Sandesh Kadur / Western hoolock gibbon (Hoolock hoolock) male hanging from branch, Assam, India.

Tim Laman / Skywalker hoolock gibbon (Hoolock tianxing) formerly described as Eastern hoolock gibbon (Hoolock leuconedys) Gaoligong Mountains National Nature Reserve, Yunnan Province, China.

Tim Laman / Skywalker hoolock gibbon (Hoolock tianxing) formerly described as Eastern hoolock gibbon (Hoolock leuconedys) Gaoligong Mountains National Nature Reserve, Yunnan Province, China.

Staffan Widstrand / Wild Wonders of China / Eastern hoolock gibbon (Hoolock leuconedys) hanging from a branch in its arms, resting between forays through the rainforest, Gaoligongshan Nature reserve, Yunnan, China

Bernard Castelein / Hoolock Gibbon (Hylobates hoolock) male hanging from a branch. Assam, India, February.

Kevin Schafer / Western hoolock gibbon (Hoolock hoolock) female with young swinging through tree, Gibbon Wildlife Sanctuary, Assam, India, Endangered species

Xu Jian / White browed gibbon hanging {Hylobates hoolock} Yunnan, Chin

Dong Lei / Hoolock gibbon (Hoolock tianxing) vocalising whilst sitting in tree. Gaoligong Mountain National Nature Reserve, Yunnan Province, China.

Patricio Robles Gil / Hoolock gibbon (Hylobatidae hoolock) female Kaziranga National Park, Assam, India.

Patricio Robles Gil / Hoolock gibbon (Hylobatidae hoolock) female with infant, Kaziranga National Park, Assam, India.

Staffan Widstrand / Wild Wonders of China / Skywalker hoolock gibbon (Hoolock tianxing) perched in a tree in Xiang Bai village, Dehong, Yunnan, China

Staffan Widstrand / Wild Wonders of China / Skywalker gibbon monkey (Hoolock tianxing) hanging from tree, Tongbiguan Nature Reserve, Dehong prefecture, Yunnan province, China, May.