One-male unit (OMU) is the basic social unit in multi-level societies of the Sichuan snub-nosed monkeys (Rhinopithecus roxellana). From October, 2001 to December, 2005, we studied dominance relationships between OMUs in a free ranging group in the Qinling Mountains, central China. The group was comprised of 6 to 8 OMUs that were cohesively associated. We analyzed a total of 2366 replacement interactions among these OMUs during eight different study periods. The results suggested a linear dominance relationship among the units in each study period. We suggest three factors that may influence dominance relationships among units: long-term association, competition for food trees and influence of provisioning. The results showed time positive ranks in the group, which is units associating for longer time in the group tend to have higher dominance rank. In addition, we reported for the first time that two cases of 'merger of OMUs', in which one resident male replaced the other, and merged two units into one. We discussed the dynamics of merger of OMUs.
