Abstract: Black-crested gibbon is the least studied species of Family Hylobatidae in field. Several reports demonstrated that it is significantly different from other gibbons, especially on social structure, group composition, and home range. This article presents the findings of a 10-month field study on concolor gibbon in Wuliang Mountain and Ailao Mountain, and the analysis of its mating system. According to our observation, the gibbon is monogamous and polygynous. The reasons are those: 1 .weaker female territoriality and group territoriality than other gibbons permit the male to approach more other adult females, 2. larger territory affords more individuals to live in the same group, 3. the male tends to be polygynous, but it depends on whether the females can benefit from the strategy, 4. concolor gibbon is the earliest form to speciate in Hylobatidae, and perhaps, polygyny in concolor gibbon represents the primitive behavioral character of gibbons, 5. worsened habitat makes the females more competitive for food and resting sites, 6. hunting and predation have direct influence on concolor gibbon's social structure and mating system.

Key words: Black-crested gibbon; Mating system; Territorial behavior; Ecology