Juvenile rank acquisition is associated with fitness independent of adult rank

Abstract

Social rank is 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 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 very consistent: approximately 80% of juveniles acquire the exact rank expected under the rules of maternal rank inheritance. The 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 Elo-deviance scores, a novel application of the Elo-rating method, to calculate each juvenile's deviation from the expected pattern of maternal rank inheritance during development. Despite variability in rank acquisition among 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 is associated with long-term fitness consequences for these individuals: juveniles 'underperforming' their expected ranks show reduced survival and lower lifetime reproductive success than better-performing peers, and this relationship is independent of both maternal rank and rank achieved in adulthood. We also find that multiple sources of early life adversity have cumulative, but not compounding, effects on fitness. Future work is needed to determine if variation in rank acquisition directly affects fitness, or if some other variable, such as maternal investment or juvenile condition, causes variation in both of these outcomes.

Keywords: dominance; early life adversity; lifetime reproductive success; rank acquisition; social behaviour; survival.

Figure 1.

(a) Histogram of Elo-deviance at den independence (n = 465). (b) The relationship between the juvenile's mother's rank and the juvenile's rank at the onset of adulthood (n = 102). According to maternal rank inheritance, points should lie directly below the dashed line (denoting where mother's rank and juvenile's rank are exactly equal). In this study, 77.5% of juveniles acquired the exact rank predicted by maternal rank inheritance. Elo-deviance at den independence (colour) did not affect the rank attained by the onset of adulthood. Taken together, these plots show transient variability in rank acquisition at the end of the den-dependent life-history stage that fails to manifest in rank differences in adulthood. (Online version in colour.)

Figure 2.

Survival probability as a function of Elo-deviance at den independence and maternal rank. Juveniles with Elo-deviance less than 0 showed reduced survival. Death of the mother before the juvenile reached adulthood also predicted reduced survival, but maternal rank did not predict survival after controlling for the other variables in the model. Maternal rank was modelled as a continuous predictor but plotted here categorically. (Online version in colour.)

Figure 3.

Lifetime reproductive success (LRS) as a function of both Elo-deviance at den independence and maternal rank. Juveniles with Elo-deviance less than 0 showed reduced LRS, as did those with low maternal rank (modelled as a continuous variable but plotted here categorically). Models with lifespan included suggest that the relationship between Elo-deviance and LRS is mediated by the relationship between Elo-deviance and survival. (Online version in colour.)

Figure 4.

Survival probability as a function of the number of adverse conditions faced by juveniles during early life. The adverse conditions considered here were Elo-deviance less than 0 at den independence, low maternal rank and death of mother before offspring reached adulthood. (Online version in colour.)