Social alliances improve rank and fitness in convention-based societies

Abstract

Social hierarchies are widespread in human and animal societies, and an individual’s position in its hierarchy affects both its access to resources and its fitness. Hierarchies are traditionally thought of in terms of variation in individual ability to win fights, but many are structured around arbitrary conventions like nepotistic inheritance rather than such traits as physical strength or weapon size. These convention-based societies are perplexing because position in the hierarchy appears to be gained irrespective of individual physical ability, yet social status strongly affects access to resources and fitness. It remains unclear why individuals abide by seemingly arbitrary conventions regarding social status when they stand to benefit by ignoring these conventions and competing for top positions or access to resources. Using data from wild spotted hyenas collected over 27 y and five generations, we show that individuals who repeatedly form coalitions with their top allies are likely to improve their position in the hierarchy, suggesting that social alliances facilitate revolutionary social change. Using lifetime reproductive success as a fitness measure, we go on to demonstrate that these status changes can have major fitness consequences. Finally, we show that the consequences of these changes may become even more dramatic over multiple generations, as small differences in social rank become amplified over time. This work represents a first step in reconciling the advantages of high status with the appearance of “arbitrary” conventions that structure inequality in animal and human societies.

Juvenile rank acquisition influences fitness independent of adult rank

“…In this system, juveniles come to acquire the rank directly below that of their mothers and above those of their older siblings; this system is found in many Cercopithecine primates as well as in spotted hyenas. Prior work found that rank acquisition by this process is highly predictable: most (78.1%) females acquired the exact ranks predicted by maternal rank inheritance with youngest ascendency [13], and were consistently able to dominate lower-born adult females when they were around 18 months old [29]. Here, we show that there is considerable variation in the process of rank acquisition, independent of the rank the juvenile ultimately acquires.…”

“…Aggressive interactions among individuals of all age classes were collected using all-occurrence sampling [36]; aggressive interactions were collected up until June 2016 for two clans, December 2016 for one clan, and March 2017 for the fourth clan. We used the aggressive interactions among adult females to infer maternal ranks (i.e., rank of a juvenile’s mother relative to other mothers) as in [13,37]; we used the aggressive interactions among juveniles to measure variation in rank acquisition using the Elo-deviance method. In all cases we used, only aggressive interactions in which the recipient displayed submissive behavior.…”

“…In addition to Elo-deviance, we also included maternal rank (calculated as the rank the juvenile’s mother held in the year of the juvenile’s birth), and coded it categorically as ‘high’ and ‘low’ rank based on they were in the upper or lower half of the hierarchy. Rank relationships among females were inferred yearly for all adult females who were at least 1.5 years old at the start of the calendar year using the Informed MatReorder method, as in previous studies [13,37,39]. To control for the possible influence of variable sampling on Elo-deviance measures, we included the number of interactions used to calculate Elo-deviance as a predictor in each model.…”

“…For example in many species, the social ranks of adults are well predicted by certain phenotypes such as body size or physical markings, or certain conventions such as age or tenure [7–12]. Social factors, such as support from conspecifics or presence of kin, also influence dominance rank [5,13–15]. Winner- and loser-effects, where individuals that win (lose) a particular interaction show increased probabilities of winning (losing) subsequent interactions, have also been demonstrated to affect hierarchy formation in a number of species [16,17].…”

Mapping Kenyan Grassland Heights Across Large Spatial Scales with Combined Optical and Radar Satellite Imagery

“…We hope that this map will prove useful for ecological research within the Reserve. The Reserve is home to a plethora of research projects, focusing on spotted hyenas (e.g., [48]), lions (e.g., [49]), cheetahs (e.g., [50]), baboons (e.g., [51]), Martial Eagles (e.g., [52]), ungulates (e.g., [44]), and river Although our overall and class-level accuracies were consistently high, the confusion between shrub and tree cover is worth mentioning. That is, riverine forest, upland forest, grass Acacia, and shrub were sometimes confused, resulting in a 5% decrease in the overall accuracy (Table 3).…”