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.”