Male squirrel monkeys made to see in living color

Squirrel monkey in Bolivia. Photo: author

Squirrel monkeys, like many New World Monkeys, are a bit unusual when it comes to their vision. Males are dichromatic, meaning that they do not have the ability to see red wavelengths but can see blue and green wavelengths. Females on the other hand, may be dichromatic or they may be trichromatic like us, able to see blues, greens, and reds. Males are dichromatic because trichromatic vision requires two copies of a certain opsin gene, a gene that is found on the X chromosome. Thus, females, who have two Xs, likely (but not always) carry two copies of the gene. The majority of female squirrel monkey can see shades of green and shades of red. Male monkeys have the gene that codes for seeing green colors but not the gene that allows them to see red.

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

Now, there may be hope for those who have dichromatic color vision and it's thanks to research done in squirrel monkeys. Two male squirrel monkeys have been made to see all three wavelengths.

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


Works cited:
 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.