From Gold to Blue: The Surprising Secret Inside Reindeer Eyes

From their thick coats to hooves specially adapted for walking on snow, reindeer are equipped with an impressive range of features that help them survive in the Arctic. One of their most remarkable adaptations is hidden inside their eyes.

Studies have found that during the summer, the reflective layer inside a reindeer’s eye appears golden, while in winter it turns a deep blue. This is not a change in iris color. Instead, the transformation takes place in a specialized layer behind the retina, altering the way the eye reflects light.

How Does It Happen?

Behind the reindeer’s retina is a light-reflecting layer known as the tapetum lucidum. A similar structure is found in other animals adapted to low-light conditions, including cats and dogs.

Light that enters the eye but is not immediately absorbed is reflected by this tissue and sent back through the retina, giving the light-sensitive cells another opportunity to detect it. This is why the eyes of certain animals appear to glow when illuminated in the dark. In reindeer, however, this reflective layer does not remain the same throughout the year.

During the Arctic summer, daylight is present for nearly 24 hours a day. At this time of year, the layer behind the retina reflects a golden color. This arrangement is suited to the bright environment and helps the reindeer see with greater clarity and sharpness.

When winter arrives and the Arctic is plunged into prolonged darkness, the animal’s visual needs change. Every bit of available light becomes valuable for finding food, navigating the landscape and detecting potential danger.

From Gold to Blue

During winter, the reindeer’s reflective eye layer turns blue. Researchers have found that the transformation is connected to changes in the microscopic fibers within the tissue and the way they reflect and scatter light.

During the long periods of winter darkness, a reindeer’s pupils remain dilated for extended periods. This affects the pressure inside the eye, producing changes in the tissue behind the retina. As a result, the color of the reflected light shifts from gold to blue.

The blue layer scatters more light within the eye, allowing the retina to capture more of the limited light available. There is a trade-off: the image may become somewhat less sharp. But in the darkness of an Arctic winter, sensitivity to light is more valuable than exceptionally sharp vision.

A World That Looks Completely Different

Reindeer vision differs from human vision as they can see ultraviolet light. Snow strongly reflects ultraviolet light, while certain objects absorb it and therefore appear much darker by contrast. This ability may help reindeer identify food, detect signs left by other animals and notice details that would be extremely difficult to distinguish against a white, snowy landscape.

The changes that take place in the reindeer’s eyes during winter may further enhance their ability to make use of the limited light available during the darkest months.

Then, as winter ends and daylight returns to the Arctic, the reindeer’s eyes gradually shift back to their summer state, and the deep blue reflective layer becomes golden once again.