As their name suggests, vocal communication forms an important part of their social behavior. They have an enlarged basihyal or hyoid bone which helps them make their loud vocalizations. Group males generally call at dawn and dusk as well as interspersed times throughout the day. The main vocals consist of loud, deep guttural growls or "howls." Howler monkeys are widely considered to be the loudest land animal. According to Guinness Book of World Records, their vocalizations can be heard clearly for 20 miles (32 km). It is hypothesized that the function of howling relates to intergroup spacing and territory protection, as well as possibly mate-guarding.
These large and slow moving monkeys are the only folivores of the New World monkeys. Howlers eat mainly top canopy leaves, together with fruit, buds, flowers, and nuts. They need to be careful not to eat too much of certain species of mature leaf in one sitting, as some of the leaves they eat contain toxins that can poison the monkey.[6] Howler monkeys are also known to occasionally raid birds nests and chicken coops and consume the eggs.
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