The howler monkey earned
its name because of its amazingly loud voice. In fact, the howler
monkey is the loudest land animal on the planet. Male howler
monkeys inflate their throat into a resonating balloon, which allows
them to make calls that can be heard up to 3 miles away. It is
thought that the calls of the howler monkey are used to maintain
spacing between different groups of monkeys.
Each social group of howler monkeys
consists of 10-20 members organized in a strict hierarchy, with a
dominant male as the leader. Group members spend nearly 2/3 of
their time sleeping; this is due to the energy-poor diet of the
monkeys. Much like the sloth, howler monkeys feed mainly on
leaves. This means that they are folivores, or leaf-grazers. These
leaves, however, are a poor source of nutrition because most of the
calories in the leaves are bound up in hard-to-digest cellulose. To
gain a sufficient amount of energy, an adult howler monkey thus
needs to ingest more than 15% of its body size per day in leaf
matter.