London, April 2: Why is it that some infants start crying as soon as they are separated from their mothers? Well, an American neuroscientist perhaps knows the reason.

Christina Barr of the National Institute of Alcohol Abuse and Alcoholism in Bethesda, Maryland, says that she has identified a gene mutation that determines whether or not infant monkeys make a fuss when their mothers are missing.

She strongly believes that her finding may shed light on why some children are cry-babies, while some are more independent.

"There are some kids that go with the flow and there are some that are very reluctant (to leave their mothers)," New Scientist magazine quoted her as saying.

Christina says that the bodies of infants and young animals release natural drugs called opioids when they spend time with their mothers, and molecular receptors on brain cells sop up these chemicals to provide temporary feelings of pleasure.

She has revealed that opiates act via the same receptors as morphine, and minute doses of the drug, not enough to sedate, quieten young monkeys after their mothers leave.

Previous studies showed that 25 to 30 per cent of macaques have a genetic mutation in the mu-opioid receptor that makes them more sensitive to the brain chemicals.

With a view to establishing whether the mutation affects maternal attachment, Barr and her colleague Steve Suomi observed 97 six-month-old macaques, born in captivity, after they were separated from their mothers for several four-day stretches.

The researchers observed that all the infants howled the first time they faced life without their mother, but most of the animals grew accustomed to the separation and made less noise the next time they were separated.

However, it was not so for the animals with one or two copies of the opioid-enhancing mutation that continued to cry out for their absent mothers.

While such monkeys seemed to cherish reunions, and tended to spend more time with their mothers after each separation, macaques without the mutation did not make up for lost time.

"These animals were much more likely to want to stay in social contact with their mothers," says Barr.

She believes that the mutation might be helpful in keeping animals safe from predators and other dangers in the absence of their mothers.

"If you're going to be the animal that gives up really quickly, maybe she'll never find you, but if you keep crying she'll hear you and come get you," she says.

She has also revealed that the same mutation is associated with the high of alcohol and drugs, saying that a study has established that given the chance, male macaques with the opioid receptor mutation drink more alcohol than normal monkeys.

"What is needed now is work at the human level to see if attachment style also varies," says Jaak Panksepp, a neuroscientist at Washington State University in Pullman, US.

Barr agrees: "Early in evolutionary history there could have been some additional value to having a kid that was more attached. Now that we have drugs and alcohol, it can make us more vulnerable to addiction."

The study has been reported in the journal Proceedings of the National Academy of Sciences. (ANI)