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Snake-Spotting Theory Brings Primate Vision into Focus

Do camouflaged predators explain why monkeys, apes and other primates evolved superior eyesight?

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Scientists are still trying to figure out why primates have excellent vision. (langur monkey photo courtesy of flickr user Troup Dresser)

We humans aren’t alone in our aversion to snakes. Our primate cousins also fear serpents. And for good reason—snakes eat primates. Snakes have been preying on primates for millions of years, and some researchers think they might be the reason we—and our fellow primates—have such good eyesight.

Good vision is a hallmark of the primate order. Compared with many other mammals, primates have more closely spaced, forward-facing eyes that allow for a lot of overlap between each eye’s visual field, which in turn gives primates 3-D, or stereoscopic, vision and a good sense of depth perception.

In the early 20th century, scientists attributed primates’ keen sense of sight to their arboreal lifestyle. The ancestors of primates needed to accurately judge the distances between tree branches before taking a leap, so the theory went. But that hypothesis lost favor in the 1970s after biological anthropologist Matt Cartmill, now at Boston University, pointed out that many other acrobatic, tree-dwelling animals like squirrels get by without such an advanced visual system.

Cartmill offered his own explanation, called the “visual predation hypothesis”: early primates needed superb visual skills to hunt and grab insects. Another hypothesis is that primates needed to see well to pluck fruits from the ends of tree branches.

More recently, snakes came into the picture. In 2006, anthropologist Lynne Isbell of the University of California at Davis argued that early primates were stalked by constricting snakes, and it was highly beneficial to see these camouflaged predators before it was too late. Later, some monkeys and apes in Africa and Asia started to live alongside venomous snakes, which led to even more visual advancements.

But the idea may not hold up, according to the authors of a recent study in the Journal of Human Evolution. Led by behavioral ecologist Brandon Wheeler of the Cognitive Ethology Laboratory at the German Primate Center, the team tested the snake hypothesis by looking at variations in modern primates’ visual skills (in terms of stereoscopic vision, as measured by the closeness of the eyes) to see if the primates with the best eyesight had the longest evolutionary history of coexisting with snakes and the greatest likelihood of encountering and being attacked by them.

The team didn’t find any correlations between snake exposure and primate vision, concluding that snake attacks did not drive the evolution of better eyesight. Still, the researchers say, detecting snakes was definitely a beneficial side effect regardless of why better vision evolved.

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