Inferring patterns of aggression from red howler monkey injuries

Venezuelan red howler monkeys in a semideciduous habitat typically survive injuries and disabilities. Intraspecific physical aggression was the most frequently observed and inferred cause of injury. Thirty-eight percent of 119 howlers of all ages examined during capture had scars or other evidence of "damage." Overall, the sexes did not differ significantly in total number of injuries or number of individuals classified as "damaged." The incidence of injury was not independent of age-sex class. The subadult male class had the highest percentage of "damaged" individuals. However, when total injuries per age class were examined for each sex separately, only females showed a pattern that was significantly different than expected based on age (i.e., exposure to injury factors). Subadult females experienced more injuries than expected, whereas adult females had fewer injuries. Troop status (resident troop, natal troop, or extratroop) was not significantly related to the number of injuries in adult and subadult males. Thin finding was not surprising, because adult and subadult males of all status classes are involved in aggression related to breeding competition. Extratroop females had more injuries than expected, and natal females had the fewest injuries. The higher incidence of injuries on subadult females and extratroop females is consistent with aggression-mediated emigration of some females and observed resistance to female immigration by resident females. Overall, 74% of injuries were located on anterior-ventral portions of the body, consistent with the face-to-face fighting observed in howlers. Sociobiological costs of aggression in red howlers are difficult to assess because many howlers, despite frequent and severe injury, subsequently survive and reproduce. We recommend caution in inferring mortality from injuries, especially when social mobility and emigration out of the study area are common.