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A lion cub goes slack in its mother's grip (Photo by  / )

Some of us might vaguely recall the sense of being carried, or lulled to sleep in a parent’s arms. Even if we don’t, we know the motion is synonymous with comfort and security. Recently, researchers published that tapped into the physiological effects of carrying on babies, and have linked it to the protective maternal instincts that mothers in the wild display when they scoop up their offspring by the scruff of the neck.  

Those images of parenthood litter wildlife programs, and every time the mother lion—or whatever mammal is the focus of the show—lifts her offspring, its limbs goes heavy and limp, as if its overbearing parent has sapped every scrap of willpower it had left. But that scenario exists in another place too, and that’s what gave lead researcher , a Japanese neuroscientist at the , near Tokyo, the necessary clue.

Kuroda knew that lab mice also go limp and calm when picked up at the nape of the neck, a quirk long-appreciated by scientists, since it lets them effortlessly transfer mice from one place to another. She explained to that she first latched onto the idea when she noticed the response of her own lab mice. “When I picked the pups up at the back skin very softly and swiftly as mouse mothers did, they immediately stopped moving and became compact. They appeared relaxed, but not totally floppy, similar to a human baby.” Kuroda was curious about the potential links between that observation, and the way human babies respond to their parents’ soothing motions.

By recording pulse rates from human babies left alone in a crib, held within a seated parent’s arms, and cradled in an enfolding embrace as the parent walked about, Kuroda was able to show that not only did crying and wriggling decline—most noticeably when parents walked around with their infants—but pulse rates dropped as well. An infant left alone in a crib had the highest pulse rate, babies held on a chair had a reduced rate, but those carried around and lulled by a soothing motion had the calmest pulse. She also found that similarly to a mouse pup or a lion cub, human babies almost automatically calm down when their mothers step in.

Kuroda associated the calming effect with the similar passiveness that overcomes mice pups, and other creatures in the wild—especially after she mirrored the human baby study with a comparable study in mice, in which mothers had to ‘rescue’ their pups from nearby. The researchers found, through tiny electrodes wired to the pups, that upon contact with their mothers, their hearts noticeably slowed. “No one has looked at [this aspect] of maternal behavior in such detail,” said , a neurobiologist at the in Germany, who was not an author on the study, to .

So maternal contact makes babies calmer—but what’s in it for the mother? “This infant response reduces the maternal burden of carrying and is beneficial for both the mother and the infant,” Kuroda told . The research suggests we might learn a thing or two from creatures that willingly hoist up their young: holding and comforting children appears to go further toward calming them down, and uses up less of the mother’s energy, than does leaving babies to ‘’. Evolutionarily speaking, a compliant newborn after all is easier to carry to safety and protect.