October 24 marks International Gibbon Day, a global celebration of these adorable, singing and acrobatic primates. This day is also important because they are the most endangered of group of apes, according the World Wildlife Fund.

There are 20 different species of gibbons and they are endemic to the tropical and subtropical rainforests of south Asia. This includes the Hainan gibbon—the rarest primate in the world and found only on Hainan Island in China. Their numbers dipped as low as 8 in the 1980s. But through conservation efforts, there are now 27 individuals in the wild, said Wenbo Zhang, who works with Cloud Mountain Conservation, an NGO that focuses on gibbon conservation in China.