Facts So Romantic

What If Only Females Could See Color?

Photograph by Michael Sale / Flickr

Have you ever wondered how your life might be different if you could see beyond the visible light spectrum—into ultraviolet or infrared? For one thing, you might be immune, or less susceptible, to implicit racial bias. Inna Vishik, an applied physicist at U.C. Davis, says if you weren’t limited to the typical range of colors most humans see, “everyone would be the same color (except for people with a fever)”—yellowish. You’d also be able to know which places have great wifi and cell phone reception, she says, and whether you “*really* should wear sunscreen today.” No doubt if you had this special ability, it would benefit not just yourself, but your family and friends, too.

Something like this scenario has actually been discovered in nature, albeit not with humans. Within a certain population of tree-dwelling primates in Madagascar—Verreaux’s sifaka, a kind of lemur, to be precise—a recent study found, nearly one in four females has trichromatic color vision (like humans). Unlike most other members of their sex—and all the males—these females can tell red and green apart, perceiving color much as we humans do. And the perks of this genetic gift may extend to the entire lemur group, says Carrie Veilleux, a biological and molecular anthropologist at the University of Texas at Austin, and the study’s lead author.

“I wanted to look at other kinds of power—more leverage, economic kinds of power.”

“If you’re a dichromat, you can’t tell ripe red fruit against a green background. It looks just like the green background,” Veilleux says. “The female that’s trichromat would be able to detect that popping out and go toward it.” In their paper, Veilleux and her co-authors argue that the existence of these lemurs support the “benefit of mutual association” hypothesis: These special females can spot juicy patches of fruit in the boughs of their leafy habitats their group might otherwise miss, and swing over to them; and when they do, the rest of the group tags (and snacks) alongside them.

Trichromatic color vision is typically a trait species either have or lack. Today’s humans, apes, and (some) monkeys, for instance, all have three types of cone cells in the retina. They’re sensitive to different parts of the visible light spectrum. The female sifaka, though, show how some animals can chance on trichromacy even if most of the other members are dichromats, meaning they have only two types of cone cells.

“In New World monkeys, and in some lemurs, there’s only one gene that codes for the two different cones,” Veilleux says. If a female sifaka inherits a different form of the gene on each of its X chromosomes—as in humans, only females have two X chromosomes—its eyes are endowed with all three cones, in a phenomenon called “polymorphic trichromacy.” For a female sifaka to be trichromatic, in other words, she needs to inherit each of these versions of the gene, as opposed to, redundantly, two of the same. “It’s crazy to think that one single base-pair difference is what can make something be able to tell red and green apart,” Veilleux says.

That’s why the male sifaka is doomed to what we would call color blindness; with just one X chromosome, it has no chance of scoring both complementary versions (which code for “medium” and “long” wavelength perception) of the critical gene. As for the third cone (“short”), it’s generated autosomally—that is, in a chromosome that all lemurs inherit, regardless of sex.

The study’s authors analyzed nine years’ worth of data on dozens of lemurs for “three potential fitness proxies,” to gauge how much the female trichomats benefited their fellow dichromats: “body mass index during the dry season, reproductive output, and infant survival.” The gains were clear. Lemurs who were either trichromatic, or living alongside trichromats, ended up shedding less body mass than those in dichromatic groups during the May-to-November dry season, a time “when they’re so energetically stressed” from rearing their young with less food available, says Veilleux. Pre-existing observation data also showed that one group of trichromat females and their hanger-ons—the only trichromat group to be tracked in the data set—spent more time feeding on fruit compared to three groups of dichromat lemurs.

The researchers looked at the lemurs’ genetic material to figure out which of them had richer color vision. Enafa Jaonarisoa, a Malagasi specialist, sedated the lemurs from a distance, using a specially treated dart expelled from a blow pipe. (He “can hit a matchbox at 30 feet,” says an anthropologist who has worked with Jaonarisoa, though not on this study). After a minute of wooziness, the lemur drops perhaps a dozen feet from a tree into a sheet held open by a few attendants. Members of the research team take two small biopsy punches from the critter’s ear, conduct measurements and a health check, and “then we release ‘em, and they’re back in maybe three hours, back in their group and doing fine,” says Rebecca Lewis, one of the study’s co-authors who runs the Ankoatsifaka Research Station within the national park these lemurs call home.

Lewis thought the team’s findings lined up pretty well with what they knew about lemurs’ social hierarchy: Among Verreaux’s sifaka, it’s the females who call the shots. They initiate foraging expeditions more often, draw more followers, and take them farther afield than males do. That power discrepancy in color vision is part of what attracted her to participate. “In animals we think of who’s biggest and baddest in terms of power,” Lewis says. “And I wanted to look at other kinds of power—more leverage, economic kinds of power.”

The researchers were quick to explain that their sample size was relatively small, and that they had to be “careful” with their conclusions, according to Lewis. But Brenda Bradley, who directs the Primate Genomics Lab at George Washington University, calls the study exciting. She sees color vision in primates as a textbook example of evolution in progress. “There aren’t that many studies where people actually can look at that in the wild,” Bradley says. “It’s great that they find at least some suggestive evidence that maybe trichromatic color vision is providing some kind of advantage.”

Coincidentally, Bradley had sequenced the genomes of nearly 100 Verreaux’s sifaka in a different Malagasy forest in 2005, as a postdoc. She and her colleagues were specifically looking for color vision, without any luck. “We went out there, collected all these samples, and genotyped them and then we realized, ‘Oh, there is no variation. There is no polymorphism. They’re all red-green colorblind,’” Bradley says. She’s since discovered six more examples of polymorphic trichromacy among other lemur species with her colleagues; the study will be published in Biology Letters. “As primatologists, we have this tendency to characterize what seems biologically true for a species at one site. And you think, ‘Oh well, this is what is true for this species.’ And then to find out that no, it’s completely different when you go a couple hundred miles north into a different type of forest…I think that’s really exciting.”

Pierre Bienaimé is a writer and musician based in New York. He’s on Twitter as @ScribblerSounds.

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WATCH: The philosopher of color Mazviita Chirimuuta on what perceiving color is.

Image credit: Michael Sale / Flickr

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