Scientists in Japan have found that the heart rate of crying babies slows down when they are in put in the arms of their mothers and carried about – but not if the mother remains still, the study found.
Kumi Kuroda and colleagues at RIKEN Brain Science Institute in Saitama discovered the same behavioural trait in young mouse pups and postulated that the phenomenon in human babies stems from an ancient evolutionary response to being handled.
“From humans to mice, mammalian infants become calm and relaxed when they are carried by their mother. This infant response reduces the maternal burden of carrying and is beneficial for both the mother and the infant,” Dr Kuroda said.
The study, published in the journal Current Biology, showed that in addition to slowing the heart rate, carrying babies around caused the involuntary activation of movement detectors in the nervous system which are controlled by the cerebellum region of the brain.
Dr Kuroda made the discovery after noticing that mouse pups became quiet and relaxed when they were gripped and moved gently by their collar, in much the same way that their mothers carry them from one nest to another.
“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, and kept the limbs flexed,” Dr Kuroda said.
“This calming response in mice appeared similar to me to soothing by maternal carrying in human babies,” she said.
Knowing how and why babies are likely to be calmed by carrying them could help to avoid cases of child abuse when parents become frustrated and anxious as a result of babies continuing to cry for long periods, Dr Kuroda said.
“A scientific understand of this infant response will save parents from misreading the restart of crying as the intention of the infant to control the parents, as some parenting theories such as the ‘cry it out’ type of strategy suggests,” she said.
“Rather this phenomenon should be interpreted as a natural consequence of the infant’s sensorimotor systems,” she added.