Ethological scars? Exposure to multiple negative events over a lifespan may predict abnormal repetitive behaviour in laboratory-housed rhesus macaques

Abstract We tested the hypothesis that, in rhesus macaques used in biomedical research, the performance of abnormal repetitive behaviours (ARBs; e.g. pacing, hair-plucking, eye-poking) reflects the cumulative burden of negative experiences. For 240 macaques living in two facilities, we did this by cataloguing, weighting and summing exposure to 12 types of negative events or practices (two current, 10 past), over animals’ entire lifespans. We then assessed their aggregated impact on current behaviour. When all recorded forms of ARB were pooled into one measure, the hypothesis was strongly supported at one facility: pooled ARB increased with lifetime negative experience scores in a dose-response type manner, and reflected combined effects of both current and past negative experience scores. At the other facility, current negative experience scores had little apparent impact on pooled ARB; however, past negative experience scores were still highly predictive. This novel approach, inspired by psychological research on humans, adds to growing evidence that ARBs can reflect the cumulative impact of the number and severity of negative experiences (both past and present). We hope that it encourages similar studies of farm, zoo, pet and equine populations to investigate other animals’ ARBs, perhaps yielding new indicators of overall quality of life.