Determinants of Harem Size in a Polygynous Primate: Reproductive Success and Social Benefits.

Abstract

Simple SummaryThe size of primate ‘harems’ varies considerably, both inter- and intra-specifically. Previous studies have shown that females prefer high-quality males and that high-quality males are superior in inter-male competition, leading to them having a larger harem size. Based on eleven years of observations of Yunnan snub-nosed monkeys (Rhinopithecus bieti), we documented longitudinal stability in the distribution of harem sizes between 2010 and 2020. These demographic properties are the outcome of male and female social investment decisions that affect their reproductive performance and success. Male reproductive success was positively related to harem size, while constraints on individual social benefits and social investments limited harem size. Our findings advance our understanding of the socioecological determinants of harem size variation in polygynous primates.We used long-term data on the variation in harem size in Yunnan snub-nosed monkeys to research the effects of harem size on reproductive success and the ratio of grooming received to given (RGRG). The results suggest that harem holders derive reproductive benefits commensurate with harem size, whereas the females’ reproductive success is unaffected by harem size. Males of larger harems groomed less and had higher RGRG than males of smaller harems. In the case of females, grooming given increased, and RGRG decreased with an increase in harem size. The males’ reproductive success seems to be a driver of harem size maximization. From the females’ perspective, dwindling social benefits appear to set the upper limit for harem enlargement. We also showed that males of monogamous units (‘single-female harems’) invested more into grooming their female, presumably to prevent unit disintegration and loss of mating privileges.