Of all the eyeballs in Glen Jeffery’s office, only a very small minority are his.
“Oh, I have an office full eyes,” Jeffery, a neuroscientist at University College London, told me. Over Skype, he fished out one of his favorites from an opaque vial: the size of a golf ball and lined with white fabric, it looked a bit like a poached egg with a slate-colored yolk. The prized specimen was a reindeer eye, an organ that captivated Jeffery for decades due to the alluring metamorphosis it undergoes each year in the arctic home of animals.
During the summer, when the sun spends months above the horizon, the inner parts of the animals’ eyes, a structure called the tapetum lucidum, glow a shimmering gold. But as the landscape plunges into perpetual winter darkness, their eyes turn a rich blue. The change is not easily noticeable in live reindeer, which simply strut about. But the first time Jeffery dissected summer and winter eyes in his lab and noticed the difference, “I nearly fell out of my seat,” he said. “I had never seen an animal capable of change his tapetum.
It was about 20 years ago. Since then, Jeffery and his colleagues have discovered that the eye transformation plays a pretty big role: it tunes the organs to the most relevant colors of light in each season, enhancing the reindeer’s ability to detect short blue wavelengths of light. the light that dominates the dreary Arctic winters, then turns their eyes back to the summer shadow that guides them through the sunny months. The change – a malleability that has never been documented in any other animal, before or since – upends traditional notions of animal vision. “We think of the eye as a pretty rigid organ,” Kate Myrna, a veterinary ophthalmologist at the University of Georgia, told me. “You are born with what you get; if anything, your vision deteriorates over time. But rule-breaking reindeer don’t just use their eyes to make sense of the world, like other animals do. They also use the world to give more meaning to their eyes.
The reindeer’s color-changing feat is subtle enough to escape the notice of scientists for many years. The tapetum lucidum, is stuck at the back of each eye behind the retina, where it helps many nocturnal mammals see their surroundings better at night. Humans don’t have a tapetum, but many of us have seen one of its tricks: the tapetum is what makes the eyes of cats, dogs, and raccoons shine in nighttime photographs. It reflects the passing light through the retina behind towards this, giving the front of the eye a second chance to catch a glimpse of it – and, in the case of cameras, to ping-pong a bright flash. “It’s like having night vision goggles that nature gives you,” Myrna told me.
The reindeer take these night vision goggles and tint them on a seasonal schedule, further increasing their powers. The tapetum is made up of a bunch of long collagen fibers, aligned lengthwise and suspended in liquid. In the summer, when the animals’ eyes glow gold, the fibers of the tapetum are loosely packed, giving the collagen “fairly good reflectivity over a wide range of wavelengths”, performing well in all light which is in the sky, says Robert Fosbury, an astronomer at University College London who collaborated with Jeffery to unravel the physics governing seasonal reindeer vision. Red, orange, yellow, green – all sorts of colors of light hit the tapetum and bounce off, as if ricocheting off a standard bathroom mirror.
But the tapetum’s catch-all rainbow reflectivity, so handy in the summer, becomes much less useful as Earth’s orbit plunges the planet’s North Pole in the winter. Twilight – a period when sunlight, passed through ozone, is filtered until it is mostly blue – can extend to 11:00 at once, throwing the snow-draped tundra in teal tones.
And so the reindeer’s eyes fit. The fibers of the tapetum, once loose and disorganized, come together until they almost touch. This reduces the range of light the tapetum can reflect, tightening around blue wavelengths, to the rhythm of the setting sun. “It gives a more contrasty view,” Fosbury told me, essential for navigating light-starved snow. “Without that extra blue sensitivity, they would see very, very little.” Tasty lichens would be nearly impossible to spot; predators would become an inevitable peril. Given the importance of tapetal variability for reindeer, “other animals living in a similar situation could also have a dynamic, color-changing tapetum,” says Fabiano Montiani Ferreira, a veterinary ophthalmologist at the Federal University of Parana, Brazil. “Which is very exciting.”
Exactly how the fibers of the tapetum approach each other is still unclear, even two decades later. Fosbury and Jeffery are fairly certain that the fluid in this part of the eye somehow wicks away from the tapetum in winter, packing the collagen threads together. One possibility is that a prolonged change in eye pressure may effect it: in the lingering darkness of arctic winters, reindeer pupils eventually dilate wildly, extending more than 13 times their summer size – an attempt to maximize the amount of light entering the eye – for weeks or months at a time. The strain blocks a set of tubes needed to drain fluid from the eye, causing the front of the organ to swell like a balloon. The expansion pushes the tapetum back and wrings it out like a sponge.
But this is not the only possible explanation. “A change in the water balance” could also sap the fluid between the fibers and pack them tightly, Caroline Zeiss, an ophthalmic pathologist at Yale, told me. Such a process might be the closest analog to a new experiment Fosbury recently performed in the lab to test how the tapetum might change. He allowed several reindeer eyes that had been dissected over the summer to air dry, until most of the fluid suspending the fibers of the tapetum had evaporated. The collagen threads came together – and rotated the reflective powers of the structures from most colors to almost entirely blue in about an hour and a half – a super-accelerated version, Fosbury thinks, of what happens in the noggins of reindeer in the wild, either through dehydration or sponge-like pressure.
However it happens, the color change may not be easy for reindeer to handle. Jeffery began to wonder if repeatedly switching between gold and blue leaves behind some eyeball wear. Add to that the excruciatingly long periods of pupil dilation, and reindeer could experience something bordering on mild glaucoma for much of their lives. And with humans continually encroaching on their habitats, animals are starting to have other problems as well. Several years ago, Jeffery and his colleagues discovered a group of reindeer whose tapeta shone not blue, nor gold, but green in winter, an intermediate hue that he says has been tainted by intermittent exposure to light pollution. If it’s futz with the eyes’ ability to transition cleanly between seasons, it could leave the reindeer stuck in visual limbo, neither “properly adapted for light or dark,” Jeffery said. Even the flexible eyes of reindeer, much like night-vision goggles, may not measure up to the actual human tools we conjure up to co-opt the night.