In reindeer, part of the ultraviolet light passes through the eye, while most daytime mammals block it. Why does this animal let in what we cannot see at all?
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Most mammals active by day block UV light with their cornea and lens, which protect the eye. In reindeer, these structures let through up to about 60% of UV, enough to stimulate the cones responsible for colour vision. Physiological studies show that the retina responds to it.
In winter, reindeer feed mainly on a lichen, Cladonia rangiferina. This lichen absorbs UV, while snow reflects it, so to a reindeer the lichen looks dark against a bright background.
One hypothesis suggests this vision may have evolved to spot the lichen under snow, saving energy during foraging. This idea has not yet been demonstrated through behaviour.
