Female dispersal, social organization, and infanticide in langurs: are they linked to human disturbance?

Female dispersal in gregarious animals can involve the desertion of a site, desertion of a social group, or both. Group desertion may be related to inbreeding avoidance. Group fidelity may result from cooperation among females in a group. Site fidelity will be more likely when food can be monopolized and when the population density is close to habitat saturation. The degree of habitat saturation was approximated with a measure of human disturbance. The influence of these various factors on the incidence of female dispersal was investigated for langur populations using data from the literature. The results suggest that female dispersal in langurs involved site desertion, not group desertion. The incidence of female dispersal may affect the social organization of langurs. I propose that when females do not disperse, male takeovers prevail, whereas in populations where female dispersal regularly occurs bisexual groups are disbanded or new groups are formed, a process I call female split-merger. Male takeover is thought to occur when site fidelity is high, female split-merger when site fidelity is low. These processes were indeed found to occur in these circumstances. The dispersal of females might prevent infanticide, whereas male takeover might promote it. Indeed, in studies with male takeover, more infants fell victim to infanticide than in studies with female split-merger. Therefore, female dispersal in langurs is an effective female counterstrategy to infanticide. The factor that had the most profound effect on female dispersal, social organization, and infanticide was habitat saturation. Habitat saturation was measured as the degree of human disturbance, and its influence on the behavior of langurs is probably of relatively recent date. This may lead to an evolutionary transient situation and may explain the discrepancy between current socioecological theories and the behavior of langurs in populations lacking female dispersal.