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

Anup Shah / Siamang gibbon male calling, pouch inflated

Anup Shah / Grey gibbon portrait

Rod Williams / Young male Black Concolor gibbon portrait {Hylobates concolor} occurs Vietnam, N Laos, Isle of Hainan

Rod Williams / Face portrait of female Black gibbon, occurs SE Asia

Robin Chittenden / Siamang Gibbon (Symphalangus syndactylus) Thrigby Hall zoo, Norfolk, England, UK, November.

Bernard Castelein / Malayan Giant Squirrel (Ratufa bicolor) climbing tree, Gibbon Wildlife Sanctuary, Assam, India

Matthew Maran / Black gibbon {Hylobates concolor} captive female, Endangered Primate Rescue Center, Cuc Phuong National Park, Vietnam

Matthew Maran / Black gibbon {Hylobates concolor} captive female, Endangered Primate Rescue Center, Cuc Phuong National Park, Vietnam

Matthew Maran / Black gibbon {Hylobates concolor} captive female, Endangered Primate Rescue Center, Cuc Phuong National Park, Vietnam

Anup Shah / Siamang gibbon male displaying

Anup Shah / Grey gibbon

Anup Shah / Grey gibbon female and juvenile

Anup Shah / White-handed gibbon juvenile in tree, Singapore Zoo

Dave Watts / Female Crested black / Concolor gibbon {Hylobates concolor} standing up on hind legs. Endangered species native to the forests of South China to Vietnam.

Roland Seitre / Kloss's gibbon (Hylobates klossii) female sat on ground. Captive, endemic to the Mentawai Islands off the west coast of Sumatra, Indonesia. Endangered species.

Joel Sartore / Photo Ark / Southern yellow cheeked gibbon (Nomascus gabriellae) male, head portrait, Singapore Zoo. Captive, occurs in Southeast Asia. Endangered.

Joel Sartore / Photo Ark / Agile gibbon (Hylobates agilis agilis) head portrait, Singapore Zoo. Captive, occurs in Indonesia. Endangered.

Joel Sartore / Photo Ark / Pileated gibbon (Hylobates pileatus) male, portrait, Gibbon Conservation Center, California. Captive, occurs in Southeast Asia. Endangered.

Joel Sartore / Photo Ark / Northern yellow-cheeked gibbon (Nomascus annamensis) male, portrait, Endangered Primate Rescue Center, Vietnam. Captive. Endangered.

Joel Sartore / Photo Ark / Siamang (Symphalangus syndactylus) head portrait, Los Angeles Zoo. Captive, occurs in Southeast Asia. Endangered.

Joel Sartore / Photo Ark / Southern white-cheeked gibbon (Nomascus siki) male, head portrait, Endangered Primate Rescue Center, Vietnam. Captive. Endangered.