The costs of competition: Injury patterns in 2 Asian colobine monkeys

Aggression rarely escalates to physical conflict because doing so puts individuals at risk of injury. Escalation only pays off when the potential benefits outweigh the potential costs, that is, when resources critical to fitness are at stake. Here, we investigated the occurrence of injury in 2 Asian colobine species: Nepal gray langurs (Semnopithecus schistaceus) and Phayre's leaf monkeys (Trachypithecus phayrei crepusculus). In both species, younger individuals are higher-ranking and might have greater incentive to fight. However, Nepal gray langurs have a strict breeding season, which may magnify male mating competition, and Phayre's leaf monkeys, unlike Nepal gray langurs, have female-biased dispersal, which may increase female injury risk during subadulthood. Using long-term data on observed injuries (Nepal gray langurs: n = 208; Phayre's leaf monkeys: n = 225), we modeled the monthly occurrence of injury (Y/N) and found that males received more injuries than females in both species. Also, subadults generally experienced frequent injury, as young individuals likely face challenges when competing for group membership and/or establishing rank. In Nepal gray langurs, males received 3 times more injuries during the mating season, suggesting strong competition for mates during this period, and females experienced more injuries before conception, suggesting competition to meet the nutritional requirements for reproduction. Unexpectedly, females in smaller groups received more injuries in Nepal gray langurs. Overall, these results indicate that injuries are most likely when fighting may aid in establishing group membership, achieving high rank, and reproducing. Future research should investigate the influence of injuries on fitness outcomes.