additional subspecies of the mantled howler are sometimes recognised, but these are more generally recognised as subspecies of the Coiba Island howler, Allouatta coibensis. However, mitochondrial DNA testing of their status has been inconclusive: Azuero howler, Alouatta palliata trabeata, in Panama; Alouatta palliata coibensis, in Panama. Physical description The mantled howler's appearance is similar to other howler monkeys of the genus Alouatta except for coloration. The mantled howler is primarily black except for a fringe of yellow or golden brown guard hairs on the flanks of the body earning the common name "mantled" howler monkey. When the males reach maturity, the scrotum turns white. Females are between in body length, excluding tail, and males are between . The prehensile tail is between long. Adult females generally weigh between , while males typically weigh between . Average body weights can vary significantly between monkey populations in different locations. The brain of an adult mantled howler is about , which is smaller than that of several smaller monkey species, such as the white-headed capuchin. The mantled howler shares several adaptations with other species of howler monkey that allow it to pursue a folivorous diet, that is, a diet with a large component of leaves. Its molars have high shearing crests, to help it eat the leaves, and males have an enlarged hyoid bone near the vocal cords. This hyoid bone amplifies the male mantled howler's calls, allowing it to locate other males without expending much energy. Behavior Social structure The mantled howler lives in groups. Group size usually ranges from 10 to 20 members, generally 1 to 3 adult males and 5 to 10 adult females, but some groups have over 40 members. Males outrank females, and younger animals of each gender generally have a higher rank than older animals. Higher-ranking animals get preference for food and resting sites, and the alpha male gets primary mating rights. Animals in the group are generally not related to each other because most members of both sexes leave the group before sexual maturity. Grooming activity in the mantled howler is infrequent and has been shown to reflect social hierarchy, with dominant individuals grooming subordinates. Most grooming activities are short and are typically females grooming infants or adult males. Aggressive interactions between group members are not often observed either. However, studies have shown that aggressive interactions among group members do occur, and are probably not often