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.