Howler monkeys are among the largest of the New World monkeys. Fifteen species are currently recognized…These monkeys are native to South and Central American forests… As their name suggests, vocal communication forms an important part of their social behavior. They each 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 3 mi (4.8 km). The function of howling is thought to relate to intergroup spacing and territory protection, as well as possibly to mate-guarding.
Increase your volume to hear the deep guttural growls or "howls" in this video.