Pacing behaviour in laboratory macaques is an unreliable indicator of acute stress

Pacing behaviour, the most frequent stereotypic behaviour displayed by laboratory rhesus macaques ( Macaca mulatta ) is often used as an indicator of acute stress. In this study, we investigated how reliable this welfare indicator is by testing the reaction of macaques to the stressful event of being exposed to a fight between conspecifics housed in the same room but in a different cage. Pacing, agitated locomotion, and stress-related displacement behaviours were quantified before, during and after fight exposure, based on video recordings of 13 socially-housed macaques in their home cage. Displacement behaviours increased after fight exposure, confirming that the events were experienced as stressful by the focal individuals. The occurrence of pacing did not increase during or after the fights. Instead, agitated locomotion increased during the fights. These results suggest either, that pacing as an indicator of acute stress is prone to false negative results, increasing in some stressful situations but not others, or that agitated locomotion has been mistaken for pacing in previous studies and that pacing is in fact unrelated to current acute stress. Both interpretations lead to the conclusion that pacing is unreliable as an indicator of acute stress in laboratory rhesus macaques.