Seasonally Changing Eyes

In the Arctic, where reindeer can experience months of continuous darkness, they cope with this challenge by changing part of their eyes to blue.

Reindeer in the highest regions of the Arctic achieve an optical feat: they alter their eye structures to find better food and avoid predators during the long, dark months of polar twilight.

In the summer, the Tapetum Lucidum, a mirror-like layer at the back of the reindeer's eyes, shines like a golden opal with turquoise lines. However, in winter, this layer turns into a dark, rich blue.

Years were spent solving the mystery of why and how this iris color change occurs, requiring the expertise of an astrophysicist, a neuroscientist, and numerous reindeer eyes.

Most animals have specialized adaptations to survive and hunt in low light conditions. One such adaptation is the Tapetum Lucidum, a reflective layer behind the retina. When animals live in dark environments, every bit of light is crucial. However, some of these light particles might be missed by the retina. This is where the Tapetum comes in, reflecting light back through the retina, giving the light-absorbing pigments another chance to capture it. This is especially advantageous for animals that hunt in the dark.

Veterinary ophthalmologist Braidee Foote notes that for nocturnal hunters like cats, the Tapetum can enhance light reflection to more than double the photoreceptor input. The Tapetum typically shines in a yellowish-golden or greenish hue, causing the eerie glow of cat or raccoon eyes at night.

However, some animals, like reindeer, have eyes that turn blue in winter. This phenomenon is likely related to maximizing light absorption in the blue and near-blue spectrum during the long, dark winter days. This adaptation helps reindeer see better and survive in these harsh conditions.

The reindeer tapetum consists of small collagen fibers that form a variable reflective crystal. In the summer, the loose collagen fibers in the eyes create a crystal mirror that best reflects reddish light as they float freely in the fluid. In winter, the collagen fibers are packed more tightly, changing the crystal shape and absorbing nearby UV light, thereby primarily reflecting blue light.

This UV vision can aid reindeer in finding food during snowy winter months. The lichens that form the basis of their winter diet absorb UV light, making them appear dark against the UV-reflecting white snow. Similarly, the fur of wolves and polar bears also absorbs UV light, creating a stark contrast against the snow and making it easier for reindeer to detect predators.

In darkness, reindeer likely dilate their pupils, which blocks a small drainage hole for eye fluid. This increases intraocular pressure, compresses the collagen bands, and alters the shape of the crystals. In summer, the pupils return to normal.

However, these unique adaptations can be problematic for reindeer. Nowadays, high-voltage power lines cutting through traditional Sami herding areas leak UV light that appears as "fireworks" to the reindeer. This causes them to avoid these areas and leads to contentious court battles to protect Sami herding grounds.