Living deep in the rainforest of Hainan Island in south China is a group of Hainan gibbons. As the only gibbon species on the planet to currently be experiencing a steady population resurgence, they serve as living testament to what can be achieved when humans live in harmony with their natural surroundings. But as one of Earth's most agile primates, the Hainan gibbons present a huge challenge to the guardians who are tracking and protecting them.