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Being able to assess pain in nonhuman primates undergoing biomedical procedures is important for preventing and alleviating pain, and for developing better guidelines to minimise the impacts of research on welfare in line with the 3Rs principle of Refinement. Nonhuman primates are routinely used biomedical models however it remains challenging to recognise negative states, including pain, in these animals. This study aimed to identify behavioural and facial changes that could be used as pain or general wellness indicators in the rhesus macaque (Macaca mulatta). Thirty-six macaques scheduled for planned neuroscience procedures were opportunistically monitored at four times: Pre-Operative (PreOp), Post-Operative (PostOp) once the effects of anaesthesia had dissipated, Pre-Analgesia (PreAn) on the subsequent morning prior to repeating routine analgesic treatment, and Post-Analgesia (PostAn) following administration of analgesia. Pain states were expected to be absent in PreOp, moderate in PreAn, and mild or absent in PostOp and PostAn when analgesia had been administered. Three potential pain indicators were identified: lip tightening and chewing, which were most likely to occur in PreAn, and running which was least likely in PreAn. Arboreal behaviour indicated general wellness, while half-closed eyes, leaning of the head or body shaking indicated the opposite. Despite considerable individual variation, behaviour and facial expressions could offer important indicators of pain and wellness and should be routinely quantified, and appropriate interventions applied to prevent or alleviate pain, and promote positive welfare.
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