The influence of regularly changing enrichment on the cognitive judgement bias of laboratory rats

The provision of environmental enrichment is a frequently used refinement method to improve welfare of laboratory animals. Compared to barren housing conditions, enrichment could decrease signs of impaired welfare, such as stress hormone concentrations and stereotypies. Moreover, the provision of enrichment was found to positively affect animals' emotional states, reflected by more optimistic judgements in cognitive bias paradigms. To investigate the impact of environmental enrichment beyond the classical comparison between barren and enriched housing, the aim of this study was to examine the influence of two different enrichment regimes on the cognitive judgement bias of laboratory rats (Rattus norvegicus f. domestica). Twenty-one individuals were housed either under consistent enrichment or regular enrichment change for several months. Afterwards, they participated in a cognitive judgement bias test, which is based on ambiguous cue interpretations and enables a classification of rats as being optimistic or pessimistic. Interestingly, we found that rats from the enrichment-change group responded more optimistically in an ambiguous situation than rats from the consistent-enrichment group. It seems that the regular enrichment change could induce a more positive emotional state in our rats and, therefore, represents a simple but effective approach to improve welfare under laboratory conditions.