Sheep exhibit a positive judgement bias and stress-induced hyperthermia following shearing

Abstract

The detection of judgement biases may improve welfare evaluations by measuring the cognitive component, particularly the valence, of affective states. Judgement biases have been successfully demonstrated in various laboratory animals but only recently in sheep. Chronic stressors have been found to induce a negative judgement bias and a short-term stressor (restraint and isolation stress) a positive judgement bias. Here we examine the impact of the short-term stress of shearing on judgement bias, haematology, plasma cortisol and stress-induced hyperthermia (SIH) in sheep. Twenty-four Merino ewes were trained to spatially differentiate between positive and negative reinforced bucket locations with a go/no-go approach response. Judgement bias was tested by offering sheep three ambiguous, unreinforced bucket locations and the learnt locations. The response of the shorn sheep ( n = 12) was compared to that of the control sheep ( n = 12) with two cohorts of sheep tested on consecutive days. Following the analysis of the core temperature (SIH) responses, a subsequent study was performed to determine SIH to shearing without judgement bias testing. The cohort × treatment × bucket location interaction approached significance ( P = 0.056). In cohort 1, shorn sheep displayed a more positive judgement bias, approaching the ambiguous bucket locations more than the control sheep ( P = 0.033). However there was no difference in judgement bias in cohort 2 ( P = 0.908). Decreased eosinophil count ( P ≤ 0.001), increased plasma cortisol concentration ( P ≤ 0.001) and stress-induced hyperthermia ( P ≤ 0.001) in the shorn sheep all confirm that shearing was an acute stressor. This study suggests that sheep display a more positive judgement bias following release from shearing. This could reflect a more optimistic affective state induced by release from an acute stressor, or an altered risk perception. These findings support previous studies which suggest that testing judgement bias is a possible method for determining the cognitive component of affective state in sheep.