The Social System, Behaviour and Communication of the Golden Monkey (Cercopithecus mitis kandti)

Abstract

Forest guenons live in polygynous groups where males disperse on reaching sexual maturity and females remain within their natal group for life. During the mating season, the resident male regularly faces reproductive competition from extra-group males leading to extreme male-male competition for access to females. The golden monkey (Cercopithecus mitis kandti) is a little-understood, endangered guenon endemic to the Virunga Massif and Gishwati-Mukura National Park (Rwanda), known to live in large groups. The aim of this study was to provide the first behavioural account of this species and to describe and quantify its social system and communication. Behavioural and spatial data were collected five days a week over 15 months, using focal, instantaneous point samples and ad libitum sampling, from the habituated Kabatwa group in the Volcanoes National Park. A total of 31 adult males were individually identified within the home-range, and categorised as resident (n=11), transient (n=4), influx (n=10) or non-resident (n=6). Elo-ratings showeda consistently stable, tolerant, egalitarian hierarchy in resident males, who displayed low levels of non-contact aggression with more severe agonism directed towards extra-group males. Social network analysis revealed closer spatial relationships between resident males during the non-mating season in the forest and consistent agonistic networks. The mating season, defined through behavioural observations and estimating conception based on birth dates, extended over 5 months and females were observed copulating outside their conception window. Males followed females and both sexes mated with several partners over the season; the use of gestural displays were common in this reproductive context, as well as in agonistic contexts. The highly seasonal births coincided with bamboo shooting season when the group spent the majority of time in closed bamboo, with increased ground feeding; ground-feeding was frequent in this population (28% overall), and also reflected foraging for potatoes in agricultural land adjacent to the forest. The group spent about 50% of their time feeding; bamboo plant parts constituted 65.3% of their foraging effort throughout the year rising to 81.4% on bamboo shoots alone when they were available. Males exhibited a wide variety of sex-specific calls in different contexts; with some related to dominance rank. Vocalisations were examined through GLMMs and these were often different in form or context compared with other guenons, notably the single and double boom, nasal scream and male grunt. In summary, golden monkeys observed in this study had a multi-male, multi-female social organisation; both sexes were promiscuous during the mating season, and births were highly seasonal. Resident males were tolerant of each other, exhibited a stable egalitarian hierarchy with a broad communication repertoire including sex specific calls and gestures. Discussion focuses on comparison with other guenons, the unusually large group size and the unique, all-year-round, multi-male society observed in this golden monkey group. Further research is warranted on other groups, to explore female social structure and relationships, behavioural development, and to establish an understanding of life history strategies.