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

Abstract

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 the 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. The 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.

Significance statement

Injuries in wild animals often result from intraspecific aggression, so their distribution may reveal both causes and consequences of social conflict. We studied the distribution of injuries in wild monkeys to examine how closely injuries matched known patterns of intra-specific aggression and, thus, to better understand where such conflict is costly. After verifying that most injuries were indeed caused by conspecifics, we examined how injury rates varied with individual attributes (age, sex, female reproductive state, and dominance rank), season, and socio-demographic context (group size and composition). Most of these variables predicted injury rates as expected from previously known patterns of aggression. Unexpectedly, however, injury rates were higher in smaller groups and unrelated to female dominance rank, suggesting the importance of inter-group competition as a source of injury and the possibility that within-group competition imposes costs other than enhanced injury risk.