DISPERSAL, PAIR FORMATION AND SOCIAL-STRUCTURE IN GIBBONS (HYLOBATES-LAR)

We report observations on reproduction, natal dispersal, pair formatio n, and group structure based on longitudinal observations of several w hite-handed gibbon (Hylobates lar) groups spanning 18 years. Our obser vations are at odds with the traditional view that gibbons live in nuc lear family groups consisting of a pair of adults and their offspring, and that parents exclude young from the family territory when they re ach adult size. In the relatively dense Khao Yai study population, dis persing young usually obtain mates by replacing adults in existing ter ritories, which creates non-nuclear families. Six subadults, five male s and one female, matured and dispersed at an average age of 10 years, or about 2 years after reaching adult size. Average natal dispersal d istance was 710 m, or between one and two territories away. At least t wo dispersing males replaced adults in neighboring groups. In one case , forcible displacement of the resident male resulted in a group which included a young juvenile presumably fathered by the previous male, t wo younger juveniles (probably brothers) from the new male's original group, and (later) offspring of the new pair. Social relations within this heterogeneous group remained harmonious: the adults groomed all t he young and play occurred between all preadult members. In only two o ut of a total of seven cases of dispersal seen did two subadults pair and disperse into new territorial space. Nonreproducing subadults whic h delay dispersal may be tolerated by the adults provided that they co ntribute benefits to the adults or their offspring. Possible benefits include behaviors such as grooming, social play with juveniles, and su pport of the adult male in defending the territory. Delayed dispersal is probably advantageous in a saturated environment where there is no room for floaters, but subadults may also gain indirect fitness benefi ts by aiding siblings and other relatives.