May 21, 2013
Carrying home the evolutionary advantage
May 20th 2013, 17:31 by A.A.K. | MUMBAI
A GENTLE pat on the back and a short walk can put a crying baby to sleep. Four-legged newborns, too, calm down when their mothers hold them by the nape of their necks to carry them. Precisely why, though, has remained a mystery. Kumi Kuroda, of RIKEN Brain Science Institute, in Japan, and her colleagues decided to find out.
The researchers monitored the heart rates of twelve human infants aged between one to six months under three conditions: while they lay by themselves in a crib, immediately after they were picked by their mothers, and while they were carried around. Specifically, they measure the heart rate variability (HRV), or the tiny differences in the interval between successive heart beats, since previous research showed that lower HRV is tied to higher stress.
As the researchers report in Current Biology, true to stereotype, the babies' HRV increased significantly when they were carried, suggesting less stress. The results persisted when controlled for things like crying, which can increase the heart rate considerably and skew HRV readings. Given the ethical constraints on experiments involving human babies, Dr Kuroda and her team then turned to mice.
After ascertaining that murine newborns, too, show the same physiological response to carrying as human infants do, they examined the sensory mechanisms which might give rise to the phenomenon. First, they used a local anaesthetic to numb the skin on back of the mice's neck. This blocked the neural signals that are produced by touch. Anesthetised mice were more fidgety while carried than those that were not.
Next, Dr Kuroda gave the mice a large dose of vitamin B6, to strip them of the ability to judge the relative position of their body parts, or proprioception, as it is known in the argot. Mice plied with the vitamin were also friskier than those that weren't, judged both by observed behaviour and physiological measures such as HRV.
Finally, the researchers used a drug to suppress the mice's parasympathetic nervous system. This is responsible for unconscious "rest-and-digest" or "feed-and-breed" activities like sexual arousal, salivation or urination, that occur when the body is at rest. When parasympathetic nerves were impaired, the carrying-induced, stress-lowering spike in the HRV was not seen, suggesting that the cardiac effect resulting from carrying did indeed stem from the parasympathetic nerve.
Crucially, the nervousness caused by pups' impaired senses meant that murine mothers took longer to carry them to part of the enclosure considered safe. Because the young often squirm when sensing danger, such impairment would make whisking them away from potential threats harder. The calming mechanism, then, appears to have evolved to facilitate such extraction. Although human offsrping are not exposed to as many existential threats as their murine counterparts, their parents will no doubt be grateful anyway.
Sumber: Economist, 20 May 2013