Reinhardt, V. 1988. Preliminary comments on pairing unfamiliar adult male rhesus monkeys for the purpose of environmental enrichment. Laboratory Primate Newsletter 27(4), 1-3.

A total of 7 different male-male combinations had to be tested during the period of non-contact familiarization in order to obtain 5 dyads (10 males) in which the partners exhibited clear dominance-subordination relationships (Table I). Pairing the respective males with each other on day 6 in a different double cage did not result in fighting in any case. Dominance relationships, however, as evidenced during familiarization were confirmed by unidirectional avoiding (3 pairs) or fear grinning (2 pairs) on each occasion within the first 6 minutes. Throughout the 1 to 4 month follow-up periods, fighting occurred in only one case, on day 5 of pair formation. The respective partners inflicted no serious wounds on each other, and their momentarily tense relationship re-stabilized within the following week. Signs of depression were not witnessed in any of the 10 subjects, but social grooming was a regular interaction between the companions (Figure 1). Temporary separation (10 minutes to 24 hours) of a partner after the first month of pair formation had no adverse impact on the subsequent compatibility of the pair.Compatibility of carefully preselected adult rhesus monkey males that had never lived together was 100%. This high intermale compatibility was unexpected and probably related to the fact that potential companions were put together only if they had established a clear dominance-subordination relationship during the time of non-contact familiarization. Under these circumstances, they apparently had no reason to fight over dominance upon pairing. When unfamiliar adult rhesus monkey males are paired they typically get involved in intense dominance-determining aggression (Maxim, 1976). Non-contact familiarization is apparently a safeguard against the risks involved in such encounters.Given that rhesus monkeys are social animals with an inherent need for social contact and interaction, the present method offers a simple, inexpensive, yet safe way of optimal environmental enrichment for singly caged rhesus monkeys. The present pilot study tested only males that had never lived together; there is little doubt that pairing males who had lived in the same group in the past would involve even less risk.This and previous (Reinhardt et al., 1987a, b) successful attempts to facilitate socialization of previously singly caged rhesus monkeys of both sexes indicate that the pending USDA regulations, requiring that nonhuman primates be housed with compatible conspecifics whenever possible, are realistic.