Age-dependent shaping of the social environment in a long-lived seabird: a quantitative genetic approach

Individual differences in social behaviour can result in fine-scale variation in spatial distribution and, hence, in the social environment experienced. Given the expected fitness consequences associated with differences in social environments, it is imperative to understand the factors that shape them. One potential such factor is age. Age-specific social behaviour-often referred to as 'social ageing'-has only recently attracted attention, requiring more empirical work across taxa. Here, we use 29 years of longitudinal data collected in a pedigreed population of long-lived, colonially breeding common terns (<i>Sterna hirundo</i>) to investigate sources of variation in, and quantitative genetic underpinnings of, an aspect of social ageing: the shaping of the social environment experienced, using the number of neighbours during breeding as a proxy. Our analyses reveal age-specific declines in the number of neighbours during breeding, as well as selective disappearance of individuals with a high number of neighbours. Moreover, we find this social trait, as well as individual variation in the slope of its age-specific decline, to be heritable. These results suggest that social ageing might underpin part of the variation in the overall multicausal ageing phenotype, as well as undergo microevolution, highlighting the potential role of social ageing as a facilitator for, or constraint of, the evolutionary potential of natural populations.This article is part of the discussion meeting issue 'Understanding age and society using natural populations'.