Colorful primates don't have better color vision, study finds

The findings, published this week in the Biological Journal of the Linnean Society, suggest that red skin and/or red-orange fur may be beneficial for use in social communication even in primate species that don't have particularly good colour vision.

It's long been assumed that primates' colourful skin and fur is linked to their enhanced colour vision, and the results may have implications for understanding why these traits exist in different species.

Lead author Robert MacDonald from the University of Bristol explained: "There is a profusion of colour in the animal kingdom -- think of the striking feathers of a bird of paradise, or the array of vivid hues on display in a coral reef.

"Mammals, though, don't tend to be so colourful, and are usually quite muted shades of black, brown, or grey.

"Primates such as monkeys, apes and lemurs are the exception to this. Several primate species have really vibrant coloration, in particular bright red skin on the face or anogenital region which can change intensity to signal things like fertility or rank in the dominance hierarchy, or red-orange fur.

"Primates also happen to have unusually good colour vision in comparison to other mammals; while all other mammals are red-green colourblind, meaning red and green appear as the same colour to them, some primates (including humans) can differentiate between shades of red and green. This enhanced colour visual system is generally thought to have evolved in order to more easily spot ripe red fruit or nutritious young red leaves among foliage, but it also makes it easier to spot the vibrant red colours that some primates exhibit."

Primates are known to use their red colour traits for communication with other members of their species, for example in signalling information about fertility or rank in the social hierarchy. It seems intuitive that having a better colour visual system that allows these traits to stand out more might have facilitated the evolution of these traits in the first place -- it would make sense for a species with better colour vision to evolve to be more colourful to take advantage of this ability.

The team set out to definitively investigate whether the evolution of enhanced colour visual system in some primates that allows the differentiation of red from green has facilitated the evolution of red colour traits.

Using photographs, the researchers categorised each species of primate in terms of having or not having particular colourful traits (e.g. red skin on the genital region or face, red-orange fur on different parts of the body). They then compared this colour information with each species' colour visual ability, taking into account the primate family tree, as well as a few other factors which might also influence coloration or colour visual ability such as whether they're nocturnal or diurnal and the size of the social group they live in. The aim was to find out whether species that have better colour vision are more likely to have red colouration, after controlling for other potential influencing factors.

Robert explained: "The fact that we didn't find that species with better colour vision are more likely to be colourful contradicts some long-held assumptions about the origins of the striking variation in colour we see within primates, and means we might have to take a closer look about what colourful red skin or fur is being used for in individual species. It shows that despite the large amount of work that has gone into investigating primate colouration in recent years, we still don't fully understand the pressures that have shaped the evolution of colour in our own closest relatives."