Effects of age, reproductive state, and the number of competitors on the dominance dynamics of wild female Hanuman langurs

Abstract

Female dominance hierarchies form as a result of individual differences in resource holding potential, social processes such as winner-loser effects or coalitions, and ecological conditions that favor contest competition. Contest competition is assumed to result in despotic, nepotistic, and stable hierarchies. However, female Hanuman langurs are exceptions to this pattern, with data from provisioned populations indicating despotic, yet individualistic (age-inversed) and unstable hierarchies despite strong within-group contest. We present data on hierarchical linearity, stability, and the determinants of female rank and rank change in a population of unprovisioned, wild Hanuman langurs (Ramnagar, Nepal). Based on 12 490 dyadic displacement interactions collected over 5 years from a medium-sized group (P group, mean = 6.9 adult females) and a large group (O group, mean = 13.6 adult females), stable periods (P group, ; O group, ) were identified and dominance hierarchies constructed with the program MatMan. In both groups, dominance hierarchies were linear (), with high directional consistency within dyads. Rank was negatively related with age, while the presence of maternal kin had no effect. Reproductive state affected dominance rank in the larger group, with females ascending the hierarchy prior to conception, and dropping in rank after birth. Ranks were unstable, with group size and the number of juvenile females driving the effect (GLMM, ). These results match earlier findings for provisioned populations. In female Hanuman langurs, competition seems most intense around conception and during gestation, creating rank instability, which is further exacerbated by the number of adult as well as maturing females.