Why are howler monkeys so loud?

Howler monkeys, which are considered by theĀ Guinness Book Of World RecordsĀ as the loudest land animal, can produce such a loud call as they have an enlarged hyoid bone in their neck.

This bone usually resembles a horseshoe and aids tongue movement and swallowing, but in howler monkeys it has developed into a large shell-like bone in the throat that grants extra, engorged resonance to any produced noise.

The reason howlers utilise this bone so frequently to create loud howls, however, is that it is through this form of communication that territorial boundaries are established, with rival clans marking their territory each day at dawn and dusk.