Reproductive Characters and Mating Behaviour of Wild Nomascus hainanus

We report for the first time the reproductive behaviors of Hainan gibbons (Nomascus hainanus), based on 29 mo of field observations. Receptive females initiated courtship displays. Copulatory patterns involved the male mounting dorsoventrally from above and behind. Multiple intromissions with thrusting were brief, lasting <10 s per copulation. We observed multiple copulations ≤4 times/d, but could not confirm multiple ejaculations. We also observed postconception proceptivity and copulations in sexually active females. We estimated the gestation period to be 136–173 d. Infants became independent at 1.5 yr, and the natal group could drive out maturing offspring at ca. 5.5 yr. The interbirth interval is ca. 24 mo. Our limited data also suggest that mating activities peak in the rainy season. The Hainan gibbons are polygynous, with a 1 male-2 females mating system. We hypothesize that suboptimal habitat quality and limited forest area may contribute to the current mating structure, but more work needs to be done over a longer period to understand better the sociosexual behaviors of this critically endangered species.