What it is like to be an optimist: temporal stability of cognitive bias in rats and its link to other individual traits

Originating from human psychology, the concept of optimism/pessimism has become increasingly popular in animal welfare science. Typically, so-called judgement bias tests are used to study optimism levels in response to different affect manipulations. Recently, however, evidence is growing that judgement biases not only reflect affective states but might also cover a trait dimension meaning that the optimism level of an individual is stable over time. Contributing to this emerging field of research, we here aimed to comprehensively characterize optimistic and pessimistic rats by repeatedly testing a cohort of Lister Hooded rats in a tactile judgement bias test. To investigate potential links to other behavioural traits, we also assessed the rats' anxiety-like and exploratory behaviour, their laterality and their vocal response to human-induced play behaviour. Furthermore, we studied concentrations of basal faecal corticosterone metabolites to record hypothalamic–pituitary–adrenocortical activity. Therefore, we took all measures twice to assess the stability over time. In contrast to previous studies, we did not find optimism levels to be temporally stable in the present study, encouraging further research on potential factors that might affect or modulate the stability of optimism levels within and across different life phases. Furthermore, we did not detect suites of correlated behaviours, calling for more research on the impact of context and/or species on the emergence of such links. Nevertheless, our results contribute to the accumulating evidence that certain interindividual differences in behaviour, such as exploratory locomotion, vocalizations, anxiety-like behaviour and laterality as well as hypothalamic–pituitary–adrenocortical activity, are indeed stable across time and hence can be considered aspects of an animal’s personality.