Season-specific carry-over of early-life associations in a monogamous bird species

Social bonds can have important fitness consequences. Although there is increasing evidence that social bonds carry over across contexts, few studies have investigated whether social bonds formed early in life are carried over to adulthood. For example, juveniles of monogamous species go through a major life-history stage transition, pair formation, during which the pair bond becomes a central unit of the social organization. At present, it remains unclear if pair members retain their early-life social bonds after pair formation and, if so, when they display these preferences most strongly. Here we investigated whether bonds formed early in life carry over into adulthood and whether carry-over was dependent on season, in a monogamous species. Moreover, we investigated the role of familiarity, genetic relatedness and aggression on the perseverance of social bonds. We studied the social structure before and after pair formation in captive barnacle geese (Branta leucopsis), a highly social, long-lived, monogamous species. We constructed social networks of groups of geese before pair formation, during the subsequent breeding season, and in the following wintering season. From these networks, we identified the preferred associations among individuals, and how these are carried over during seasonal changes. We found that early-life associations in females were lost during the breeding season, but resurfaced during the subsequent wintering season. In males, the early-life associations persisted across both seasons, and were mediated by genetic relatedness and familiarity. The high level of aggressiveness of males, but not females, in the breeding season suggests that males may have played a key role in shaping both their own social environment and that of their female partners. By taking advantage of a captive population in which animals had no temporal or spatial restrictions to re-associate after pairing and for which we had complete data on their social history, we show that early-life social relationships can be maintained well into later life. Such bonds can be sustained even if they are temporarily disrupted, for example due to reproductive behaviour. Our findings therefore highlight that the early-life social environment can have life-long consequences on individuals' social environment.