Comparing proportional and ordinal dominance ranks 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 affects these outcomes 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—ordinal rank and proportional or ‘standardized’ rank—to predict 20 distinct traits in a well-studied wild baboon population in Amboseli, Kenya. We propose that ordinal rank best predicts outcomes when competition is density-dependent, while proportional rank best predicts outcomes when competition is density-independent. We found that for 75% (15/20) of the traits, one of the two rank metrics performed better than the other. Strikingly, all male traits were better predicted by ordinal than by proportional rank, while female traits were evenly split between being better predicted by proportional or ordinal rank. Hence, male and female traits are shaped by different competitive regimes: males’ competitive environments are largely driven by density-dependent resource access (e.g., access to estrus females), while females’ competitive environments are shaped by both density-independent resource access (e.g. distributed food resources) and density-dependent resource access. However, traits related to competition for social and mating partners are an exception to this sex-biased pattern: these traits were better predicted by ordinal rank than by proportional rank for both sexes. We argue that this method of comparing how different rank metrics predict traits of interest can be used as a way to distinguish between different competitive processes operating in animal societies.

High social status males experience accelerated epigenetic aging in wild baboons

“…Ordinal dominance ranks were assigned on a monthly basis to every adult based on these matrices, such that low numbers represent high rank/social status and high numbers represent low rank/social status 33,69 . Although most analyses of data from the Amboseli baboons have used ordinal ranks as the primary measure of social status, in some analyses proportional rank (i.e., the proportion of same-sex members of an individual's social group that he or she dominates) has proven to be a stronger predictor of other trait outcomes 93,94 . In this study, we chose to use ordinal ranks, but proportional and ordinal dominance rank were highly correlated in this particular data set (R 2 = 0.70, p = 1.13 x 10 -58 ).…”

Early life adversity and adult social relationships have independent effects on survival in a wild animal model of aging

“…Low maternal social status was assigned when the mother’s proportional dominance rank in the month of her offspring’s birth was in the lowest quartile of dominance ranks. Proportional dominance rank ranges from 0 (lowest ranking female) to 1 (highest ranking female) and indicates the proportion of adult females in a study subject’s social group that she dominated in agonistic interactions ( 46 ). Low maternal social connectedness was assigned when the mother’s social connectedness to other females was in the lowest quartile of the distribution of social connectedness values during our study subjects’ first two years of life.…”

“…A female dominance matrix was created for each month based on these win/loss outcomes, and female ordinal dominance ranks were assigned by minimizing entries below the diagonal ( 45 ). We then scaled these ordinal rankings by group size and assigned to each female a ‘proportional dominance rank’ ( 46 ), calculated as [1 – (ordinal rank – 1) / (number adult females – 1)]. A female’s proportional dominance rank represents the proportion of adult females that she dominates.…”