Costs of social conflict: do injuries mirror patterns of aggression in blue monkeys?

In wild animals, injuries often arise from intraspecific contests, and therefore may reveal challenges and selection pressures related to social conflict. We evaluated whether known aggressive patterns predicted injury patterns in wild blue monkeys, in which most injuries of known cause resulted from conspecifics. After describing injuries and their causes in 12 groups monitored over 13 years, we compared injury distributions across age-sex class and season. Adult males were much more likely, adult females about equally likely, and immatures much less likely to sustain injuries relative to random expectation. Among adult males, injuries were most common in the mating season. Using multivariate models, we examined how individual, social, and demographic factors predicted injury rates in adult females and immatures. Rates were higher for adult females versus all immatures, for individuals in smaller groups, and for those living in groups frequently exposed to novel adult males and with few immature members. Among older juvenile and adult females, these trends persisted; additionally, pregnant and lactating females were injured less often than others, but there was no evidence that dominance rank influenced injury rates. For juveniles, injury rates increased similarly with age in both sexes, providing only limited evidence that heightened aggression motivates natal dispersal by males. Most findings supported the hypothesis that aggression increases injury risk. Costs of within-group aggressive competition may manifest in ways other than injury, however, especially for females. We highlight the importance in comparative analyses of considering both within- and between-group aggression and demographic variables as influencing injury risk.  

Methods
Data were collected from November 2006 through December 2019 via field observations of wild, individually-recognized blue monkeys (Cercopithecus mitis stuhlmanni) living in multiple groups in the Kakamega Forest, Kenya. The study groups were part of a long-term project that began in 1979. During the 2006-2019 period, each group was monitored on a near daily basis by members of an experienced field team that noted any injuries during a census of group members carried out on each observation day. Each injury was assigned to the date on which it was first noted. Further details about the data collection are available in the associated publication.