A comparison of dominance rank metrics reveals multiple competitive landscapes in an animal society

Abstract

Across group-living animals, linear dominance hierarchies lead to disparities in access to resources, health outcomes and reproductive performance. Studies of how dominance rank predicts these traits typically employ one of several dominance rank metrics without examining the assumptions each metric makes about its underlying competitive processes. Here, we compare the ability of two dominance rank metrics—simple ordinal rank and proportional or ‘standardized’ rank—to predict 20 traits in a wild baboon population in Amboseli, Kenya. We propose that simple ordinal rank best predicts traits when competition is density-dependent, whereas proportional rank best predicts traits when competition is density-independent. We found that for 75% of traits (15/20), one rank metric performed better than the other. Strikingly, all male traits were best predicted by simple ordinal rank, whereas female traits were evenly split between proportional and simple ordinal rank. Hence, male and female traits are shaped by different competitive processes: males are largely driven by density-dependent resource access (e.g. access to oestrous females), whereas females are shaped by both density-independent (e.g. distributed food resources) and density-dependent resource access. This method of comparing how different rank metrics predict traits can be used to distinguish between different competitive processes operating in animal societies.

Genetic ancestry predicts male-female affiliation in a natural baboon hybrid zone

“…Alternatively or additionally, female choice and female-female competition may be more important in predicting grooming and proximity than mating behavior. Whereas only one or a few females experience estrus at any given time (Bercovitch, 1983; Bulger, 1993; Levy et al, 2020), all adult females are available as, and may actively be searching out, grooming partners. Notably, grooming relationships in baboons are more often initiated and maintained by females than by males, whereas males primarily absorb the costs of mate-guarding in a mating context (Alberts et al, 1996; Nguyen et al, 2009; Packer, 1979b; Palombit et al, 1997; but see Weyher et al, 2014).…”

Distinct gene regulatory signatures of dominance rank and social bond strength in wild baboons

“…To investigate rank-gene expression associations, we extracted ordinal dominance rank values concurrent with blood sample collection, which represent rank as integer values where rank 1 denotes the top-ranking individual, rank 2 denotes the second highest-ranking individual, and so on. We note that previous analyses in this and other social mammals show that alternative rank metrics sometimes confer improved predictive power [110,111]. In the Amboseli baboon population, this is especially observable in females, where proportional rank (i.e., ordinal rank scaled by group size) is more closely associated with fecal glucocorticoid levels and injury risk than ordinal rank [110].…”

“…We note that previous analyses in this and other social mammals show that alternative rank metrics sometimes confer improved predictive power [110,111]. In the Amboseli baboon population, this is especially observable in females, where proportional rank (i.e., ordinal rank scaled by group size) is more closely associated with fecal glucocorticoid levels and injury risk than ordinal rank [110]. In this data set, substituting ordinal rank for proportional rank produces highly concordant effect size estimates (R 2 for baseline male, LPS male, baseline female, and LPS female rank effects = 0.75, 0.79, 0.88, 0.85, respectively), so we reported the results for ordinal rank for both sexes.…”

“…Sex-specific dominance ranks are assigned each month for each social group in the study population based on the outcomes of dyadic agonistic interactions observed on a near-daily basis [104,109]. Dominance rank assignments produce a hierarchy structure that minimizes the number of cases in which higher ranking individuals lose interactions to lower ranking ones [110]. To investigate rank-gene expression associations, we extracted ordinal dominance rank values concurrent with blood sample collection, which represent rank as integer values where rank 1 denotes the top-ranking individual, rank 2 denotes the second highest-ranking individual, and so on.…”

Costs dictate strategic investment in dominance interactions

“…The second step is to use the hierarchy-inference subset of the data to generate a hierarchy. This can be done using a range of methods, and can generate either rank- [27,28] or score-based [29] hierarchies. We suggest using the method proposed by Sánchez-Tójar et al [30], which combines Elo scores with a temporal randomisation procedure that has been shown to increase accuracy for static hierarchies.…”