Rearing infant monkeys (Macaca nemestrina) in pairs produces deficient social development compared with rearing in single cages

Abstract

Many scientists and colony managers assume that social housing is a beneficial living condition for all captive primates. Several older studies of primate development question the generality of this assumption. We recently tested this assumption by comparing the social development of pigtailed macaque infants raised in pairs and those that were raised in individual cages. All animals received 30 min of daily socialization in a playroom. Infants paired from postnatal week 3 through month 4 developed a playroom behavioral repertoire consisting largely of mutual clinging, fear, and social withdrawal. This was especially true of females. Unlike the singly caged infants, pair-reared monkeys did not successfully adapt to living in a large social group at 8-10 months of age. In this situation, pair-reared infants were subordinate and spent almost all of their time huddling on the pen floor. It was concluded that rearing macaque infants in pairs produces a behavioral repertoire that is maladaptive with respect to social development.

Social development in nursery-reared pigtailed macaques (Macaca nemestrina)

“…Elevated amounts of passive social contact appear to represent deviant behavior in pigtail macaques. In a previous study conducted at the IPRL M. nemestrina that were housed in pairs during development had much higher levels of Social Passive, in the form of clinging, than infants raised individually with daily peer contact [Ruppenthal et al, 1991]. Pair-reared infants also showed depressed levels of behavioral changes and social and environmental exploration, and did not make a successful transition to group housing at 8 months of age.…”

“…Some forms of laboratory rearing do produce individuals with marked behavioral deficits [Mason, 1960;Harlow & Harlow, 1965Alexander, 1966;Harlow et al, 1966;Champoux et al, 1991;Ruppenthal et al, 1991]. There are, however, nursery-rearing regimens that produce subjects with relatively normal behavioral repertoires.…”

“…The order of observation of focal subjects was randomized. All observations were taken on weekdays between 0700 and 1700 h. Behaviors were entered into a keyboard as a four-digit code [described in Ruppenthal and Sackett, 1992]. The behavioral categories composing the coding system were mutually exclusive and exhaustive.…”

“…Infants born at the IPRL were separated from their mothers at birth and raised according to a standard nursery protocol [Ruppenthal & Sackett, 1992]. Infants born at the PFS were raised according to the nursery protocol once they entered the IPRL.…”

“…In the IPRL nursery, several husbandry practices that occur during the developmental period encompassed by this study may represent environmental stressors [Ruppenthal & Sackett, 1992]. Introduction to the playroom and first contact with peers during weeks 4-7 is one potential stressor, as it involves exposure to both unfamiliar surroundings and unfamiliar conspecifics.…”