The most celebrated experiments on the early social development of mammals are those of H.F. Harlow and his colleagues on the rhesus monkey, Macca mulatta. When these small and appealing creatures are born they cling to their mother's fur, but their mothers also hold them. The young monkey soon begins to explore its mother, both by looking at her and also by the use of the mouth and the hands. There is also much imitation of the mother from an early age: if the mother looks at something, so does the baby; if the mother manipulated an object, the baby follows suit. There is a period of about three months of very close attachment to the mother; another three months follow during which there is greater independence, but also a great deal of resort to the mother; finally there is complete separation. Rhesus monkeys are convenient animals to study in the laboratory, for even those which have been born in the wild do well in captivity, they also behave normally towards other monkeys, and mate and rear young successfully. The attachment of mother monkey to young, and young to mother is intimate and specific. When a baby is disturbed, it returns to its own mother, not any female. Similarly, a mother usually rejects a young monkey which is not her own (through a mother with a very young baby may adopt another of the same age and nurse both). A mother has two distinctive signals which attract her baby to her: one is to present her rear and look back between her legs; the other is a type of grin. Both are very effective in recalling a straying infant.
Harlow and his colleagues have done a long series of experiments on the effects of early deprivation of maternal care, and of isolation from other monkeys. An early group of observation on infant monkeys was given the opportunity to cling to a substitute or model mother. In some experiments, two models were offered: one was covered with cloth and was soft, while the other was of wire. Even when the monkeys were fed on the wire mother, they spent most of their resting time clinging to the cloth mother. Hence the habit of resorting to the mother is not a simple consequence of being fed by her. For a person with human feelings this is not a surprising conclusion, but it is valuable to have it demonstrated experimentally. The presence of an inanimate substitute mother has a considerable effect on behaviour. Without one, a baby monkey, though well fed, does little exploration and is terrified by strange objects; with one, there is a more usual amount of exploration, broken by resort to the mother-object. Completely deprived monkeys are liable to sit in a corner rocking - just like some severely abnormal human babies do. The deprived monkeys are also liable to inflict injury on themselves.
Despite the immense interest of the detailed account of the behaviour of these infants, the most important observations have been on their behaviour later in life. Six months of isolation have a calamitous effect on social and sexual behaviour. When deprived monkeys are put together in groups, they - unlike ordinary monkeys - fall to form quickly a stable status system. There is an exceptional amount of conflict, and deaths may result.
The effect on sexual behaviour is even more severe. Of all the males reared on model mothers, without encounter with any real monkeys, none has succeeded in reproducing. Physiologically they are indistinguishable from other adult males, but in Harlow's words, they are sexually destroyed. They still have sexual impulses, since they approach females in a sexual manner, but their subsequent behaviour is not organised or oriented in such a way that they can achieve coitus.
The isolated females, too, at first seemed completely incapable of breeding. They were, however, put with exceptionally gentle, patient, unhurried and skilful males, and eventually some had babies. But they proved incapable of behaving as normal mothers. They were either indifferent to their infants, or hostile towards them. The babies made desperate and pathetic efforts to approach their mothers, but were often beaten or knocked down. (It is a relief to know that only a few of these females have been studied). Eventually the young learnt to approach their mothers from behind, and climb onto their backs. The mothers sometimes groomed their young. Two of the mothers would not allow feeding, and the young had to be fed from a bottle.
How did these young behave? The young reared by hostile mothers were over-aggressive, and exceptionally active and precocious sexually. They were not grossly abnormal like their mothers, but had they been human they might well have been described colloquially as 'neurotic'. An obvious question is whether it is essential for a young monkey's social development that it should have access to a mother, as distinct from other monkeys. There have been experiments in which young monkeys had experience only of others of their own age. Those monkeys evidently develop normally. They cuddle each other a great deal, and of course they play. These interactions seem largely or wholly to make up for lack of mothering.
One thing that most people will wish to know is the implications of the work on monkeys for man. Harlow and his colleagues are very properly cautious about this. The main implication is, once again, that behaviour which in the past would have been called 'instinctive', is much influenced in its development by conditions in early life. This is a new conclusion: it was reached by Sigmund Freud (1856-1939) as a result of studies of an entirely different sort.
