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

Andrew Walmsley / Male Siamang gibbon (hylobates / Symphalangus syndactylus) vocalising, with throat pouch extended, Captive, New Zealand, Endangered species

Andrew Walmsley / Male Siamang gibbon (hylobates / Symphalangus syndactylus) vocalising, with throat pouch extended, Captive, New Zealand, Endangered species

Andrew Walmsley / Pair of Siamang gibbon (Hylobates / Symphalangus syndactylus) vocalising together, note the male's extended throat pouch, Captive, New Zealand, endangered species

Eric Baccega / Two Siamang Gibbons (Hylobates / Symphalangus syndactylus) calling, vocal pouches inflated, Endangered, captive, from SE Asia

ZSSD / Siamang (Hylobates syndactylus) is an arboreal, black furred gibbon native to the forests of Malaysia, Thailand, and Sumatra

Anup Shah / Siamang gibbon (Hylobates / Symphalangus syndactylus) male hanging from arm, captive, Endangered

Rod Williams / Siamang gibbon {Hylobates / Symphalangus syndactylus} young female feeding, captive, from Malaysia and Sumatra, Endangered

Roland Seitre / Siamang (Symphalangus syndactylus) hanging from branch over water to drink. Captive, occurs in Indonesia, Malaysia, Thailand. Endangered species.

Roland Seitre / Siamang (Symphalangus syndactylus) hanging from branch over water to drink. Captive, occurs in Indonesia, Malaysia, Thailand. Endangered species.

Roland Seitre / Siamang (Symphalangus syndactylus) hanging from branch over water to drink. Captive, occurs in Indonesia, Malaysia, Thailand. Endangered species.

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