Through the eye of a snake
Although snakes have good eyesight, their eyes are most adept at detecting movement. Hence the old warning: Stand perfectly still when you encounter a snake. It’s good advice. Even better advice is backing off as quickly as possible! Once you’re 5 m or more away from any snake, you are relatively safe and unlikely to get bitten. Snakes are unlikely to strike at stationary objects or at people, unless they resemble or smell like prey.
Only two snakes in South Africa are believed to have superior vision and are capable of seeing stationary prey: The boomslang and the twig or vine snake. These snakes have binocular vision like humans; most other snakes have monocular vision, which means the eyes don’t register images in co-ordination with each another.
Snakes do not have moveable eyelids. Instead, a fixed transparent shield, shed with the rest of the skin during the sloughing process, covers the eye. This is a feature we can use to distinguish snakes from legless lizards, as most lizards have eyelids.
You can tell a lot about a snake’s behaviour by looking at its eyes. Take the pupil, for example: Snakes with vertically elliptic (cat-like) pupils are generally nocturnal, whereas snakes with round pupils are diurnal, meaning they are active during the daytime.
The position of the eye is also important. Many water snakes such as the dusky-bellied water snake, and desert snakes like the Peringuey’s adder, have eyes on the top of the head. For water snakes, this allows them to ambush prey and watch out for predators while submerged; for desert-dwelling snakes, it means they can bury themselves in the sand with only the top of their head and eyes exposed to ambush passing lizards.
The diameter of the eye compared to the head tells us about the snake’s lifestyle. Arboreal snakes, like the spotted bush snake and the boomslang, have large eyes with good vision, while underground snakes, like Bibron’s blind snake, have small eyes with poor vision.
And lastly, did you know that diurnal snakes, like mambas and cobras, have colour vision, while most nocturnal snakes only see in black and white?