How Reindeer See Ultraviolet Light: Arctic Winter Vision Explained

Reindeer can see ultraviolet (UV) light, which most mammals cannot. In humans and many other mammals the lens of the eye blocks UV before it reaches the retina, but in reindeer UV passes through to light-sensitive cells that respond to it. Seeing UV helps reindeer in the dim, blue-tinted light of the Arctic winter, when snow reflects UV strongly and things that absorb it, such as lichen, wolf fur and urine, stand out as dark patches.

Why can’t most mammals see ultraviolet light?

Most mammals cannot see UV because the cornea and lens of the eye filter it out. UV is light with wavelengths shorter than violet, below about 400 nanometers, and it carries more energy than visible light. That energy can damage the retina over time, so blocking it protects the eye. Many insects, birds, fish and reptiles do see UV, and some small mammals such as certain rodents and bats also detect it.

Humans are a useful comparison. The human lens absorbs almost all UV. People whose natural lens has been removed, for example in some older cataract surgeries without a UV-blocking replacement, have reported seeing UV as a faint whitish violet.

How do reindeer eyes let ultraviolet light through?

Reindeer eyes let UV through because their lens and cornea transmit it instead of absorbing it. Researchers studying reindeer eyes in the early 2010s found that light in the UV range reaches the retina and that the retina responds to it with electrical signals, the same kind of response it gives to visible light. Reindeer do not appear to have a separate UV cone. Their existing light-sensitive cells pick up some UV along with the visible colors they are tuned to.

Like most hoofed mammals, reindeer have two types of color-sensing cones, so their color vision differs from the three-cone vision of most humans. UV sensitivity adds to what they can tell apart, especially against snow.

What do reindeer see with UV in the snow?

Against snow, reindeer see UV-absorbing objects as dark shapes on a bright background. Fresh snow reflects a large share of UV light. The EPA notes that snow can reflect up to 80 percent of UV, which is why skiers get sunburned and snow blindness is a risk. Anything that absorbs UV therefore contrasts sharply with the snow around it.

Things that would stand out to a reindeer include:

• Lichens, the reindeer’s main winter food, often called reindeer moss. They absorb UV and look dark against snow, even where a human eye sees pale gray on white.
• Wolf fur. White or pale fur that blends into snow for a human absorbs UV, so a wolf could appear as a dark outline.
• Urine. Urine absorbs UV, so marks left by predators or other reindeer show up on the snow.

Spotting food and predators faster matters in winter, when reindeer must find lichen under snow and wolves hunt them across open ground.

Why is ultraviolet vision useful in the Arctic winter?

UV vision is useful because the Arctic winter has very little direct sunlight, and much of the light that remains is short-wavelength blue and UV. Above the Arctic Circle the sun stays below the horizon for days or weeks around the winter solstice. Daylight is replaced by long twilight, when sunlight scatters through the upper atmosphere and reaches the ground as diffuse, blue-shifted light.

In that light an eye that uses UV as well as visible wavelengths collects more of what is available. Snow cover adds to the effect by bouncing UV back up. For an animal that stays active all winter instead of hibernating, every bit of extra contrast helps. Reindeer are one of many species with special winter adaptations; others slow down instead, as this guide to hibernation, torpor and brumation explains.

Do reindeer eyes change color between summer and winter?

Yes. The tapetum lucidum, the reflective layer behind the retina that makes animal eyes shine in headlights, shifts from golden in summer to deep blue in winter. The tapetum bounces light back through the retina for a second chance to be detected. In the long winter darkness reindeer pupils stay wide open, pressure inside the eye rises, and the fibers in the tapetum pack more tightly, which changes the color of light it reflects.

The blue winter tapetum scatters more light inside the eye. That makes the eye more sensitive in dim conditions, at the cost of a less sharp image. Reindeer appear to trade some sharpness for the ability to see anything at all in polar twilight.

Doesn’t UV damage reindeer eyes?

How reindeer avoid UV damage is still an open question. In the Arctic summer, with near-constant daylight and bright snowfields in spring, their eyes receive a large dose of UV. Scientists have suggested that the low overall light of winter limits the risk for much of the year, and that reindeer eyes may have protective or repair mechanisms not yet described. No firm answer exists yet. For humans, UV off snow can burn the cornea within hours, so snow goggles or sunglasses that block UV are standard advice on snowfields.

Quick answers

Do reindeer see the northern lights in ultraviolet?

The visible auroras are mostly green and red light from oxygen and blue and purple from nitrogen, all of which reindeer can see. Auroras also give off some UV, but most of it is absorbed high in the atmosphere. To see an aurora yourself, check the northern lights forecast for tonight.

Are reindeer and caribou the same animal?

Yes. Reindeer and caribou are the same species, Rangifer tarandus. In North America wild herds are usually called caribou, and in Europe and Asia the animals, wild or herded, are called reindeer.

What other mammals can see ultraviolet light?

Several small mammals, including some rodents, bats and marsupials, detect UV. Dogs and cats let a little more UV through their lenses than humans do, and may sense some of it, although how much they perceive is not well understood.