The behavior of singly-caged, yearling rhesus monkeys is affected by the environment outside of the cage

Abstract

Numerous studies have demonstrated that the behavior of singly caged laboratory primates can be positively affected by inanimate enrichment within the cage. The environment outside of the cage may also influence the behavior of singly caged rhesus monkeys. To test this, we compared two cohorts of yearling rhesus ( Macaca mulatta); one that had only limited stimulation in the environment outside of the cage (singly caged in indoor rooms), and one that had considerable naturalistic and social stimulation in the extra-cage environment (singly caged in an outdoor building). Half the animals in each housing condition received a three-phase enrichment program and the rest served as controls. Subjects that had sensory access to social groups of conspecifics spent significantly more time drinking, feeding, and behaving abnormally, and significantly less time inactive and playing than did subjects that only had sensory access to other singly caged rhesus. Enriched subjects in both settings spent significantly more time playing and less time self-grooming than did controls. There was also a significant interaction for time spent in play, with enriched subjects living indoors playing the most. Although subjects that received considerable stimulation from outside of the cage spent more time engaged in abnormal behavior, most was pacing; usually in response to human manipulations of the other monkeys in the colony. Subjects housed indoors, with limited extra-cage stimulation spent less time pacing, but more time in potentially injurious abnormal activities. Among enriched subjects only, those housed indoors spent less time drinking and behaving abnormally and more time playing and using enrichment. Inanimate enrichment within the cage led to consistent positive changes in behavior for singly housed rhesus. Although the environment outside of the cage was shown to influence behavior, it was not clear whether exposure to numerous social and naturalistic stimuli was preferable to more limited stimulation for promoting well-being. Although the increased stimulation associated with a complex extra-cage environment is usually considered beneficial, not all such stimulation is positive. Frequent disturbances to social groups in view of singly caged subjects resulted in increased pacing, but the complex extra-cage environment also provided subjects with more opportunities to express and observe components of the species-typical behavioral repertoire.