Rudolph the Blue-Eyed Reindeer
Other reindeer also undergo this phenomenon, not just our red-nosed friend.
Howdy! It’s Joey, back with more Fun Fact Friyay. Today’s fact is slightly different based on the season.
Reindeer eyes change color during the winter from yellow to blue.
Fun Fact Friyay has welcomed a bit of reindeer trivia over the years. Maybe you love rattling off Santa’s reindeer like Donner and Blitzen and whatever the other six are.
I don’t believe the reindeer have ever been measured in height, but I sure hope Dancer is the most tiny of them all.
Why, it seemed like just yesterday when we were recounting the birth of Rudolph, who is the most famous reindeer-slash-advertising campaign to ever exist.
Rudolph’s big old red nose gets all the fame, but the celebrity reindeer, like all reindeer, would also be experiencing a change in his eye color.
Glen Jeffery is a neuroscientist who studies animal vision. His Norwegian colleagues sent him a bag full of reindeer eyes (for science, not as some kind of cruel prank). After dissecting the eyes, Jeffery was shocked to see that the animals that had been killed during the summer had yellow eyes, while those killed in the winter had blue eyes.
That led Jeffery down a delightful path of discovery that reindeer eyes adapt during harsh winter weather. Animals have a “cat’s eye,” scientifically known as the tapetum lucidum. It’s a mirrored layer behind the retina, which helps the animals see in dim conditions by reflecting light passing through the retina back onto the tapetum, giving light-detecting cells two opportunities to catch those photons.
When it’s dark outside, your iris muscles contract to dilate pupils and let your eyes take in more light. Since reindeer live in Arctic and subarctic regions, where the winters are harsh, bitterly cold, and generally quite dark, their pupils dilate for months at a time.
That causes the eyeballs to swell due to pressure, which squeezes fluid between the collagen fibers in the tapetum. Typical collagen fibers reflect yellow wavelengths. The reindeer’s more tightly packed wintry fibers reflect blue wavelengths.
Jeffery’s team also found the blue eyes are more sensitive, which they posit is due to blue light reflecting a bunch of times before getting captured. Researchers don’t necessarily agree on that point, though it’s still a fun rabbit hole to go down.
Unfortunately, if you’re gazing longingly into a reindeer’s eyes at any time of year, you won’t see this phenomenon yourself. The change in the tapetum occurs in the back of the eye, behind the retina.
And the tapetum is nowhere near the iris. So, much like the Goo Goo Dolls’ most popular song of the same name, a reindeer might say, “I don’t want the world to see me” (changing eye colors).
However, if you visit a place like the Natural History Museum in London (or just click the link that’s coming up after this parenthetical), you can see a reindeer eyeball that’s been cut in half. It’s kind of eerie, but the iridescent blue layer is super impressive.
I have not seen such an eyeball in person, though I did visit the Surgeons’ Hall Museum in Edinburgh last year. They had MANY medical specimens and oddities, including a human stomach that had somehow gotten a massive hairball lodged in it.
I also tried my hand at a handful of activities, including performing VR surgery and trying to keep my hands steady while a machine measured how much they were shaking.
Let’s just say we should all be glad we can channel nervous, hand-twitching energy into writing instead of life-saving surgery.
Brillaint breakdown of Arctic adaptation. The tapetum lucidum mechanism is wild: something as simple as sustained pupil dilation triggering structural changes at the fiber level. I'd always heard about night vision in animals but never knew the seasonal dimmer switch was an actual thing. Kind of makes me rethink how much phenotypic plasticity is baked into survival at those latitudes.