Mirror image stimulation and behavioural development in stumptail macaques
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.