Social Rank, Female Competition for Food, and Behavioral Time-Budgets in the Golden Snub-Nosed Monkey Rhinopithecus roxellana Multi-Level Society

Abstract

Competition within primate groups often translates to a social hierarchy, with high-rank individuals gaining privileged access to resources, especially food. Golden snub-nosed monkeys Rhinopithecus roxellana live in a multi-level society, with multiple one-male units (OMUs), each containing a single adult male and several females, forming a breeding-band. High-rank males have increased access to high-value foods and spend less time feeding and more time being groomed by females, potentially enhancing social cohesion within their OMUs. The adults of each OMU mainly feed and socialize together, with food competition predominantly acting at the OMU level. We thus predicted that adult females by association attain the rank and feeding privileges of their OMU leader males, and make similar time-budget trade-offs. By food-provisioning a wild breeding-band during winter and spring, when natural foods are abundant or limited, respectively, we found that females of high-rank OMUs ate more provisioned foods at higher rates, especially during winter when provisioned foods had increased value. In winter, females of low-rank OMUs fed for longer on natural foods and females of high-rank OMUs longer on provisioned foods. Females of high-rank OMUs spent longer being groomed by other OMU members, especially during winter. Our results are consistent with females attaining the feeding privileges of their OMU leader male, enhanced during winter due to increased value of provisioned foods, high thermal demands, and reduced natural food availability. Importantly, the feeding privileges attained by females of high rank OMUs were less pronounced than those previously found for leader males, possibly due to higher tolerance between females of different OMUs. We suggest that behavioral time-budget effects of food competition in female R. roxellana enhance cohesion in high-rank OMUs, contributing to OMU integrity and, hence, the social structure of this multi-level society.