Short- and long-term effects of endogenous cortisol on personality traits and behavioural syndromes

Animals express consistent individual differences in some behaviours, termed animal personality, but behaviours can also vary considerably within individuals, within minutes or hours, due to environmental stimuli. Consistent among-individual variation is often assumed to be mediated by hormonal mechanisms. Hormones are also involved in flexible and fast responses towards environmental stimuli. Even though basic mechanisms by which hormones regulate behaviours are known, much of the quantitative patterns underlying hormone–behaviour interactions within and among individuals remain unclear. Here, we conducted two experiments to investigate the immediate, short-term effects of experimentally elevated cortisol titres on well-known animal personality traits (experiment 1) and their potential long-term effects (experiment 2) in the medium-sized cavy, Cavia aperea. Therefore, we tested how personality traits related to stress coping, novelty seeking and social behaviour react within hours to elevated cortisol. In experiment 2, we tested whether a 3-week elevation of cortisol affects the same personality traits after cessation of the hormone treatment. We investigated effects on the mean levels of behaviours, that is, the personality type, the temporal consistency (i.e. repeatability) and among-individual correlations of traits. In experiment 1, we found cortisol led to more aggressive behaviour and more passive stress coping while other traits were unaffected. In experiment 2, we found no long-term effects. Both measured hormones, cortisol and testosterone, showed correlations with several personality traits; these correlations, however, were unaffected by the cortisol treatment. Animals receiving the cortisol treatment showed higher repeatability for one stress-coping trait and lower repeatability for testosterone concentration. Interestingly, the sexes differed in only few mean trait expressions but showed different correlation structures across traits. Taken together, our data indicate that personality traits in adult individuals are very consistent and only react with short-term fluctuations to internal hormonal signals.