|Squirrel monkey in Bolivia. Photo: author|
When we refer to color blindness in humans, often what we really mean is that the person has dichromatic color vision. True color blindness in humans, where the person can only see in black and white, is very rare, although something similar does occur in another New World Monkey. Night monkeys have very poor color vision. As their name suggests, these monkeys are active only at night. Thus, seeing reds, greens, and blues isn't very advantageous given their environment. Over time, owl monkeys have evolved so that they have fewer cones, which allow the eye to see colors, and more rods, which allow the eye to distinguish blacks and whites. To further adapt for their mainly nocturnal lifestyle, the size of their eyes has also expanded. Again, being monochromatic is rare in humans, yet dichromatic color vision does occur in one out of twelve men.
|Visible Wavelengths Dichromats lack red cones|
Mancuso and colleagues injected the gene that would allow two male squirrel monkeys to see red into a virus, and then they injected that virus behind the retina. (Review your eye anatomy here.) Two years later, the monkeys have had no side effects and are able to see all three wavelengths. Squirrel monkey brains, like our own, are quite plastic, meaning they can change. These two animals can now distinguish reds from greens and their brains have apparently rewired themselves so that seeing and processing information about the red wavelength can be done.
Why do humans see in all three colors? There isn't a certain answer. Multiple hypotheses have been proposed. Peter Lucas, my former professor, believes our ability to see reds was advantageous in helping our ancestors see young red leaves, which are especially nutritious, against a background that is mostly made up of shades of green (Lucas et al., 1998). Our closest living relatives, chimpanzees and gorillas, consume a diet mainly composed of leaves. Or perhaps we see reds because red fruit is often ripe and therefore we can distinguish ripe fruit against a green background (Allen, 1879; Mollon, 1989; Fleagle, 1999). Yet, if this were the complete answer, wouldn't we expect New World Monkeys, who consume fruit, to see red wavelengths? Another hypothesis suggests that our ability to see shades of red occurred so that male Old World Monkeys could more easily discern the sexual swellings of females that were in estrous (Liman and Innan, 2003).
Regardless of why trichromatic vision evolved in our ancestors, it seems that those humans unable to see all three wavelengths may one day be able to enjoy the Christmas colors following gene therapy. Trials in humans are underway and we have two male squirrel monkeys to thank for that.
Links of interest:
Color vision in primates
Color vision in humans
Night monkeys and their morphology
A molecular view of how human color vision evolved
Allen, G. 1879. The colour-sense: its origins and development. Trubner, London.
Fleagle, J. G. 1999. Primate adaptation and evolution. 2nd ed. Academic Press, San Diego, CA.
Liman, E. R., & Innan, H. (2003). Relaxed selective pressure on an essential component of pheromone transduction in primate evolution. Proceedings of the National Academy of Sciences, 100(6), 3328-3332
Lucas, P. W., Darvell, B. W., Lee, P. K. D., Yuen, T. D. B., & Choong, M. F. 1998. Colour cues for leaf food selection by long-tailed macaques (Macaca fascicularis) with a new suggestion for the evolution of trichromatic colour vision. Folia Primatologica, 69(3), 139-154.
Mancuso, K. et al. Nature advanced online publication, doi:10.1038/nature08401 (2009).
Mollon, J. D. 1989. “Tho'she kneel'd in that place where they grew…” The uses and origins of primate colour vision. Journal of Experimental Biology, 146(1), 21-38.