In the Arctic regions, where reindeer live, their staple food is a white lichen, Cladonia rangiferina, generally known as reindeer moss, although it isn’t really moss. To humans, the white lichen would not be visible from afar in the snowy Arctic. Yet the reindeer’s eyes have evolved in a manner that enables them to spot their food even in the dark, a new study has found.
The study, conducted by researchers from Dartmouth College and University of St. Andrews, has been published in the journal i-Perception. It comes at a time when reindeer are the flavour of the season, with children all over the world waiting for Santa Claus set to ride in with gifts on his reindeer-driven sleigh.
The researchers found that Cladonia rangiferina absorb ultraviolet light. So do a few other lichen species that supplement the reindeer’s primary diet. And reindeer can see light in the ultraviolet spectrum.
A membrane called the tapetum turns blue in the reindeer’s eye during winter. This helps amplify light and enable the reindeer to see better. As a result, the reindeer see the white lichen as dark patches where humans and most other mammals would have not seen them at all.
The researchers discovered this using apparatus that mimicked reindeer vision. They carried out their observations on reindeer and lichen in the Cairngorms mountains of Scotland. Although reindeer had gone extinct in the region, they were reintroduced from Scandinavia subsequently.
The researchers suggest that the reindeer’s eyes are primed to enable them to locate Cladonia rangiferina during the winter, when it would otherwise be most difficult to spot. One effect of the blue tapetum is that in winter, reindeer see the Arctic surroundings in purple. Surfaces that reflect UV light, such as snow, shine brightly before reindeer eyes. On the other hand, objects that absorb UV light, such as lichen, appear dark enough to be located.