Which mammal has eyes that change colour with the seasons?

Scientists think the blue colour helps to capture even more light. Animals that live in the Arctic experience dramatic changes in light levels, with long hours of bright light in the summer and almost total darkness in winter.

One theory is that in the winter, pressure inside the animal's eye builds due to the effort of keeping the pupils dilated and large for months on end.

The pressure squeezes fluid out of the tapetum lucidum, which is formed mostly of collagen fibres. As result, the fibres pack together more tightly and start to reflect blue wavelengths of light instead of yellow.

The blue eyes become over a thousand times more sensitive to light than the yellow summer ones, making reindeer vision perfectly adapted to its unforgiving habitat.