Release from restraint generates a positive judgement bias in sheep

Abstract

The study of judgement biases in animals has attracted interest as a way of potentially measuring emotional states by being able to detect pessimistic-like or optimistic-like evaluations of their environment. While judgement biases have been successfully identified in laboratory species, no such research has been reported in livestock species. Twenty ewes were trained to learn a spatial location task that required a go/no-go response according to the location of a bucket in a pen. One bucket location was associated with a feed reward, and the other associated with a negative reinforcer (no food and the presentation of a dog). Ten sheep were then subjected to a 6 h restraint and isolation stress (RIS), for three consecutive days. Following RIS on each day, all sheep were tested for biases in judgement by measuring their response to three previously unseen bucket locations and the learnt locations. Serum cortisol, haematological parameters, and behaviour of the sheep in an open field test were also recorded. Restrained and isolated sheep were more likely to approach the ambiguous bucket locations compared to control sheep (P = 0.008), suggesting RIS-treated animals had a more optimistic-like judgement bias. This was despite serum cortisol concentrations showing that sheep were highly stressed by the RIS treatment (P = 0.019). This finding provides evidence that it is possible to measure judgement biases in a livestock species. When released from RIS the sheep may have had a more positive emotional state, or a lesser perception of risk, than that exhibited by control sheep.