Short reportInvestigating attentional processes in depressive-like domestic horses (Equus caballus)
Introduction
One of captive/domestic animals’ responses to confinement is to become inactive and unresponsive to external stimuli (Fuchs and Flügge, 2002), as reported for many lab/farm species experiencing inappropriate living conditions (e.g. fur-farmed mink; tethered sows; caged dogs, Cronin, 1985). Humans’ inactivity is one of the behavioural markers of clinical depression (APA, 2013), a complex heterogeneous mental disorder diagnosed by the co-occurrence of affective, cognitive and behavioural symptoms (APA, 2013). Symptoms include deficits in selective attention (i.e. ability to focus perception on one stimulus while filtering out other simultaneous stimuli, Posner et al., 1980), as evidenced e.g. by slower reactions in an auditory oddball task (Kemp et al., 2010). Most reports of horses' impaired welfare (Burn et al., 2010, Pritchard et al., 2005) or pain (e.g. Ashley et al., 2005; review in Hausberger et al., in press) indicate that some horses are unresponsive to environmental stimuli. A ‘withdrawn’ state was recently described more formally (Fureix et al., 2012): during a withdrawn bout, horses remain totally motionless (immobility with no neck, head, and ear movements); displaying a flat-necked posture; wide open, rarely blinking eyes; and backwards-pointing ears. Compared to non-withdrawn horses from the same stable, withdrawn horses react less to human approach and tactile stimulation, and consume less sucrose (Fureix et al., 2015), a sign of anhedonia i.e. loss of interest or pleasure (Willner et al., 1992), which is a core symptom of human clinical depression. We hypothesized that if withdrawn horses were in a depression-like condition their selective attention would be altered (Greimel et al., 2015). Therefore we submitted 12 withdrawn horses and 15 non-withdrawn control horses from the same stable to unfamiliar auditory stimuli.
Section snippets
Ethical note
This study complied with French laws related to animal experimentation and the European directive 86/609/CEE.
Subjects
Twenty-seven horses (Nmares = 6; Ngeldings = 21; aged 5–20 years, , 77% French Saddlebred) from the same riding school were observed in June 2012. The horses were kept in 3 m × 3 m individual straw-bedded stalls in a barn, allowing visual contact with conspecifics. Each stall was cleaned every morning, and was equipped with an automatic drinker. Animals were fed hay once a day
Results
Neither sex nor type of stimulus nor observation group influenced reactions and durations of attention significantly (sex: Mann–Whitney U tests, P > 0.05; stimuli: Friedman tests, 1.2 < F <7.2, P > 0.05; group: Kruskall–Wallis tests, 6.8 < H< 10.2, P > 0.05), therefore data were pooled for subsequent analyses.
Discussion
Playback of unfamiliar auditory stimuli elicited reactions of non-withdrawn horses but of only half of the withdrawn horses on the first day. Durations of attention of reactive non-withdrawn horses were longer than those of reactive withdrawn horses. Non-withdrawn horses’ durations of attention decreased on the following days, but those of withdrawn horses did not vary significantly.
On the first test day, withdrawn horses seemed to have “switched off” auditory stimuli compared to non-withdrawn
Acknowledgments
We would like to thank the managers of the riding school for allowing us to work with their horses and all staff for their help and cooperation. This work was supported by a doctoral grant from the French Research Minister (CR) French Fyssen foundation postdoctoral fellowship (CF), a grant from the French Canada Research Fund, and from the Institut Français du Cheval et de l’Equitation (IFCE). The project benefitted from discussions from the GIS CCS. The funders had no role in the study design,
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Present address: Centre for Behavioural Biology, Department of Clinical Veterinary Science, University of Bristol, Langford House, Langford BS40 5DU, UK.