Juvenile rank acquisition influences fitness independent of adult rank

Abstract

Social rank has been identified as a significant determinant of fitness in a variety of species. The importance of social rank suggests that the process by which juveniles come to establish their position in the social hierarchy is a critical component of social development. Here, we use the highly predictable process of rank acquisition in spotted hyenas to study the consequences of variation in rank acquisition in early life. In spotted hyenas, rank is inherited through a learning process called maternal rank inheritance. This pattern is highly predictable: ~80% of juveniles acquire the exact rank predicted by the rules of maternal rank inheritance. This predictable nature of rank acquisition in these societies allows the process of rank acquisition to be studied independently from the ultimate rank that each juvenile attains. In this study, we use a novel application of the Elo-rating method to calculate each juveniles deviation from expected pattern of maternal rank inheritance during development. Despite variability in rank acquisition in juveniles, most of these juveniles come to attain the exact rank expected of them according to the rules of maternal rank inheritance. Nevertheless, we find that transient variation in rank acquisition in early life predicts long term fitness consequences for these individuals: juveniles underperforming their expected ranks show reduced survival and lower lifetime reproductive success than better-performing peers. Finally, we present evidence that this variability in rank acquisition in early life represents a source of early life adversity, and that multiple sources of early life adversity have cumulative, but not compounding, effects on fitness.