Behavior

All species live in multimale, multifemale groups, although one-male, multifemale groups of Alouatta are also common. In the three well-known atelin genera (Ateles, Brachyteles, and Lagothrix), males are philopatric, while females disperse from their natal groups to join other groups of males. In Alouatta, both males and females disperse from their natal groups, usually to establish new troops. Female red howler monkeys (Alouatta seniculus) may be retained in their natal troops, while males sometimes disperse in pairs to establish new troops together.

Alouatta are renowned for their loud, long-distance roars. Neighboring troops engage in howling displays, often, but not exclusively, at the boundaries of their ranges. Howler monkey calls can be heard by humans as far as 0.6 mi (2 km) away. Ateles and Brachyteles have large repertoires of vocalizations, including a long-distance call that resembles a horse's whinny, and an alarm call that resembles a dog's bark. They also have softer, less far ranging "chuckles," which may help them to maintain contact with one another while they are spread out during travel and foraging.

None of the species defend exclusive territories, although encounters between groups, especially of Alouatta, can be highly aggressive. In Lagothrix, Ateles, and Brachyteles, large neighboring groups exploit large, overlapping home ranges. Home range overlap is greater at high population densities. Home range sizes vary from 25 acres (10 ha) in Alouatta to over 2,220 acres (900 ha) in woolly monkeys and muriquis living in continuous forest in the Amazon and southern Atlantic forest, respectively.

All of the genera are primarily diurnal. Observers that leave the monkeys asleep at dusk often find them in the exact same positions the next morning. Activity patterns differ with climate and season. In general, bouts of morning traveling and feeding are followed by mid-day siestas, and then more traveling and feeding before the groups settle down for the night. Howler monkeys devote up to 70% of their daylight hours to resting, and travel shorter distances each day than the other genera. Spider monkeys and muriquis devote about half of the day to resting, and can travel up to 1.9 mi (3 km) in a day. Woolly monkeys are intermediate in their resting and traveling habitats, at least at the sites where they have been studied to date.

The Venezuelan red howler monkey (Alouatta seniculus) lives in groups of about ten. (Photo by Rick Sullivan. Bruce Coleman, Inc. Reproduced by permission.)
The Mexican black howler monkey (Alouatta pigra) uses the hyoid bone to amplify its call. (Photo by Tom & Pat Leeson/Photo Researchers, Inc. Reproduced by permission.)
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