Long-term repeatability and age-related plasticity of female behaviour in a free-living passerine

Abstract

Behaviour is often both repeatable among individuals (i.e. personality) and plastically adjusted within individuals according to environmental conditions or age. Yet, little is known about the repeatability and age-related plasticity in behavioural traits across the lifetime of free-living animals, which is, however, crucial for understanding the development and evolutionary consequences of personality in the wild. Here, we explored long-term (co)variation and age-dependent expression of two female behaviours, female–female aggression and hissing behaviour, using longitudinal data of free-living great tits, Parus major . Both behaviours were repeatable across years but did not covary among females into a behavioural syndrome. In contrast to hissing behaviour, female aggression declined with age on the population level, which was due to within-individual plasticity in aggression and not selective disappearance. Moreover, individual females differed in their level of plasticity, with more aggressive females showing a steeper decline in aggression than less aggressive females, resulting in a pattern where females became more similar in aggression with age. At the same time, the repeatability of aggression increased with age, mostly driven by a decrease in within-individual variance across age. Large between- and within-individual differences in same-sex aggression in early life and a higher repeatability in later life may be functionally linked to age-dependent requirements and constraints. Overall, our findings suggest that how female aggression will respond to selection might depend on when during the life cycle selection is operating, which has important consequences for the understanding of the evolutionary dynamics of animal personality in the wild. • We studied long-term repeatability and age-related plasticity of female personality. • Female-female aggression and hissing behaviour were repeatable but did not covary. • Aggression showed age-related plasticity at the population and individual level. • Repeatability of aggression increased with age. • Effects of selection on behavioural phenotypes may depend on age.