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Abstract Changes in population size are driven by environmental and social factors. In spite of repeated efforts to identify the constraints on an unusually low-density population of blue monkeys ( Cercopithecus mitis ), it remains unclear why this generalist species fails to thrive in Kibale National Park in Uganda. While an unidentified disease may occasionally obstruct conception, it does not seem to limit overall reproductive rates. Infanticide at this site is infrequent due to the long tenures of resident males. Our analyses indicate that the single biggest constraint on blue monkey densities may be feeding competition with grey-cheeked mangabeys ( Lophocebus albigena ): across Kibale, the densities of these two species are strongly and negatively correlated. Though further analysis is needed to understand the timing and strength of feeding competition between them, we conclude that blue monkeys at Ngogo experience competitive exclusion from grey-cheeked mangabeys, possibly resolving the 50-year mystery surrounding this population. 

Abstract BackgroundSocial isolation is a key risk factor for the onset and progression of age-related disease and mortality in humans. Nevertheless, older people commonly have narrowing social networks, with influences from both cultural factors and the constraints of senescence. We evaluate evolutionarily grounded models by studying social aging in wild chimpanzees, a system where such influences are more easily separated than in humans, and where individuals are long-lived and decline physically with age. MethodologyWe applied social network analysis to examine age-related changes in social integration in a 7+ year mixed-longitudinal dataset on 38 wild adult chimpanzees (22 females, 16 males). Metrics of social integration included social attractivity and overt effort (directed degree and strength), social roles (betweenness and local transitivity) and embeddedness (eigenvector centrality) in grooming networks. ResultsBoth sexes reduced the strength of direct ties with age (males in-strength, females out-strength). However, males increased embeddedness with age, alongside cliquishness. These changes were independent of age-related changes in social and reproductive status. Both sexes maintained highly repeatable inter-individual differences in integration, particularly in mixed-sex networks. Conclusions and implicationsAs in humans, chimpanzees appear to experience senescence-related declines in social engagement. However, male social embeddedness and overall sex differences were patterned more similarly to humans in non-industrialized versus industrialized societies. Such comparisons suggest common evolutionary roots to ape social aging and that social isolation in older humans may hinge on novel cultural factors of many industrialized societies. Lastly, individual and sex differences are potentially important mediators of successful social aging in chimpanzees, as in humans.Lay summary: Few biological models explain why humans so commonly have narrowing social networks with age, despite the risk factor of social isolation that small networks pose. We use wild chimpanzees as a comparative system to evaluate models grounded in an evolutionary perspective, using social network analysis to examine changes in integration with age. Like humans in industrialized populations, chimpanzees had lower direct engagement with social partners as they aged. However, sex differences in integration and older males’ central positions within the community network were more like patterns of sociality in several non-industrialized human populations. Our results suggest common evolutionary roots to human and chimpanzee social aging, and that the risk of social isolation with age in industrialized populations stems from novel cultural factors.