Like most mammals, lemurs are usually colorblind. However, a small number of female lemurs see in color. A study of how this trait survives has helped illuminate the evolution and survival of color vision. It also reveals the way rare traits within a group benefit social animals.
Most humans, even those with partial color blindness, are trichromatic, having three types of cone cells in our eyes that detect different wavelengths of light. Being able to see color can be very useful, but there is a price to pay for it, possibly in loss of edge detection and capacity to spot camouflaged predators. Consequently, even many mammals with trichromatic ancestry have lost a cone type, leaving them unable to distinguish colors towards the middle of the spectrum.
Curiously, some lemurs, including Madgascar's Verreaux's sifaka (Propithecus verreauxi), have a trichromatic minority, which are always female. Professor Rebecca Lewis of the University of Texas, Austin, found color vision pays off for these rare individuals during hard times, and their entire troop (or conspiracy if you prefer) benefits as well.
"Previously, researchers trying to understand why trichromacy evolved have focused on color vision in New World monkeys," said Lewis in a statement. "However, studies of wild monkeys were unable to find clear benefits for trichromacy. Our work on lemurs suggests that being able to distinguish red from green can be particularly important when times are tough and that trichromatic females are better able to feed themselves and their babies."
Irrespective of their own vision, the offspring of trichromatic mothers are 22 percent more likely to survive the May-November dry season when food is scarce and the sifaka typically lose weight.
In Scientific Reports, Lewis went a step further. Verreaux's sifaka live in female-led troops of two to 13 members. Only seven of the 31 females (and no males) Lewis tested were trichromatic. Nevertheless, over nine years of observations, other troop members benefited from the presence of even one member with color vision. During dry seasons, the body mass of dichromatic lemurs was higher if they had a trichromat in their troop than if they did not. This appears to be a consequence of greater and more extended fruit consumption, attributed to the ease with which trichromats can lead the troop to fruiting trees.
As first author, Dr Carrie Veilleux, also of UT Austin, noted that differences in fitness among long-lived animals are often too small to measure easily, so these findings are striking.
More widely, the work demonstrates how diversity usually beats homogeneity. The weirdo who sees strange things, but can't spot a fossa in the bushes, might just prove everyone's savior.
What beautiful eyes you have - even if most of your species-mates can't appreciate them. Hajakely/Shutterstock