As the seasons change, Arctic reindeer (Rangifer tarandus; pictured) alter the colours at the backs of their eyes.

Credit: KIA HANSEN

In cats and some other mammals, a reflective surface called the tapetum lucidum reflects light through the retina a second time and improves sight in dim light. Glen Jeffery of University College London and his colleagues studied this structure in reindeer captured within two weeks of the summer and winter solstices.

The tapetum lucideum changed from golden in summer to deep blue in winter. The winter colour seems to scatter more light across the retina.

Proc. R. Soc. B 280, 20132451 (2013)