Louisa Bierbaum, Viktoria Siewert, Carolin Mundinger, Sylvia Kaiser, S. Helene Richter

It is long known from human psychology that people differ in their perception of the world, with some judging ambiguous information more positively (i.e., “optimists”) and some more negatively (i.e., “pessimists”). About 20 years ago, this knowledge was transferred to animal welfare science to assess emotional states in animals by quantifying optimistic or pessimistic judgement biases. More recently, with increasing interest in animal personalities, researchers have also begun to explore whether differences in optimism and pessimism remain consistent across time. While first evidence suggests that optimism and pessimism represent a stable trait, less research has focused on their consistency within and across specific life phases. Using laboratory rats, we therefore aimed at systematically investigating the consistency of optimism and pessimism within and across two life phases during adulthood. Specifically, a cognitive judgement bias test relying on tactile cues was conducted twice during early and full adulthood, respectively. Temporal consistency within and across life phases was assessed by analysing the repeatability of individual optimism levels. While we did not detect consistent individual differences in optimism levels during early adulthood, they stabilized across phases and remained consistent during full adulthood. These findings align with human psychological research, suggesting that personality traits consolidate over lifetime. Despite this consolidation process, however, we also found considerable within-individual variation in overall optimism levels, indicating a high degree of behavioural plasticity. We therefore encourage future research on the ecological relevance of consistent versus flexible decision-making and highlight the significance of considering life phase when assessing behavioural traits.