Insights from the judgment bias paradigm: Social group and tank size does not affect mental state in female guppies

Optimal holding conditions are key to animal welfare. How stressful husbandry is perceived by the animal can be determined via assessment of an animal's mental state - where it is positioned on the continuum between optimistic and pessimistic state - and can be measured using the judgment bias paradigm. In this test, individuals are trained to distinguish a rewarded from an unrewarded cue before being presented an ambiguous, intermediate cue. The response time to the ambiguous cue is then indicative of mental state. A shorter latency suggests a more positive (optimistic) mental state, a longer latency a more negative (pessimistic) mental state. Here, we used the judgment bias paradigm to assess the impact of standard laboratory housing conditions on the mental states of female guppies (Poecilia reticulata). As it is debated which holding conditions confer optimal welfare, we tested the impact of husbandry on mental state by keeping animals for three weeks in small or large social groups in either small or large tanks. We found that the different standard housing conditions we used did not lead to differences in mental state. As an unexpected side result, we found that female guppies seem lateral. Our findings of comparable mental state across housing conditions suggest that guppies either perceive the tested conditions as equally stressful or alternatively, that guppies are relatively resilient to the combination of group and tank sizes tested in this study. We conclude that the judgment bias paradigm can be a useful tool to assess fish welfare. This article is protected by copyright. All rights reserved.