Range expansion as a response to increasing group size in the Yunnan snub-nosed monkey.

The Yunnan snub-nosed monkey (Rhinopithecus bieti) is characterized by a larger home range and group size than other colobine species. We investigated variations in home range size of a free-ranging group of R. bieti from 1998 to 2007 in the Baimaxueshan National Nature Reserve in Yunnan, China. Group size increased from 160 to 450 over the 10 years. Results also showed a home range shift and yearly home range expansion, although this expansion was very limited. The study group covered a 33.78-km(2) area during the 10-year period, with some areas abandoned and others used repeatedly. The yearly home range increase stopped in 2004 and decreased thereafter. Despite increasing group size usually being coupled with larger yearly home range (>30 km(2)), the group confined itself within a relatively small and stable area (<18 km(2)) during 2005-2007, with a significant portion of their accessible home range discarded. Consequently, the study group showed the highest recorded population density among extant groups of R. bieti. Reasons why the group settled into a substantially smaller home range area remain unclear.