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Breast-feeding makes new mothers mama bears

Everyone knows not to get between a bear and her cubs, but if mama bears used bottles maybe they’d be a little more mellow.

A study published in the September issue of Psychological Science found that nursing mothers are roughly twice as aggressive as bottle-feeding moms and women without children when confronted by a threat.

“Maternal defense does not involve nursing mothers going out and looking for bar fights, but when they have a helpless baby, they’re more likely to defend themselves when the fight comes to them,” said Jennifer Hahn-Holbrook, a postdoctoral fellow at UCLA’s Department of Health Psychology.

Similar aggression has been observed in lactating monkeys, rats, mice, deer, hamsters, lions, prairie voles and sheep. When reading a book about how vicious predators become loveable moms, Hahn-Holbrook noticed that many of our preconceptions about mothers being quite docile were actually wrong for other species. She wondered if the same thing would hold true for humans.

She and other researchers recruited 18 nursing mothers, 17 women who were feeding formula to their babies and 20 non-mothers. The women were told they’d be playing a competitive computer game against a research assistant posing as a rude and aggressive study participant. When the women “won” a round of the game, each got to choose how long and loudly they would blast their opponent with an annoying sound.

After accounting for other differences, the researchers found that breast-feeding mothers delivered sound blasts to the rude opponent that were more than twice as loud and long as those administered by non-mothers and nearly twice as loud and long as those by bottle-feeding mothers.

The study suggests that lactation—and not just motherhood in general—kicks maternal protection into overdrive. During the confrontations, for instance, nursing moms exhibited lower blood pressure levels than the other two groups of women. That can actually dampen fear and stress responses and give them a little extra moxie to defend their offspring, the study concluded.

“We interpreted this as breastfeeding being nature’s way of helping moms calmly but effectively deal with potential threats,” Hahn-Holbrook said. But in a day and age when we’re not exactly likely to be chased by saber-toothed tigers, does the aggression factor add any benefit?

“That’s completely beyond the scope of our study, but I’m sure there are plenty of contexts in which moms could use a little extra help in that regard,” she said. “This wouldn’t just come up in terms of predators but might also encourage a mom to run back into a burning building and save an infant. I definitely think that moms generally are inspired to do that, but I wonder if lactation would just give moms a little extra push and a little extra courage.”