Mirror image stimulation (MIS) is reported to elicit persistent social responses in monkeys, in contrast to most humans and great apes, who exhibit self-recognition. The abnormal features of a mirror image as a social stimulus have generally been ignored in monkey reports, whereas research with other animals has identified some important differences between MIS and other stimuli. Differential agitation during separations in peer-reared and mirror-reared infant stumptail monkeys suggests that even the limited opportunity for physical contact with a reflection renders it a sub- optimal attachment-eliciting stimulus. Mirror-rearing appeared to only slightly diminish responsiveness to pictures of conspecifics, compared to peer-rearing. Animals reared with no form of social stimulation exhibited less responsiveness to pictorial stimuli, and engaged in more abnormal and self-directed behaviours than mirror- or peer-reared animals, indicating that a mirror can at least partly compensate for the absence of a true social companion during rearing. The extent of abnormal behaviours in alone-reared stumptail monkeys appears to be considerably less than that reported in rhesus monkeys. The mirror was reacted to as a social partner by mirror-reared animals, and correlations between behaviours, and between measures of a single behaviour, were similar in mirror- and peer-reared groups. However, a live cagemate received 50% more social behaviour than did a reflection, with play behaviours producing group differences in rate, duration, bout length, and variability. MIS or a peer behind Perspex reduced separation agitation in pair-reared but not group-reared infants. In comparison to a peer behind Perspex, MIS received positive responses in mirror-reared and pair-reared animals, whereas group-reared animals reacted more ambivalently to the abnormal animal represented in the mirror. Those mirror-reared animals who received additional experience of a peer behind Perspex during rearing reduced responding to the mirror, whereas responsiveness in mirror-only-reared animals persisted. Peer-only-reared animals were also highly responsive to MIS, possibly due to novelty. Alone-reared subjects, when tested in a familiar setting, were the most responsive of all the subjects to MIS. None of the subjects exhibited self-recognition, even although some had approximately 3,500 hours of experience of a triple mirror image effect, and an additional six months group mirror experience. Some results were obtained with small numbers of subjects, so caution is required in interpretation.

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