A bull snake slithers toward a burrow in Mammoth Hot Springs in Yellowstone National Park. (Bonnie Jo Mount/The Washington Post)

Snakes have been preying on primates and their ancestors for more than 100 million years. The first to do so were constrictors, which squeeze their victims to death. Several million years later, venomous snakes appeared and began immobilizing their targets via their sharp fangs.

We humans are primates, of course, and scientists believe this long history of danger from snakes has given us a built-in fear of the reptiles and very good snake-spotting skills. The idea that primates’ brains and visual systems evolved to be hypersensitive even to camouflaged snakes — increasing our chances of survival — is known as the Snake Detection Theory, and it’s been demonstrated in several studies.

In visual search experiments, for example, people pick out snake photos faster than they do spider images, and far faster than they do pictures of innocuous things like fruits or flowers. This superior snake-seeing and -fearing is true even for kids who don’t have a lot of experience with snakes, as well as for monkeys raised in captivity who had never encountered a snake.

Charles Darwin carried out a highly nonscientific test on himself during a visit to the London Zoological Gardens in 1872. “I put my face close to the thick glass-plate in front of a puff adder . . . with the firm determination of not starting back if the snake struck at me,” he later wrote. “But as soon as the blow was struck, my resolution went for nothing, and I jumped a yard or two backwards with astonishing rapidity. My will and reason were powerless against the imagination of a danger which had never been experienced.”

A new study published in PLOS ONE lends further credence to the idea that we are programmed to spot snakes — and fast. Using an image-manipulation tool, researchers presented people with a series of 20 images of a snake, bird, cat or fish that ranged from totally blurry to absolutely clear.

“In the images, the animals are ‘camouflaged’ in a uniform way, representing typical conditions in which animals are encountered in the wild,” co-author Nobuyuki Kawai said in a statement.

Here’s the series that reveals a cute and harmless (to us humans, anyway) kitty:


(Nobuyuki Kawai)

And here’s the series featuring that scary snake:


(Nobuyuki Kawai)

Participants were able to identify snakes pretty well by the sixth image, and with a rate of 90 percent by the eighth. The other, harmless animals weren’t generally made out until the 10th image or later. Participants had the toughest time identifying birds, which the authors said required further study to explain.

“This suggests that humans are primed to pick out snakes even in dense undergrowth, in a way that isn’t activated for other animals that aren’t a threat,” said another co-author, Hongshen He.

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