The “new world” monkeys of South and Central America range from large muriquis to tiny pygmy marmosets. Some are cute and furry, others bald and bright red, and one even has an extraordinary moustache. Yet, with the exception of owl and howler monkeys, the 130 or so remaining species have one thing in common: a good chunk of the females, and all of the males, are colour blind.
This is quite different from “old world” primates, including us Homo sapiens, who are routinely able to see the world in what we humans imagine as full colour. In evolutionary terms, colour blindness sounds like a disadvantage, one which should really have been eliminated by natural selection long ago. So how can we explain the continent of the colour blind monkeys?
I have long wondered what makes primates in the region colour blind and visually diverse, and how evolutionary forces are acting to maintain this variation. We don’t yet know exactly what kept these seemingly disadvantaged monkeys alive and flourishing – but what is becoming clear is that colour blindness is an adaptation not a defect.
Pygmy marmoset. Coldmoon Photoproject/Shutterstock
The first thing to understand is that what we humans consider “colour” is only a small portion of the spectrum. Our “trichromatic” vision is superior to most mammals, who typically share the “dichromatic” vision of new world monkeys and colour blind humans, yet fish, amphibians, reptiles, birds and even insects are able to see a wider range, even into the UV spectrum. There is a whole world of colour out there that humans and our primate cousins are unaware of.
Yet while the eyes of insects and mammals look very different, they work in a remarkably similar way. Both capture and process electromagnetic waves reflected from objects or radiated from luminous sources. Both their eyes contain cells called rods and cones. Rods are specialised for low light levels, providing a sort of night vision. Cones are responsible for colour vision, balancing blue, red, and green to provide the perception of the visual spectrum of light. A problem in any cone type causes problems with colour perception.