Gibbons are our close relatives. Thus their living status is highly concerned to human beings. Hainan gibbon is an endemic species in China and is only distributed in the Bawangling area of Hainan Tropical Rainforest National Park, China. Only 7-9 Hainan gibbons were left in rainforest remnants on the Hainan island in the 1980s due to massive deforestation for plantations and poaching. Hainan gibbons are listed as critically endangered on the IUCN Red List of Threatened Species and topped the 25 Most Endangered Primate Species list in 2007. In the 1980s, the Chinese government established the Bawangling Nature Reserve to protect the natural forest and carry out daily monitoring and patrol of the remaining Hainan gibbon and its habitats. The authority also mobilized and encouraged domestic and foreign research institutions, all walks of life, and non-governmental organizations to participate in the research and protection of the Hainan gibbon. Consequently, the Hainan gibbon population started to grow slowly. Thirteen gibbons were discovered in 2003.

To study the factors affecting the growth of the gibbon population in Hainan, researchers from the Institute of Zoology, Chinese Academy of Sciences, conducted field research with Hainan Tropical Rainforest National Park, Hainan Institute of National Park, and other relevant institutes. In November and December of 2020 and 2021, they performed two synchronized surveys on the Hainan gibbons in the Hainan Tropical Rainforest National Park. During each survey, more than 40 investigators were divided into eight groups and systematically carried out field trips. Every morning, the survey team members, with their ears fully erected, listened to the song of the gibbons, searched for gibbons in the rainforest following the vocals, surveyed all potential habitat patches of Hainan gibbons, and carried out field total counting. They also compared the daily monitoring data with previous census data of the Hainan Tropical Rain Forest National Park.

They found 33 Hainan gibbons in the 2020 survey and 35 in 2021. The gibbons lived in five family groups designated A, B, C, D, and E., In the 2020 survey, four solitary gibbons were found near the family groups A, B, C and E. In 2021, six solitary gibbons were found near above family groups, which will be the basis for establishing a new family group of Hainan gibbons. Two solitary gibbons were discovered in 2019 but have not been seen again in the 2020-2021 synchronized survey, and their survival status remains to be determined. The current population of Hainan gibbons has increased by 169%, from 13 gibbons in two groups in 2003.

Based on the field survey data, they established a population model to fit the population dynamics of Hainan gibbon from 2003 to 2021. They analyzed the reproductive capacity of the Hainan gibbon. It was found that the Hainan gibbon population has not yet reached its full reproductive potential (theoretically, all adult females can give birth every 24 months). In January 2022, the field patrol team of the Bawangling Sub-Bureau of Hainan Tropical Rainforest National Park found another baby gibbon born in group D during their daily patrol, which once again renewed the number of gibbons in Hainan, indicating that the gibbon population in Hainan still has growth potential. However, only 47% of Hainan gibbons' reproductive potential was realized from 2017-2021. Due to the intrinsic and external factors, the Hainan gibbon did not realize its potential reproductive potential. External factors of the population, such as available habitat in terms of foods and trees for night sleeping, as well as the intrinsic factors such as nutritional, physiological, and behavioral factors of Hainan gibbons themselves, may limit the further realization of their reproductive potential, and further study of these influencing factors is necessary.

Since the establishment of the Bawangling nature reserve, regular patrols, banned logging, curb poaching, community education, encouraging the public to participate in, carry out environmental education, and a series of practical conservation actions, specific flagship species of tropical forests in Hainan gibbon recovery proves this point, on October 12, 2021 state national park authority have officially announced that the establishment of Hainan Tropical Rainforest National Park with the Bawangling Nature Reserve as an integrated part, It has further strengthened the conservation of island type tropical rainforest biodiversity with Hainan gibbon as the flagship species. The population size of the Hainan gibbon is still extremely small. However, the Hainan gibbon is the only gibbon with its population continues to increase in the world; it is still in a critically endangered state and has not yet gotten rid of the danger of extinction; make careful conservation actions, including a forest restoration plan, to ensure the sustainable survival of Hainan gibbon.

The study is published online in the International Journal of Primatology. Ph.D. candidate Quo-qi Liu of the Institute of Zoology, Chinese Academy of Sciences is the first author, Professor Zhigang Jiang is the corresponding author, co-authors of the paper including those from Guizhou Normal University, Hexi University, Hainan Tropical Rain Forest National Park, Hong Kong's Kadoorie Farm Botanical Garden, the George Washington University, Northwestern University and Institute of Hainan National Park. The project was supported by the Key Research and Development Program of the Hainan Provincial Department of Science and Technology (No. ZDYF2020179) and the Strategic Pilot Program of the Chinese Academy of Sciences (Type A, No. XDA19050204).

Paper link https://doi.org/10.1007/s10764-022-00309-8

Contact: Zhigang Jiang, jiangzg@ioz.ac.cn