Insights from the judgement bias paradigm: social group and tank size do 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 an assessment of an animal's mental state - where it is positioned on the continuum between optimistic and pessimistic states - and can be measured using the judgement bias paradigm. In this test, individuals are trained to distinguish a rewarded from an unrewarded cue before being presented an ambiguous, intermediate cue.