Towards a ‘depressive-like’ state in horses?
Résumé
Domestic horses often encounter chronic stress, which in humans leads to a variety of negative psychological effects (e.g. depression). Pilot observations performed on 59 adult horses from three riding schools (30 min total/horse) revealed that 24% of these horses presented an atypical profile in their home environment, termed withdrawn hereafter and characterized by a particular posture (stretched neck and similar height between neck and back), the absence of ear and head movements and a fixed gaze (no visible eye movements) from 17 to 97 seconds (an apparently unusual trait, mean gaze duration of horses under natural conditions: 15±4 s). Our aim here is to investigate behavioural and physiological correlates of this atypical state, by comparing withdrawn horses (n=14) with non-withdrawn animals from the same stables (n=45). We evaluated their responsiveness to their environment (tactile reactivity, standardized sudden human approaches), their anxiety levels (novel object) and their cortisol levels (two blood samples collected for each horse between 18:00 and 19:00). Withdrawn horses appeared more indifferent to environmental stimuli in their home environment than the non-withdrawn horses (lowered responsiveness to tactile stimuli, Chi-square, P<0.01; indifference towards a human approach, Mann-Whitney, P<0.01) but reacted more emotionally in more challenging situations, i.e. novel object test (chi-square, P<0.001). Withdrawn horses exhibited lower plasma cortisol levels (Mann-Whitney, P<0.05), a phenomenon sometimes reported in various taxa when chronically stressed or depressed. Females were over-represented in withdrawn horses (chi-square, P<0.01), adding to the convergence with human depression. Altogether, results suggest a ‘syndrome’ in horses that resembles depression in humans, and open a promising line of investigation of what altered welfare states could look like in horses.