Why do mandrills have colourful faces?

Male mandrills develop brilliantly coloured faces as they produce more testosterone with age, and the high ranking males have the brightest faces of all to let the other mandrills know who’s boss.

Mandrills are the biggest species of monkey with males reaching heights of 65 centimetres and they explore the forests of central Africa in groups of up to 600. In the darkness of the trees, their vivid faces and bottoms help mandrills follow each other and ensure no monkey gets left behind.

The electric blue and scarlet face of a dominant male mandrill tells females that he is a fantastic partner and will produce the strongest babies.
Females have some colour on their faces too. Her face and bottom flush a deep pink to tell the male monkeys when she is ready to mate.

You may recognise these creatures from Disney's The Lion King! Rafiki, the wise old monkey who guides Simba is in fact a mandrill!