Snakes control blood flow in their ‘eyelids’ to help them see
Kevin van Doorn
Snakes have blood flowing over their eyes. Technically, you do, too. But in your case, that blood is contained within the tiny blood vessels that send oxygen and nutrients to your eyelids. Eyelids that blink keep us from being blinded by the blood vessels and tissues making up our eyelids, while still allowing the eyelid to do its job. When you blink, your eyelids help keep dust and debris out of your eyes while keeping the surface moist.
Snakes aren’t so lucky. Instead of eyelids, snakes have what is known as a spectacle, an analogous piece of tissue. While our eyelids move, the snake spectacle is fused together. While this does help keep debris out of the eye and keeps the eye moist, it means that snakes have a tissue in front of their eyes. A tissue full of blood vessels, that never goes away. Instead, it acts like a window, and the snake’s eye can move freely underneath.
Now, this might seem OK. But blood vessels, you see, are solid little things. You can’t exactly see though them. So even though the blood vessels in the spectacle might be tiny, that close to the eye, they could interfere with what the snake is trying to see (like how you sometimes see “floaters” from the structures within your own eye). Blinded by its own blood.
But obviously, snakes are NOT blinded by their own blood flow. So how do they manage?
Kevin van Doorn of the University of Waterloo in Canada hypothesized that snakes might be able to alter the blood flow over their eyes to help them see. To examine this, he took three coachwhip snakes (Masticophis flagellum), a cute little nonvenomous snake found all over the southern United States and Mexico. He put them in clear Plexiglas boxes, big enough for the snake to feel at home. The snakes settled in and didn’t move much, which means good access to their eyes. The scientist then took images of the snakes' eyes by lighting them with infrared filters. During the first observations, the author was hidden behind a screen. The snake could see nothing threatening and was relaxed. Then the author came out and moved around in front of the snake, producing a threat so the snake’s eyes could be observed during rest and during threatening stimuli (which would presumably be when the snakes would want to have the best vision).
In a paper published in the Nov. 15 Journal of Experimental Biology, the scientists found that there was a natural rhythm to the blood flow in the snakes’ eyes that varied from snake to snake (see video below). But in all cases, the blood flow was NOT constant. Instead, there were times of vasoconstriction — reduced blood flow — and times of open blood flow. Sometimes the rhythm was constant, and sometimes there were longer periods of blood flow and fewer periods of constriction. But there was always alternation between periods of blood flow and periods where there was NO blood flow (and when, presumably, the snake would have the best vision).
This was during rest. But when the snake was threatened (by the presence of a human), the periods of blood flow began to come few and far between. The length of a blood flow period, usually around a minute, decreased to an average of 33 seconds. And the number of blood flow periods over time also dropped, showing that during a threat, there was less blood flow to the spectacle in general.
Of course, this is only one species of snake, but it suggests that blood flow to the spectacle does reduce vision, and that the snakes reduce blood flow during times of threat or stress. This means that if the snake knows it needs to make a quick getaway, it will reduce blood flow to its spectacle to clear its vision. In other words, if the blood flow to the spectacle isn’t reduced during threat, the snake might end up making a spectacle … of itself. Future research will have to determine how this control happens, and what it means for how snakes behave. But in the meantime, be grateful for your blinking eyelids, so you don’t have to make a spectacle of yourself.
Blood flows through the spectacle, or eyelid, of a juvenile corn snake when the eyelidlike covering is being regenerated. The illuminated area is the pupil of the snake's eye. The many small dark lines are scratches in the spectacle scale that accumulate during a snake's routine activities and that will be shed when the animal molts. About six seconds into the video, the snake shifts its gaze, but the vessels remain fixed, demonstrating how the eye rotates freely beneath the fixed spectacle. Credit: Kevin van Doorn