Twice a Year, Reindeer Eyes Pull Off a Wonderful Magic Trick

The Arctic can stay perpetually dark for months. Reindeer cope by changing part of their eyes from gold to blue.

a reindeer standing in snow
Till Findl / EyeEm / Getty

Of all the eyeballs in Glen Jeffery’s office, only a very small minority are his.

“Oh, I’ve got an office full of eyes,” Jeffery, a neuroscientist at University College London, told me. Over Skype, he fished one of his favorites out of an opaque vial: About the size of a golf ball and fringed with white tissue, it looked a bit like a poached egg with a slate-hued yolk. The prized specimen was a reindeer eye, an organ that has captivated Jeffery for decades because of the beguiling metamorphosis it undergoes each year in the animals’ Arctic home.

During the summer, when the sun spends months above the horizon, the inner parts of the animals’ eyes, a structure called the tapetum lucidum, gleam a shimmering gold. But as the landscape dips into the perpetual darkness of winter, their eyes turn a rich blue. The change isn’t easily noticeable in live reindeer, simply strutting about. But the first time Jeffery dissected both summer and winter eyes in his lab and spotted the difference, “I nearly fell off my seat,” he said. “I’d never seen an animal able to change its tapetum.”

That was about 20 years ago. In the time since, Jeffery and his colleagues have discovered that the eyes’ transformation serves a pretty important role: It tunes the organs to the colors of light most relevant to each season, enhancing the reindeer’s ability to detect short, blue wavelengths of light that dominate the Arctic’s dreary winters, then flipping the eyes back to the summer shade that guides them through sun-soaked months. The changeup—a malleability that’s never been documented in another animal, before or since—turns traditional notions of animal vision on their head. “We think of the eye as a pretty inflexible organ,” Kate Myrna, a veterinary ophthalmologist at the University of Georgia, told me. “You’re born with what you get; if anything, your vision gets worse over time.” But rule-breaking reindeer don’t just use their eyes to make sense of the world, as other animals do. They also use the world to make their eyes make more sense.

The reindeer’s color-changing feat is subtle enough that it eluded the notice of scientists for many years. The tapetum lucidum, is wedged in the back of each eye behind the retina, where it helps many nocturnal mammals better see their surroundings at night. Humans lack a tapetum, but many of us have seen one of its tricks: The tapetum is what makes cat, dog, and raccoon eyes glow in nighttime photographs. It reflects light that passes through the retina back toward it, giving the front of the eye a second opportunity to glimpse it—and, in the case of cameras, ping-ponging a brilliant flash back out. “It’s like having night-vision goggles that nature gives you,” Myrna told me.

Reindeer take those night-vision goggles and tint them on a seasonal schedule, upping their powers further. The tapetum is composed of a bunch of long collagen fibers, lined up lengthwise and suspended in fluid. In the summer, when the animals’ eyes shine gold, the tapetum’s fibers are packed loosely, giving the collagen “quite good reflectivity over a wide range of wavelengths,” jibing well with all the light that’s in the sky, says Robert Fosbury, an astronomer at University College London who has been collaborating with Jeffery to untangle the physics governing seasonal reindeer vision. Red, orange, yellow, green—all sorts of colors of light knock up against the tapetum and bounce right back, as if ricocheting off a standard bathroom mirror.

But the catch-all rainbow-reflectivity of the tapetum, so handy in summer, gets far less useful as the Earth’s orbit plunges the planet’s North Pole into winter. Twilight—a time when light from the sun, passed through ozone, gets filtered until it’s mostly blue—can stretch up to 11 hours at a time, casting the snow-draped tundra in tones of teal.

a grid showing reindeer tapeta and the Arctic sky
Reindeer tapeta extracted in summer (top left) and winter (bottom left), juxtaposed with photos of the Arctic sky in summer (top right) and winter (bottom right). Credit: Robert Fosbury

And so the reindeer’s eyes adjust. The fibers of the tapetum, once lax and disorganized, huddle closer until they almost touch. This narrows the range of light that the tapetum can reflect, tightening around blue wavelengths, in lockstep with the fading sun. “It gives a higher-contrast view,” Fosbury told me, essential to navigating the light-starved snow. “Without this extra blue sensitivity, they would see very, very little.” Tasty lichens would be nearly impossible to spot; predators would become an unavoidable peril. Considering how important the tapetum’s changeability is to reindeer, “other animals that live in a similar situation might have a dynamic, color-changing tapetum” as well, says Fabiano Montiani Ferreira, a veterinary ophthalmologist at the Federal University of Paraná, in Brazil. “Which is very exciting.”

Exactly how the tapetum’s fibers close in on one another isn’t yet clear, even two decades in. Fosbury and Jeffery are fairly certain that the fluid in that part of the eye somehow evacuates the tapetum in the winter, cramming the collagen threads together. One possibility is that a prolonged change in eye pressure could be carrying it out: In the persistent darkness of Arctic winters, reindeer pupils end up wildly dilated, expanding to more than 13 times their summer size—an attempt to maximize the amount of light that enters the eye—for weeks or months at a time. The effort blocks a set of tubes necessary to drain fluid from the eye, causing the front of the organ to swell like a balloon. The expansion pushes back on the tapetum, and wrings it out like a sponge.

But it’s not the only possible explanation. “A change in fluid balance” could also sap liquid from between the fibers and pack them in close, Caroline Zeiss, an ophthalmic pathologist at Yale, told me. Such a process might be the closest analogue to a new experiment Fosbury recently performed in the lab to test how the tapetum might change. He left several reindeer eyes that had been dissected during the summer out in the open air to dry, until most of the fluid suspending the fibers of the tapetum had evaporated. The collagen threads bunched together—and pivoted the structures’ reflective powers from most colors to almost entirely blue within about an hour and a half—a super-sped-up version, Fosbury thinks, of what happens in reindeer noggins in the wild, whether by dehydration or a sponge-like squeeze.

Regardless of how it happens, the color shift may not be easy for reindeer to cope with. Jeffery has started to wonder whether repeatedly toggling back and forth between gold and blue leaves behind some eyeball wear and tear. Add to that the agonizingly long stints of pupil dilation, and reindeer might experience something verging on mild glaucoma throughout much of their lives. And with humans continually encroaching on their habitats, the animals are starting to have other problems, too. Several years ago, Jeffery and his colleagues discovered a group of reindeer whose tapeta shone not blue, not golden, but green in the winter, an intermediate shade that he believes has been tainted by intermittent exposure to light pollution. Should that futz with the eyes’ ability to transition cleanly between seasons, it could leave the reindeer stuck in visual limbo, neither “properly light or dark adapted,” Jeffery said. Even reindeer’s flexible eyes, so much like night-vision goggles, may be no match for actual human tools, conjured by us to co-opt the night.