The acquisition of dominance among free-ranging rhesus monkey siblings

Abstract

In several species of Cercopithecine monkeys, younger sisters reverse dominance with older sisters during the course of maturation. Proximate hypotheses to account for the phenomenon are based on few and incomplete data, and tend to be unifactorial. This paper (1) presents systematic data on ‘youngest ascendancy’ among free-ranging rhesus monkey, Macaca mulatta, siblings; and (2) attempts to identify its proximate basis. Early social influences were inadequate to explain reversal. Older siblings resisted outranking, and only those younger siblings (including orphans) with powerful long-term support rebelled successfully. Supporters tended to be close relatives (such as the mother and other siblings) dominant to both siblings. Rebellion did not occur at some absolute age of the younger sibling. Its timing depended partly on the relative size of siblings, and partly on the extent of support the younger sibling had against the older. The study strongly suggests that the occurrence and timing of reversal depends on an interaction between factors predictive of individual fighting ability (such as body size) and those predictive of social power (such as alliances). Observed variation in the regularity with which ‘younges ascendancy’ occurs is discussed in relation to these findings.