Social organization and male residence pattern in Phayre's leaf monkeys

The genus Trachypithecus (Colobinae, Presbytini) has previously been characterized by one-male groups and both male and female dispersal. Occasionally, males may mature in their natal groups, resulting in so-called age-graded multi-male groups. Our long-term observations of a population of Phayre's leaf monkeys (Trachypithecus phayrei crepusculus) in Thailand, while revealing values in group size and composition similar to other species, indicate a hitherto undescribed social organization, in which males mature and breed in their natal group or disperse and form new groups. Groups are not age-graded and multi-male groups are one phase of a dynamic social organization changing between multi-male and one-male constellations. The ways in which our views of the social organization of Phayre's leaf monkeys changed over a period of eight years underscore the importance of long-term studies for a full understanding of the behavioral ecology of long-lived species like primates.