Maternal behavior of primiparous rhesus monkeys: effects of limited social restriction and inanimate environmental enrichment

Many studies have shown that female rhesus monkeys (Macaca mulatta) that were socially restricted while they were very young were likely to become inadequate mothers, especially to their firstborn offspring. As part of a strategy to convert a traditional breeding colony of rhesus monkeys into a specific pathogen-free (SPF) colony, females (and males) were housed in their natal social groups for their first year, singly for their second year, in male-female pairs for their third year, and in small, unimale-multifemale breeding groups for subsequent years. To test whether this social restriction adversely affected the maternal abilities of 42 primiparous females, their distance from their infants and maternal behavior were compared with eight primiparous females that had been continuously socially housed according to our standard colony practices. Focal animal observations were conducted on mothers during the first 20 weeks of their first infants' lives. Analyses revealed that socially restricted (experimental) females did not differ from socially housed (control) females in the overall adequacy of their maternal activities. Experimental primiparous females spent less time nursing and in contact with their infants and more time in non-infant-directed behavior than did control females. However, body weights of infants did not differ between groups at either 6 or 12 months of age. There were no significant differences when enriched experimental females were compared with those that were unenriched. Females that could not view social groups while singly housed, spent more time nursing than did females that could view social groups, but infant body weights did not differ. Two factors in our SPF housing strategy were likely to have contributed to the high levels of maternal competence measured in the experimental subjects: (1) allowing infants to remain in their natal groups for their first year; (2) providing visual access to social groups containing mothers and infants during the years while subjects were singly and pair housed. Rearing competent mothers was a crucial goal for establishing a cost-effective, self-sustaining SPF breeding colony of rhesus macaques that appears to have been achieved.