Snakes’ Vision Sharpens When They’re Under Stress

Snakes have no eyelids. Instead, they have spectacles—modified, transparent scales that cover and protect their eyes. The spectacles are criss-crossed with blood veins, “much like blinds on a window,” RedOrbit says. And these scales allow snakes to change the way they perceive the world depending on whether they’re relaxing, feel threatened or even if they’re shedding, new research finds.

Depending on what the snake is doing, the blood flow into its spectacles changes. When the animal is resting, the researchers observed, the blood comes and goes in patterned cycles, repeating itself over several minutes. While snakes are shedding their skin, the vessels become extra engorged. But when snakes feel threatened—like when researchers are tampering with them—they demonstrated the opposite pattern: they restricted all blood flow to their eyes.

“It took me a moment, and several repeats of adjusting my instrument, to realize the spectacle blood flow was responding to my own activity,” lead author Kevin van Doorn told the BBC.

The restricted blood flow, the researchers think, likely increases the clarity of the snake’s vision. They’re optimizing their vision for situations that require the most attention to detail—like when a giant human trying to measure the blood flow around their eyes—and, in other situations, save energy.

More from Smithsonian.com:

Five Giant Snakes We Should Worry About
Snakes: The Good, the Bad and the Deadly 

Rachel Nuwer | | READ MORE

Rachel Nuwer is a freelance science writer based in Brooklyn.