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Snakes Defend Themselves with Shape-Shifting Eyes

By Elizabeth Preston
Nov 8, 2016 12:39 AMNov 19, 2019 9:09 PM


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Superman donned glasses to disguise himself and blend in with other people. One snake hides its identity using a similar trick: when threatened, it changes the shape of its pupils. This makes it resemble a much more dangerous animal. The mock viper (Psammodynastes pulverulentus) is mild-mannered, not superpowered. It's common across much of Asia, and—as you might have guessed from its name—looks a lot like a viper. Actual vipers are a widespread family of venomous snakes. Like true vipers, mock vipers have triangular heads. Other details are different, though, such as the mock viper's round pupils. True vipers' pupils are vertical slits. Mock vipers carry only a mild venom; they're harmless to humans, for example. So imitating a more dangerous snake might help mock vipers scare off potential predators. In the summer of 2012, researchers from Thailand's Sakaerat Environmental Research Station were doing fieldwork in a forest in northeastern Thailand when they came across a small reptile basking in the sun:

[B]ased on careful visual inspection we confirmed that the snake was in fact a mock viper because it had round pupils instead of the vertical slit pupils of true vipers. However, as one team member grasped the snake behind the head, he noticed that the eyes had shifted to completely vertical slits. Thinking that he had picked up a viper, he instinctively dropped the animal, which immediately fled into the underbrush.

Had someone made a mistake? Or did the mock viper have a trick up its non-sleeve that no one had noticed before? The researchers explored the question further by methodically spooking some snakes and observing their reactions. They found that the mock viper does, in fact, have a shape-shifting pupil. When the snake is relaxed, its pupil is round. But if a threatening animal (say, a human scientist) comes close enough, it squeezes its pupil into a slit. The authors think this ruse might help mock vipers defend themselves from hungry animals. A visually oriented predator would likely have a hard time distinguishing the slit-eyed mock viper from a dangerous true viper. Mock vipers share their range in this part of the world with the similar-looking, but very venomous, Malayan pit viper. The authors think the mock viper may have evolved to mimic this snake. There could be another explanation. For example, maybe shrinking their pupils improves mock vipers' vision during an attack. Or maybe slit eyes are harder for predators to see; by shrinking its pupil to a slit, a snake might make its head blend in better with its surroundings. But the researchers note that they only saw snakes change their pupils after a human touched them, and not when someone was merely nearby. If the snakes were just trying to hide, it would make sense to disguise their pupils sooner. "Appearing dangerous might be a last resort, when all else fails," the authors write. Another snake in this region, the Malayan bridle snake, mimics a venomous snake called the Malayan krait not just in its coloration, but by hiding its head under its coils when threatened. For animals that don't have super strength or speed, a less mild-mannered disguise might be just enough to let them survive another day. Images: Silva et al.

Silva, I., Crane, M., Artchawakom, T., Suwanwaree, P., & Strine, C. (2016). More than meets the eye: change in pupil shape by a mock viper Frontiers in Ecology and the Environment, 14 (8), 453-454 DOI: 10.1002/fee.1420

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