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The first major study into the genes that control vision in snakes has found that the reptiles match their vision to their lifestyles.
An international team of snake biologists and vision experts studied various aspects of snake sight, including light-sensitive visual pigments, the genes for these pigments and the lenses found in snake eyes.
The researchers found that snakes that hunt during the day have lenses in their eyes that block ultraviolet light, allowing the reptile to see clearly in bright conditions.
Species that hunt at night have the opposite: lenses that allow more UV light in, helping them to see in the dark.
Dr David Gower, an evolutionary biologist at the Museum who led the research, says, 'There are more than 3,500 living species of snakes, with very diverse lifestyles.
'Most modern work on the genetics of vision has been done on mammals, birds and fish. But studying snakes' eyes is important for a more accurate and complete understanding of how vision functions and has evolved in vertebrates more generally.'
Snakes have diverse sets of rods and cones - the specialised cells in the retina that an animal uses to detect light. Having multiple types of visual pigments inside the cones allows animals to see in colour because these pigments are sensitive to different wavelengths of light.
The team, which included Museum researchers Dr Bruno Simões and Filipa Sampaio, studied the genes involved in producing these visual pigments in 69 species of snake.
They discovered that most snakes have three visual pigments, two of which are in cones. Snakes are therefore likely to be dichromatic in daylight, meaning they see two primary colours compared to the three that humans see.
Most snakes examined in the study are sensitive to UV light, which allows them to see well in low light conditions.
For light to reach the retina and be absorbed by the pigments, it first travels through the lens of the eye. Snakes with UV-sensitive visual pigments therefore have lenses that let UV light though.
In contrast, the research showed that most snakes that rely on very sharp eyesight in the daytime, such as the golden tree snake Chrysopelea ornata and the vine snake Ahaetulla nasuta, have lenses that block UV light.
These species have probably also tuned the pigments in their retina so that they are no longer sensitive to the short-wavelength UV light.
The team discovered that visual pigment genes have changed substantially during snake evolution, including many changes to the wavelengths of light that the pigments are sensitive to.
Snake eyes have adapted to their changing and diverse lifestyles.
The study suggests that the most recent ancestor of all living snakes had UV-sensitive vision.
Dr Gower says, '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 findings of the study, funded by the Leverhulme Trust, are published in the journal Molecular Biology and Evolution.