Senescence of personality in a wild bird

Despite a growing body of literature reporting developmental changes in personality, few studies have adopted a lifetime perspective to study age-related changes in personality traits. Since most personality traits are heritable and linked with fitness, ontogenetic changes can have evolutionary implications. In this paper, we explore age-related changes in two behavioral responses to handling, both of which are heritable and associated with fitness, in wild blue tits. We find that one of these responses (handling aggression), which is positively correlated with fitness, declines with age. We show that this age-related decline is not due to selective disappearance, but occurs also within individuals, which confirms that this personality trait undergoes a senescent decline in the wild. We further find that individuals differ significantly in their rate of ageing but found no statistical support for the presence of genotype-age interactions on personality. Future research on personality would benefit from a more explicit lifetime perspective. Our study constitutes the first demonstration of a senescent decline in a personality trait and underlines the need for future research on personality to adopt a more explicit lifetime perspective. In addition, we provide an example of how to investigate age-related plasticity in behavior while dealing with typical issues arising from the use of longitudinal data, which will be of interest for behavioral ecologists. Finally, our approach is likely to stimulate research given its relevance for both our proximate and ultimate understanding of animal personality in nature.