Question: What dramatic change in appearance—that has nothing to do with antlers—marks the peak of winter for reindeer?
Answer: Their eyes change from gold to blue
When a team of neuroscientists collected the eyes of reindeer in Norway’s Sami country, their goal was to figure out how the animals were able to adapt to the seasonal shift from months of perpetual darkness to months of midnight sun. They examined eyes taken from reindeer in the winter and a set taken in the summer. The eyes of both seasons seemed similar in size and weight, but a British peer noticed that there was one major difference: The summer eyes were gold, and the winter ones were blue.
Closer study revealed that the tapetum lucidum, a mirror-like layer behind the retina, seemed to be the source of the coloration. The tapetum is mostly collagen, its fibers arranged in rows with fluid filling the gaps; in the space between fibers, light gets reflected.
The scientists began to put together what was happening. In sun-filled summer, reindeer pupils are smaller and the spacing between fibers is ample; in winter months, reindeers have to dilate their pupils to allow more light, which decreases the space between fibers. The standard spacing reflects light as yellow; the diminished spacing returns light as blue, so the eyes mimic a case of glaucoma. In that way, blue eyes not only reveal that winter is in full swing but make clear how hard the reindeer has to work to see in the continual dark.
The scientists expanded their theory later, after the researchers collected specimens from reindeer kept in pens near populated areas. These reindeer had also experienced the winter dilation but less dramatically, because they could see the glow of streetlights. Living in a space that fell between the golden glow of summer and the depths of wintry blue, perhaps the outcome for those deer was no surprise: their eyes were green.
Fascinating Specimens: What (Else) Makes Reindeer Unique
- Though antlers are a male-only feature for most reindeer, both male and female Finnish reindeer have antlers.
- Male reindeer lose their antlers in late fall or early winter, while female reindeer shed their antlers in the summer, and whichever gender has antlers at any moment gets top rank in the feeding hierarchy.
- Reindeer wear two coats: a dense undercoat that keeps them warm, and an overcoat of hollow hairs filled with air. The top layer acts like a life jacket that allows them buoyancy when they are in water.
- Reindeer breathing is also a heating system, with a mucus membrane warming air before it enters the lungs, and then drawing moisture from the air before it’s expelled on the outward breath.
- In summer, when the tundra is moist, reindeer footpads expand and become more sponge-like to allow better traction on wet surfaces; while in the winter, they contract to reveal the sharp edge of the hoof for crushing ice and snow.
- Long hair grows between the split “toes” of reindeer hooves in the winter, covering the pads so only the rims of the hooves touch the ground.
- When reindeer walk, their knees click, a sound that helps other deer determine their size and dominance. The longer between clicks suggesting the longer—and thus larger—strides.
- Reindeer change their sleeping pattern in Arctic summer, swapping out a solar-based circadian rhythm for an ultradian rhythm based around when they eat.
- During the winter, non-breeding females have more body mass than breeding females, while breeding females have more body mass during the spring and summer. The two groups are only similar in size around September.
- Reindeer eyes adjust enough to see light in the ultraviolet range, allowing them vision that humans cannot match regardless of eye color.
Meet reindeer and the cultures who know them best during Grand Circle Travel’s new Norwegian Fjords, Lapland & Finland Voyage.