Two proboscis monkeys sit on a tree branch. One proboscis monkey sits lower on the branch, its back facing the camera. The other male proboscis monkey holds on to the branch with his arm and his mouth is wide open, making an alarm sound.

This proboscis monkey is sounding an alarm call in a tree in the Kinabatangan River, Borneo. Proboscis monkeys use a range of sounds to communicate information including honks and roars.

Image credits: Karen Foley
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