Santa Claus was certainly on to a good thing when he choose reindeer to guide his sleigh on Christmas Eve. It now appears that reindeer have a remarkable way of contending with the near-complete darkness of winter, having evolved a mechanism which increases their sensitivity to the limited environmental light.
Reindeer (Rangifer tarandus) that live in the Arctic Circle face a challenging environment. Their world undergoes a drastic shift from summer to winter, veering from almost two months of permanent daylight to two months of continuous darkness. And yet they are able to cope in both environments, partly because of a special mechanism that enables them to modify their vision under different light conditions.
Researchers previously discovered that reindeer are able to extend their visual range into the near ultraviolet (UV), allowing them to make better use of the UV-rich winter light. But now, a new study describes how a part of the reindeer eye called the tapetum lucidum (or “cat’s eye”) actually changes colour in winter, further enhancing their ability to use light on dark days.
The tapetum lucidum is a reflective surface found in some mammals that bounces light back through the retina, enhancing visual sensitivity. Like other ungulates, the tapetum lucidum is golden in reindeer. But only in summer. In winter, it undergoes an extraordinary change, becoming a deep blue colour.
This shift in colour is associated with a change in the amount of light that is reflected out of the eye. In summer, when the tapetum is golden, most light is reflected back directly through the retina. But in winter, less light is reflected back out.
This change in winter appears to be linked to the dilation of the pupil, which acts to increase intra-ocular pressure and compress collagen fibres on the tapetum lucidum. As a result of reduced spacing between collagen fibres, the tapetum lucidum appears blue in colour and reflects shorter wavelengths. This blue reflection then may scatter more light through photoreceptors resulting in increased retinal sensitivity.
This incredible eye adaptation may have originally evolved to help reindeer improve their detection of predators in winter. But especially at this time of year, it would also be nice to think that it somehow plays a part in spreading Christmas cheer around the globe.
Reference: Stokkan KA, Folkow L, Dukes J, Neveu M, Hogg C, Siefken S, Dakin SC, & Jeffery G (2013). Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer. Proc. R. Soc. B, 280 (1773) PMID: 24174115