Travelers on Amazon River cruises don't need to listen closely to hear the low, rumbling growl of red howler monkeys. Aptly named, these primates, native to Central and South America, can be heard up to three miles away.

The monkeys have large throats with a shell-like hyoid bones, a U-shaped bone between the base of the tongue and the larynx. This allows them to raise their voices to eardrum-blasting volumes, the loudest of all the howler monkeys. Their call, often heard at dawn or dusk, is a territorial call to other primates.

Measuring up to 36 inches with tails of the same length, these animals can weigh up to 22 pounds, making them the largest primate breed in the Western Hemisphere. According to the National Primate Research Center at University of Wisconsin Madison, the monkey’s fur is a brick- or orange-red, and the ventral areas of the animal are darker.

Visitors to the Peruvian Amazon may also spot a red howler on trees or on the ground. Their most important mode of transportation is clambering, though they also walk and run quadrupedally.

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