evolved a drive to seek success. The more successful individuals in any society are the ones who would traditionally leave behind more offspring and therefore get to be more represented in the next generation.”

For several years, McAllister lived with a hunter-gatherer community called the Tsimané (pronounced Chi-man-EE) in Bolivia. There, she noted that women commonly gained higher status based on the number of healthy children they could produce.

Notably, like the Hadza women in Tanzania, Tsimané women had few options beyond marrying and becoming mothers. On average, women had their first baby at 18 and eventually had as many as nine children. By contrast, 2017 data from the U.S. Centers for Disease Control and Prevention showed that women ages 30 to 34 were having the most children in the U.S., and the number of babies per mom isn’t that high.

“In our society, we don't measure a woman's worth as much by a woman's ability to mother or have children anymore. We often measure it by, What kind of job does she have? [or] Does she drive a nice car?” says McAllister.

“Here, you get a lot of men and women who just don't want kids. It's just not on their radar,” she adds. “You get a ton of men and women who never get ‘baby fever’ or this maternal instinct when they happen to be around a child. It's just that the internal psychology for how we measure success has shifted in our society.”

Still, it seems humans as a species remain biologically driven to form bonds with infants placed in their care—no matter their gender or social status.