Using the Judgment Bias Task to Identify Behavioral Indicators of Affective State: Do Eye Wrinkles in Horses Reflect Mood?

Identifying and validating behavioral indicators of mood are important for the assessment of animal welfare. Here, we investigated whether horses' eye wrinkle expression in a presumably neutral situation is a measure of mood as assessed in a cognitive judgment bias task (JBT). To this end, we scored pictures of the left and right eyes of 16 stallions for different aspects of eye wrinkle expression and tested the same individuals on a spatial JBT with active trial initiation. Eye wrinkle expressions were assessed by a qualitative assessment, i.e., the overall assessment of how “worried” horses look, the number of wrinkles, and the angle measured at the intersection of lines drawn through the eyeball and the topmost wrinkle. Correlations between the three eye wrinkle measures and the optimism index as a measure of horses' decisions in the JBT were not statistically significant, but with increasing optimism index, horses tended to be scored as looking less worried (qualitative assessment). We discuss our findings from different perspectives and make suggestions for future research, e.g., by calling for experimental induction of mood and thus greater variation within and/or between individuals and by investigating the interplay between shorter-lasting emotional and longer-lasting mood states to further explore the potential use of the JBT to validate eye wrinkles and other facial or body expressions as indicators of mood.

Once an Optimist, Always an Optimist? Studying Cognitive Judgment Bias in Mice

Individuals differ in the way they perceive the world. From human psychological research, it is known that these differences become particularly evident in ambiguous situations: while some individuals interpret ambiguous information pessimistically, others bias their inter-pretations in a more optimistic way, referred to as cognitive judgement bias (CJB). CJBs have also been studied in non-human animals as tools for the assessment of affective states. However, the ecological and evolutionary relevance of CJB has so far been overlooked. We here aimed to transfer the concept of CJB to behavioural ecology. More specifically, we investigated the causes of differences in CJB in mice, focusing on both genetic and environmental factors. Furthermore, we assessed whether individual differences in CJB are repeatable over time, addressing the question whether “optimistic” and “pessimistic” decision styles, respectively, may represent stable traits. Thus, two strains of mice (C57BL/6J and B6D2F1N) were housed in two different environmental conditions: “scarce” or “complex”. While mice living in the “scarce environment” experienced standard housing conditions, those living in the “complex environment” had regular access to a super-enriched “playground”. To calculate the repeatability of “optimistic” and “pessimistic” decision styles, we assessed CJB four times across the course of seven weeks. Moreover, we assessed anxiety-like behaviour to detect potential differences in the effects of genetic or environmental factors on CJB and anxiety. While the selected genotypes and environments influenced some aspects of anxiety-like behaviour, no influence on CJB could be detected, indicating that CJB and anxiety might represent distinct systems. Remarkably, CJB was moderately repeatable, suggesting that decision-making under ambiguity constitutes a relatively stable trait and might even be considered an aspect of animal personality.