Age-related reductions in within-individual variation increase the repeatability of mating behaviour

Abstract: Behavioural ageing patterns can vary among individuals and can predict long-term changes in the extent of among-individual variation in behaviour, resulting in age-related changes in behavioural repeatability. Understanding how repeatability changes with age is important because it offers insight into the age-specific evolvability of a trait. However, age-related alterations in behavioural repeatability can also stem from changes in within-individual behavioural variation. Nevertheless, little is known about the roles of among- and within-individual behavioural variation in age-related changes in behavioural repeatability. In this study, we repeatedly measured mating attempts of individual male water striders (Gerris lacustris) at different times and estimated individual patterns of behavioural ageing as well as age-related changes in within- and among-individual behavioural variation. We found a decline in mating attempts with age in male water striders. Additionally, individual males exhibited diverse behavioural ageing patterns, resulting in varying ranks with age and consistent among-individual variation in mating attempts. Conversely, within-individual variation in mating attempts decreased with age, leading to increased repeatability. This increased repeatability may arise from reduced mating behaviour flexibility or a decline in the maximum number of mating attempts with age. Therefore, we emphasise the importance of within-individual behavioural variation in age-related changes in repeatability, independent of changes in among-individual behavioural variation. Significance statement: Despite considerable research on age-related phenotypic changes, little attention has been given to individual-level analyses of behavioural changes with age. Here, we show that male water striders decrease their mating attempts with age and also exhibit variation in how their behaviour changes with age. The extent of individual differences in mating attempts was unchanged with age, but the extent of within-individual variation in mating attempts decreased. This pattern results in an age-related increase in behavioural repeatability. Thus, we suggest that age-related changes in behavioural variation should be considered when predicting behavioural evolution.