Mature males are black, while the young and females are pale gray. Males are also larger than females, weighing about 16.5 lbs. Howlers have greatly enlarged lower jaws which accommodate egg-shaped resonating chambers. These chambers enable howlers to make their unique reverberating calls. The calls warn nearby troops to keep away from feeding areas.
Howlers have prehensile tails which are used mostly for balance or a sense of security.
The tails have naked skin on the under surface of the tip. A baby howler's tail is not functional until about 4 weeks of age. Howler monkeys are slow moving, thus easily falling prey to humans, where they are hunted for the large amount of meat on their bodies. Most of their time is spent in trees. They live in groups of from 8 to 20, with females outnumbering males.