Infant Rats Need Mom More than Brothers and Sisters

In recent years, there has been much speculation and controversy over the effects of an animal's early experiences - especially its relationships with its mother or siblings -and later emotional development. Back in the early 1960s, Harry Harlow's classical experiments on maternal deprivation in rhesus monkeys indicated that not only was the mother needed to produce socially adequate offspring, but that the infant monkeys needed their peers around them as well. Now Manfred D. Koch and William J. Arnold of University of Nebraska have been taking another look at the question, this time using rats, and they have come up with rather different answers.

They used four groups of rats, two mother-reared, either singly or with peers, and two similar incubator-reared groups. After a couple of months' rearing, the animals were ready for testing. Koch and Arnold tested emotionality in two ways. First they watched the rats in an 'open-field " large enclosure. The more emotional rats took longer to enter the illuminated field from a small dark ante-room (rats being naturally nocturnal, were more diffident in exploring their new environment) and urinated, more often. In the second set of tests, Koch and Arnold made a note of the animals' reaction to either a ringing bell or an electric shock. The more unstable the animal the greater the response in terms of increased heart beat; it also took longer for the heart rate to return to normal.

On all counts, as might be expected from previous results, the mother-reared animals came out more emotionally stable than their maternally deprived fellows. But more interesting perhaps is to compare the responses of those animals brought up with and without their peers. It appears that the baby rats who had their mothers to themselves – the only children -are more stable than those with brothers. This is in direct contrast to Harlow's work with monkeys.

Looking at the incubator-reared rats, a different pattern emerges. Those reared in complete isolation came out - not surprisingly – as very unstable emotionally. This seems to lend support to earlier work by Levine which suggested that social isolation itself is a dominant factor in the production of emotional instability. So it seems that for the rat at least, it's preferably to be brought up as an only child with a mother; but if an orphan it's better emotionally for the rat to have his peers around him.