What a maleless group can tell us about the constraints on female transfer in Thomas's langurs (Presbytis thomasi)

During a long-term study of wild Thomas's langurs (Presbytis thomasi), the adult male of one of the bisexual study groups, group M, suddenly disappeared. Just after the adult male's disappearance, the females of group M actively avoided other groups and sometimes formed subgroups. Infants were attacked by extra-group males and 1 infant disappeared. After about 2 months, the females initiated affiliative interactions with males of neighbouring groups. Aggressive interactions between females of group M and females of other groups were also observed. Females of group M did not leave their home range in spite of efforts by adult males to lead them away. After about 4.5 months, a former all-male band member joined group M. The coincidences of this case suggest that increased infanticide risk, increased food competition, loss of knowledge of the home range and loss of established social relationships could be important constraints on female transfer. A comparison with 5 other studies of 1-male groups in which the adult male disappeared showed that proximate factors, such as the presence of infants, the size of neighbouring groups and the presence of extra-group males, influence female decisions.