How Rocking a Baby Is Like Mouth-Carrying a Mouse Pup

18 April 2013 12:40 pm

(Clockwise From Top Right) Ken Yuel; Mehgan Murphy/Smithsonian National Zoo; Senthil Palaniappun; Manfred Ebert

Safe and secure. Curling up and calming down when in mom's mouth is a common reaction among (clockwise from top left) infant squirrels, redpandas, gray mouse lemurs, and lions.

A human mother rocking a baby in her arms and a cat carrying her kitten by the scruff of its neck have the same physiological effect on both young animals and probably stem from the same maternal instinct to protect their young. That's the conclusion of a new study, which for the first time has compared the physiological impact of maternal carrying behaviors across species. The findings may lead to better parenting techniques for people and possibly to new ways to detect developmental disorders early in life.

It's "really fascinating" work, says Oliver Bosch, a neurobiologist at the University of Regensburg in Germany, who was not involved in the research. "No one has looked at [this aspect] of maternal behavior in such detail."

Japanese neuroscientist Kumi Kuroda began the study in her own home. She noticed that carrying her newborn baby boy while walking had a rapid calming effect on him. Back in her lab at the RIKEN Brain Science Institute, near Tokyo, she found that picking up mouse pups by the scruff of the neck makes them passive and easy to handle. Kuroda wondered if the same physiological processes were driving both behaviors.

She and colleagues recorded pulse rates and observed the crying and squirming behavior of 12 infants, 1 to 6 months old, as each was left alone in a crib, held by its mother sitting in a chair, and carried as the mother walked around. In various durations and combinations of the three conditions, they found that the carried babies cried and squirmed the least and had the lowest pulse rates. Those left in the crib were the fussiest; holding the baby while sitting produced in-between results. What was particularly surprising, Kuroda says, was that when a mother started walking, the infant's pulse dropped, and the crying and squirming stopped within 2 to 3 seconds, not over several minutes.

Turning to animals, Kuroda and her colleagues noted that cats, squirrels, rodents, and some other mammals carry newborns by grasping skin at the nape of their necks in their mouths. While being carried, the young become passive and assume a compact posture, with hind legs drawn to the body. The team devised an experiment in which mother mice had to rescue young pups that were taken from the nest and placed in a cup. The scientists wired the pups with custom-made minielectrodes to track heart rates, recorded their ultrasonic vocalizations, which previous research suggests are akin to an infant's cries, and captured video images. Sure enough, when a mother grabbed a pup, its heart rate plunged, vocalizations stopped, and it became passive and assumed the compact posture. Researchers got the same effect by picking pups up by gently pinching the scruff of their necks.

The three responses—cessation of crying, passivity, and a decreased heart rate—"are very similar in mice and humans. We didn't expect that," Kuroda says.

With mice, the team was able to investigate the underlying mechanisms in experiments that could not be tried with human babies. They applied a local anesthesia to pups' necks and chemically blocked neural signals produced by motion. Depriving the pups of these sensory inputs inhibited the calming effect of being carried. Surgically removing parts of the brain and testing mice with neural abnormalities allowed the team to track the adoption of a compact pose to the cerebellar cortex, which, not surprisingly, is thought to be involved in controlling posture and balance. And they found that it took longer for mother mice to rescue pups that had been anesthetized, so that they could not curl up, or drugged, so that they would not become passive. "The evolutionary value and function [of the calming response] is quite clear," Kuroda says: A passive, quiet, compact newborn would be easier for a mother to carry away from danger. The group reports its work today in Current Biology.

Kuroda believes that the findings could lead to better parenting by giving parents a better understanding of how to soothe children. They could also prove useful in identifying abnormal child responses that might give an early indication of certain developmental disorders such as autism, in which children find it difficult to adjust their posture. The team is also continuing its experiments to try to identify the center in the brain that processes the infants' sensations of being held and carried and that triggers the behavioral and physiological effects.