The howler monkey's special hyoid bone

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

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

This unique adaptation primarily serves one function: territorial defense through sound. The howling serves to keep rivals at bay without the need for physical confrontation – an effective strategy in the dense jungle.

In adult howler monkeys, the hyoid bone consists of fully ossified tissue – meaning it is true bone, not cartilage. During embryonic development, it is initially cartilaginous but becomes completely ossified during growth. Its shape resembles a porous capsule or hemisphere – perfect as a sound body.

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

Image source: SWISSskulls collection