The catarrhines (Old World monkeys – Cercopithecidae, and apes – Hominoidea) evolved colour vision, possibly to detect the best leaves and fruit. They also exhibit a great diversity of facial patterns and hues.

Charles Darwin was very interested in primate colouration, writing about it in The Descent of Man, and Selection in Relation to Sex, The Expression of the Emotions in Man and Animals and in an article published in Nature in 1876.

Further work on the subject remained thin on the ground until the start of this century.

Dr James Higham, from New York University, US, says digital photography and sophisticated computer applications have allowed scientists to finally quantify the huge variety of primate features.

His team has carried out in-depth investigations of specific species, including looking at facial colouration in drills and rhesus macaques, as well as sexual swellings in olive baboons.

Male drills (Mandrillus leucophaeus) differ significantly from females, being three times heavier with larger canine teeth and extremely bright sexual skin coloration. Researchers examining the reasons for this discovered that colour is a status symbol for males, with the strongest hues reserved for higher ranked individuals.

“Though interesting, in many ways these traits resemble other more typical mammalian traits in that they seem to be a result of male-male competition,” Dr Higham says.

Mammals have evolved lots of weapons, such as antlers, horns and large canines, aimed at improving male success in fights with other males.

In contrast, birds and other clades have evolved ornamental features such as colorful crests, feathers and patterns, aimed at attracting females.

Mandrills' (Mandrillus sphinx) bright red colour has been shown to be both related to rank and attractiveness to females independently.

Even more surprising are Rhesus macaques (Macaca mulatta), which display red skin colouring around the face, as well as the genital and hind-quarter areas, but do not follow the usual mammalian pattern of sexual adornments at all.

This is much more like the mate choice ornaments exhibited by birds than the typical mammalian trait

In 2003 a UK-based research team found that females preferred darker red faces, suggesting that colour indicated quality.

Dr Higham’s team followed up this research to show rhesus macaques colouring is not related to male dominance rank, rather it appears to be much more driven by female mate choice.

“An ornament shown by males to attract females and used in mate choice is extremely interesting," Dr Higham says.

“This is much more like the mate choice ornaments exhibited by birds than the typical mammalian trait.”

More recently a team led by Dr Constance Dubuc showed that darker skin colouring in rhesus macaques was linked to reproductive success, in both males and females, and also that the deeper reds were passed down through the generations.

“Showing that colouration is heritable and related to measures of reproductive fitness are essential elements of showing that the signal is under selection,” adds Dr Higham.

“But it’s very hard to do as it requires measures for a large number of individuals combined with an extensive genetic parentage database.”

Dr Higham’s team has also studied the differences that occur between species, pioneering the use of human facial recognition technology on non-human faces.

They tested Oxford zoologist Jonathan Kingdon’s suggestion that the guenons' (Cercopithecini) variety of facial appearances was due to their need to identify their own species and avoid mating with others.

They found that guenons' looks have evolved to become more distinctive from relatives living close by, backing up Mr Kingdon’s explanation, which had stood unsubstantiated since the 1980s.

Dr Higham is now hoping to find out exactly which features are the most useful in telling species apart. His team is also collaborating on projects investigating other Old World monkeys – including perhaps the most interesting face of all – that of the black and white snub-nosed monkey.