When it is dark, we strain our eyes to see better. Reindeer, though, simply change the colour of their eyes!
Reindeer live in the cold Arctic regions and can be found in the northern parts of Europe, Asia, North America and Greenland. From November to February, the temperature in the Arctic regions can drop below -50 degree Celsius. During these months, on most days, the sun does not rise at all, hence, it is always dark which makes it difficult for animals to see. Also, snow reflects ultraviolet light a type of light that is invisible and harmful to the human eye.
When it is dark, our pupils dilate, that is, become bigger, to allow more light to enter the eyes. Similarly, the reindeer’s eyes remain dilated throughout winter as it is always dark. Hence, there’s continuous pressure on the tissue (called tapetum lucidum) in its eyes which changes the way the eyes reflect light. Because of this change, the eyes turn blue and the reindeer is able to see well in the dark.
However, the reindeer can see the ultraviolet light as its eyes change colour from golden to blue.
This helps them to navigate their way around, find food and spot predators.