One of the four species of monkeys seen around El Remanso, howlers have a large throat sack and large hollow hyoid bone that act to amplify the sound from the vocal cords. They are the loudest land animal, with their calls clearly audible up to 3 km away.
Note the prehensile tail wrapped around the tree. It can easily support their body weight, and gives them a "fifth limb" as they navigate through the trees.