Determinants of female dispersal in Thomas langurs.

Female dispersal occurs in a number of primate species. It may be related to: avoidance of inbreeding, reduction in food competition, reduction of predation risk, or avoidance of infanticide in combination with mate choice. Female dispersal was studied for a 5-year period in a wild population of Thomas langurs (Presbytis thomasi) that lived in one-male multi-female groups. Juvenile and adult individuals of both sexes were seen to disperse. Females appeared to transfer unhindered between groups, mostly from a larger group to a recently formed smaller one. They transferred without their infants and when not pregnant, and seemed to transfer preferentially during periods when extra-group males were harassing their group. During these inter-group encounters extra-group males seemed to try to commit infanticide. Thus, the timing of female transfer was probably closely linked to infanticide avoidance. Moreover, females seemed to transfer when the resident male of their group was no longer a good protector. The observations in the present study suggest that females transferred to reduce the risk of infanticide. Female dispersal may have another ultimate advantage as well, namely inbreeding avoidance. Due to the dispersal of both females and males the social organization of Thomas langurs was rather fluid. New groups were formed when females joined a male; male takeovers were not observed. Bisexual groups had only a limited life span, because all adult females of a bisexual group could emigrate. This pattern of unhindered female dispersal affects male reproductive strategies, and in particular it might lead to infanticidal behavior during inter-group encounters.