According to the Wikipedia article:
Mandrills are noted as being exceptionally colourful for mammalian standards. Charles Darwin wrote in The Descent of Man that "no other member in the whole class of mammals is coloured in so extraordinary a manner as the adult male mandrill's".
So I know that in many species sexual dimorphism can act as a feature which is selected by sexual selection, I'm not surprised because there are other examples. But the male mandrill seems exceptional, especially according to what I've read, among mammals. Seems very curious to me, especially its face.
I was wondering if there are mainstream explanations for why the mandrill in particular should have such striking colours maybe more than any other mammals.