Big voice means less sperm

The deep, growling roar of the howler monkey may hide reproductive shortcomings, according to biologists.

A study by an international team, led by Cambridge University researchers, has revealed that the primates either develop big, deep voices, or big testes - but not both.

To study the animals' roars, the team measured the volume of a hollow bone in their throats called the hyoid bone, which acts as a resonator. The larger that bone, the deeper their voice.

Comparing anatomy across the species, the researchers noticed that monkeys with the deepest voices also had the smallest reproductive organs.

This clip, courtesy of La Senda Verde Animal Refuge in Bolivia, shows a male red howler monkey demonstrating that impressive howling roar the animals make to intimidate rivals and impress potential mates.