We all see the world differently—some of us, in vivid, varying shades and combinations of red, green, and blue. But, a small portion of the human population, most of them male, have has trouble distinguishing red from green.
Among squirrel monkeys, though, that exception is the rule: all males have red-green color deficiency.
The X-Chromosome Advantage
Among humans, color deficiency occurs because the specialized eye cells responsible for sensing color lack photoreceptors, called opsins, for red or green. When that happens, red and green apples set side by side—or football players wearing red and green jerseys—appear nearly the same: grey.
In humans, the red and green opsins are found in two locations on the X chromosome. Males have one X chromosome, while females have two. So, males have one shot at getting both red and green opsins on their X chromosome. If they inherit two green opsins, or two red opsins, rather than a green-red combination, they end up with color deficiency.
Like humans, squirrel monkeys (Sairmiri sciureus) also carry opsin genes for color vision on their X chromosomes. However, researchers think that this species only carries the opsin protein in one location on the X chromosome. Females, with their two X chromosomes, can inherit genes coded for both red and green vision—though they, too, sometimes strike out and end up seeing only two colors. But some inherit both, and they’re the ones who can see in full-color. Males, with only a single X chromosome on which to inherit either the red or the green opsin, are all born with two-color vision.
Color, or Shape?
In a 2003 study led by University of Stirling primatologist Andrew Smith, researchers found that primates who were trichromats could spot ripe fruit better than dichromats—not surprising when raw fruit tends to be green, and ripe fruit darkens into shades of red.
But since dichromacy—or two-color vision—remains prevalent in species such as squirrel monkeys and tamarins, findings suggest that some primates remain dichromats because they’re more adept at identifying shapes and forms and better at procuring other food—for example, catching prey—than their trichomatic peers.
Correcting Color Deficiency
Since color deficiency stems from a missing gene, researchers are testing to see if gene therapy may someday correct the condition. Early results are promising.
In the 2009 study published in the journal Nature, Jay Neitz of the University of Washington and his colleagues injected red opsin protein into the retinas of two male squirrel monkeys. Within five months, both male monkeys gained the ability to see in three colors, retaining their full color vision for several years.
Learn more about the senses of different species in our new exhibition Our Senses: An Immersive Experience.