The Effect of Social Rank on Reproductive Traits Depends on Rank Metric: Evidence From a Group-Living Carnivore

Group-living animals form dominance hierarchies to mediate competition among members. The social rank of individuals determines their access to resources such as food and mates, as well as their exposure and opportunities to respond to social challenges. High rank often translates into greater reproductive success, but the strength of this correlation may vary considerably depending on the rank metric and the reproductive trait studied. Here, we investigated the predictive ability of two rank metrics-ordinal and standardised ranks-assumed to capture distinct underlying mechanisms of intragroup competition on six reproductive traits for female and male spotted hyenas Crocuta crocuta. We used 28 years of individual-based data from 481 hyenas from the Ngorongoro Crater, Tanzania. Ordinal rank strongly outperformed standardised rank in its ability to predict four out of six traits. As ordinal rank reflects the absolute position of an individual in the hierarchy, its importance when predicting reproductive traits suggests the relevance of priority of access to resources for both sexes. One trait-interbirth interval-was instead best described by standardised rank, suggesting socially mediated constraints on reproductive output. Our results reveal mechanisms that have been previously overlooked in explaining rank-related success in specific reproductive traits in the spotted hyena and emphasise the importance of selecting rank metrics that appropriately reflect the competitive context in focus.