Sex differences in compatibility of pair-housed adult longtailed macaques

This research was designed to evaluate the effects of same-sex pair housing on the psychological well-being of adult wild-born longtailed macaques (Macaca fascicularis). We studied behavioral compatibility and stress as measured by urinary cortisol excretion in 15 pairs of each sex. Before they were housed together, the pairs were categorized by noncontact pairedpreference testing as preferred, nonpreferred, or randomly assigned partners. Every aspect of data analysis indicated that the success of pairing was strongly related to gender. Whereas 100% of female pairs were compatible, only eight of the 15 male pairs were still together after two weeks, and only five (33%) showed a degree of compatibility resembling that of females. The psychological well-being of virtually all females seemed to be improved during the physical contact paired-housing conditions; they spent more than one-third of the day engaged in social grooming. Paired adult males had much lower interaction rates than adult females. On average, males were initially somewhat stressed by the introduction to a cagemate as indicated by increased urinary cortisol excretion. The noncontact preference testing procedure was no more predictive of pair success than random assignment. For males, the presence of fighting combined with the absence of grooming during the first 90 min opportunity for physical contact ("introduction") was associated with pair incompatibility, but not to a statistically significant extent. For research protocols permitting social grouping of this species, the social contact requirement of the USDA Animal Welfare Rules usually can be met for adult females by pair housing. For males, pairing with other adult males often is unsuccessful; by our estimates, at least 20% of males cannot be pair-housed with other males. These sex differences in response to same-sex adults are consistent with the known socioecology of macaques. Further research is necessary to determine whether adult males have a lower need for social contact than females, or whether their needs are better met by other types of social contact. © 1994 Wiley-Liss, Inc.