The special hyoid bone of the howler monkey

Howler monkeys ( Alouatta ) are among the most vocal land animals on Earth. Their deep, piercing calls echo for miles through the tropical rainforest—especially in the early morning hours. The key to this impressive vocal apparatus lies in a special anatomical feature: the hyoid bone.

In howler monkeys, the hyoid bone (os hyoideum) is greatly enlarged, hollow, and particularly prominent in males. While the hyoid bone remains inconspicuous in most primates, in howler monkeys it forms a resonance-enhancing A chamber that acts like a natural loudspeaker, allowing the animal to achieve maximum acoustic impact with minimal effort.

This unique adaptation has one main function: territorial defense by sound. The roar serves to keep rivals away without having to physically confront them—an effective strategy in the dense jungle.

The hyoid bone of adult howler monkeys consists of fully ossified tissue—true bone, not cartilage. During embryonic development, it is initially cartilaginous, but becomes fully ossified as the monkey grows. Its shape resembles a porous capsule or hemisphere—perfect for use as a resonator.

If you want to see these animals in person, visit Basel Zoo—it's the only place in Switzerland where howler monkeys are kept. They are part of the EEP breeding program and live there alongside other South American monkey species.

Image source: SWISSskulls Collection