Group and matriline differences in the behaviour of rhesus monkey infants

A ‘Monte Carlo’ method was developed for characterizing matrilines of rhesus monkey infants (Macaca mulatta) born to their mothers within six similar captive social groups, according to behavioural measures, and for characterizing social groups according to their matrilines' median scores. Eight- and 16-week-old infants differed according to their groups, but not according to their matrilines, in measures of their activity in the vicinity of their mothers, and the frequencies with which social companions other than the mother initiated social contacts with them while they were out of contact with their mothers. Sixteen-week-olds differed according to their groups in the frequencies with which their mothers rejected them, and 8-week-olds in the frequencies of the social contacts they initiated with companions other than mother while off their mothers. When tested with their mothers, in a mildly disturbing situation away from other social companions, 52-week-olds differed according to their group of origin in time spent out of contact with their mothers. Different groups were seen as producing infants differing in ‘enterprise’ from their eighth week, in the sense of being ready to initiate social contacts with others while off their mothers at 8 weeks, and of being off their mothers in the test at 52 weeks. Cases where the 8-week measures did not reflect infant enterprise could be explained in terms of other aspects of the relationships involving the infants and their mothers in the social group. In particular, matrilines differed consistently in dominance status, and some mothers also received high levels of aggression from other adults. Mothers receiving high levels of aggression were more responsible for maintaining proximity with their 8-week-old infants, and their 8-week-olds were less often involved in social contacts with others.