New mothers quickly learn that babies quiet down when carried and rocked. Now researchers say that this calming response is actually a coordinated set of reactions, involving the nervous, motor and cardiac systems.
Dr. Kumi O. Kuroda, a neurobiologist at the Riken Brain Science Institute in Japan, led a team that used electrocardiogram measurements to monitor the heart rates of babies and mice after they were picked up and carried. Their heart rates slowed almost immediately.
"It's very difficult for adults to relax so quickly," said Dr. Kuroda, whose study appears in the journal Current Biology. "I think it's specific to infant physiology."
In the case of the mouse pups, it took only one second for the heart rate to drop. In human babies, it took about three seconds.
The researchers worked with babies under 6 months; the response was stronger in those 3 months and younger.
Both babies and mouse pups also stopped moving after they were carried, and the mice stopped emitting ultrasonic cries.
The same sort of reaction has been seen, anecdotally, in many other mammals.
"Lions sometimes carry cubs by the mouth, and it's known that these infants look very limp and relaxed, with their eyes closed," Dr. Kuroda said. "But nobody measured the infant response until now."
By the way, she added, the mother is not the only one who can have this calming effect.
"We actually also did some preliminary studies with fathers and grandmothers," she said. "And basically they can have the same effect." SINDYA N. BHANOO
This article originally appeared in The New York Times.