Snakes that hunt during the day have natural “sunglasses” – eyes that filter out ultraviolet light and sharpen up their vision, scientists have discovered.
In contrast, nocturnal snakes have eye lenses that let ultraviolet light through and help them see in the dark.
To see different colours, animals have pigments in their retinal cells that are sensitive to different light wavelengths.
A study of 69 snake species found that visual pigment genes in the animals have changed greatly through evolution to suit their lifestyles.
Most of the snakes examined were sensitive to ultraviolet (UV) light which helped them see better in low-light conditions. They also had eye lenses transparent to UV rays.
But snakes that hunted in bright daylight – such as the gliding golden tree snake (Chrysopelea ornata) and Monypellier snake (Malpolon monspessulanus) – had lenses that blocked UV light.
In the same way that skiers’ yellow goggles improve contrast, the “sunglasses” are thought to make their sight sharper.
Snakes with UV-filtering lenses also had retina pigments that were not sensitive to short UV wavelengths.
None of the nocturnal species studied had UV-filtering eye lenses.
Analysis of the eye pigments suggested the most recent ancestor of all living snakes had UV-sensitive vision.
Lead researcher Dr David Gower, from the Natural History Museum, London, said: “The precise nature of the ancestral snake is contentious but the evidence from vision is consistent with the idea that it was adapted to living in low light conditions on land.”
The research is published in the journal Molecular Biology and Evolution.