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Investigating anhedonia in a non-conventional species : are some riding horses depressed?

Abstract : Some riding horses display states of inactivity and low responsiveness to external stimuli that we term ‘withdrawn’, and that resemble the reduced engagement with the environment seen in clinically depressed people. To assess whether these animals are indeed affected by a depression-like condition, we investigated anhedonia: the loss of pleasure that is a core symptom of human clinical depression. Subjects were withdrawn horses and controls from the same stable (16 geldings and 4 mares, 7-20 years old, 85% French Saddlebred). The time individuals spent being withdrawn was determined by a trained observer using instantaneous scan sampling every 2 minutes over 1 h long periods repeated daily over 15 days. To measure sucrose intake, a classic measure of anhedonia in rodent-based biomedical research never previously applied to horses, commercially-available flavoured sugar blocks, novel to these subjects, were mounted in each stall and weighed 3, , 24 and 30 h after provision. We hypothesized that if depressed-like, withdrawn horses would consume less sucrose than controls. Horses spending the most time withdrawn did show reduced sucrose consumption (F1,18=4.65, P=0.04, in a repeated measures model also controlling for age, sex, and the time each horse spent in its stall – thus able to eat the sucrose – during testing). We then controlled for two possible alternative explanations for this pattern: neophobia towards novel foods, and generally low appetites. Hay consumption was measured over 5 days, as were subjects’ latencies to eat a meal scented with a novel odour. When included in our model, high hay consumption strongly tended to predict high sucrose consumption (F1,14=4.52, P=0.051), while long latencies to eat a novel food predicted low sucrose consumption (F1,14=8.34, P=0.012). However, statistically controlling for these two confounds did not eliminate the relationship between being withdrawn and consuming less sucrose (although reducing it to a strong trend: F1,15=4.28, P=0.056), suggesting that neither overall food consumption levels nor neophobic reactions explained our previous findings. Overall, this study illustrates the methodological challenges of investigating anhedonia in non-conventional species; reveals possible depression-like conditions in riding horses; and suggests a way of assess anhedonia in other animals showing profound inactivity (e.g. working equids in the developing world).
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https://hal-univ-rennes1.archives-ouvertes.fr/hal-01335844
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Submitted on : Wednesday, June 22, 2016 - 2:04:49 PM
Last modification on : Thursday, January 9, 2020 - 4:08:04 PM

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Carole Fureix, Cleo Beaulieu, Céline Rochais, Soizic Argaud, Séverine Henry, et al.. Investigating anhedonia in a non-conventional species : are some riding horses depressed?. 48th Congress of the International Society for Applied Ethology, International Society for Applied Ethology (ISAE), Jul 2014, Vitoria-Gasteiz, Spain. pp.978-90-8686-245-0, ⟨10.3920/978-90-8686-797-4⟩. ⟨hal-01335844⟩

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