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

Thomas Marent / Hoolock Gibbon (Hylobates hoolock) in tree, Hoollongapar Gibbon Sanctuary, Assam, India

Sandesh Kadur / Western Hoolock Gibbon (Hoolock hoolock) female carrying newborn on a fig tree, Gibbon Wildlife Sanctuary, Assam, India. Endangered.

Sandesh Kadur / Western Hoolock Gibbon (Hoolock hoolock) male, Gibbon Wildlife Sanctuary, Assam, India. Endangered.

Staffan Widstrand / Wild Wonders of China / Western Hoolock Gibbon (Hoolock hoolock) Gaoligongshan NP, Yunnan province, China

Bernard Castelein / Western Hoolock Gibbon (Hylobates / Hoolock hoolock) male hanging from branch. Endangered. Gibbon Wildlife sanctuary, Assam, India.

Dr. Axel Gebauer / Hoolock gibbon (Bunopithecus / Hoolock hoolock), Itanagar, Arunachal, India, captive

Bernard Castelein / Hoolock Gibbon (Hylobates hoolock) male foraging in a tree. Assam, India, February.

Kevin Schafer / Western hoolock gibbon (Hoolock hoolock) female and infant in tree, Gibbon Wildlife Sanctuary, Assam, India, Endangered

Kevin Schafer / Western hoolock gibbon (Hoolock hoolock) female swinging through canopy, Gibbon Wildlife Sanctuary, Assam, India