Not only is the reindeer's eye beautiful, it is a scientific marvel. The animal's eyes change from a golden colour to a deep blue as summer fades to winter, helping it to see in dramatically different light levels.
What is happening?
The part that changes colour is called the tapetum lucidum. It is a shiny, mirrored layer behind the retina that helps some animals to see in the dark.
When light enters the eye much of it hits the sensitive cells in the retina. But sometimes it misses the mark. The tapetum lucidum gives the eye a second chance to detect the light by reflecting it back towards the retina again.
It is the reason some mammals, including cats, have eyes that seem to glow when light is shined on them.
In many animals this reflective layer shines gold, permanently. While reindeer eyes are also gold in the summer months, in winter their layer turns blue.
Why the colour change?
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.
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