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Parents trying to soothe a fussy infant might want to invest in a sling to carry their kid.
In human infants and mouse pups, bumping along in their mothers’ arms (or mouths) acts as the ultimate pacifier: crying stops, fidgety bodies go still and racing hearts thump more slowly, researchers report April 18 in Current Biology.
The calm may do more than save parents’ sanity. Carrying lulls mammalian babies into a trance that may improve their chances of survival, the study’s authors suggest. Lugging around wriggling banshees is tough work; mouse mothers on the move fare better with babies that relaxed when carried.
“The idea makes sense,” says pediatrician John Harrington of Eastern Virginia Medical School in Norfolk, who was not involved with the research. “If you’re running away from a predator, you’d want your child huddled against your chest and quiet.”
Scientists have previously tried to figure out what calms crying infants by studying parent diaries, but no one has examined the physiological effects of carrying babies, says study author Kumi Kuroda of Japan’s Riken Brain Science Institute.
So the team stuck heart rate monitors onto 12 healthy babies and took videos of the infants as their mothers sat holding them, carried them around a room or laid them in a crib. The babies immediately relaxed when their mothers picked them up and walked briskly.
Young mice also calmed down when their mothers carried them, Kuroda’s group found. When the team placed pups in a cup away from their mothers, mom mice nabbed the babies by the scruff of the neck and hauled them back to the nest. Clamped in their mothers’ mouths, pups tucked their legs to their bellies and stayed still; also, their heart rates slowed.
When researchers drugged the pups or numbed their necks so they couldn’t sense that they were being carried, the animals struggled in their mother’s grasp. Moms carrying squirming babies took longer to get back to the nest than moms holding calm pups did.
In the wild, a mother who has to make a quick getaway may have to leave hard-to-hold babies behind, Kuroda says. Mammalian infants may have adapted to relax when carried.
“It’s neat to draw parallels between different species, but we have to be careful when generalizing animal studies to humans,” cautions child development researcher Rebecca Pillai Riddell of York University in Toronto. Even behaviors that look the same may have different purposes, she says.
But Kuroda says that in both humans and mice, infants’ responses to being carried seem to be a way to collaborate with their mothers. “Carrying is necessary for the infant, so the infant cooperates by helping the mother.”
She also cautions that, as all parents know, carrying an infant is not a magic switch to turn off crying. But for parents pacing all night with babies in arms, Kuroda hopes the findings offer some solace: Babies who cry when parents ease them into the crib aren’t just trying to manipulate their moms and dads.
“This way of thinking is ignorant to the baby’s physiology,” she says. Infants’ heart rates shoot up as soon as their parents stop carrying them. “It’s just like a reflex mechanism,” Kuroda says.
When held in its mother’s arms and walked briskly around a room, an infant immediately calms down and stops crying. When the woman stops walking, the baby’s heart rate goes up (interval between heart beats shown in trace at bottom of video).
Current Biology, Esposito et al.