And even though most real reindeer never pull sleighs through snowy nights, new research suggests that their eyes would be far better suited to the task than Santa's are. Unlike people, the study found, reindeer can see ultraviolet light — which probably allows them to detect food and predators in a mostly white environment.
The study makes reindeer the first large mammal known to have UV vision. And it raises questions about how animals that are highly specialized to their environments will adapt as their environments change.
"Reindeer are mammals and what we find may be related to humans," said Karl-Arne Stokkan, an Arctic biologist at the University of Tromsø in Norway. "In the view of potential climate change, we have also realized that reindeer may be an important 'signal-animal' because of their strong adaptation to an environment believed to suffer the biggest changes."
Many animals are able to see or respond to ultraviolet light, including some birds, rodents, fish, bees and bats. But UV light is invisible to the human eye and with enough exposure, can even cause damage. Most dangerous are bright, snowy and icy conditions at high elevations or high latitudes, where lots of UV intensity and reflectivity can cause snow blindness in human eyes.
Since reindeer live in the Arctic, where levels of ultraviolet light are at their highest in proportion to other wavelengths, Stokkan and colleagues wondered if the animals might have evolved a way to deal with a world full of ultraviolet radiation. In a lab experiment with LED lights, the researchers first showed that UV light passed right through the reindeer's cornea and lens into its retina, they reported in the Journal of Experimental Biology.
In human eyes, the lens instead acts as a UV filter, said Marty Banks, a vision scientist at the University of California, Berkeley. That prevents ultraviolet light from reaching the retina, where it can cause damage.
Next, the researchers used anesthetized reindeer to demonstrate that cells in the retina responded electrically to UV signals. Similar tests on other kinds of deer have previously come up negative, Stokkan said.
Ultraviolet vision may be a specialized adaptation to life in the snowy Arctic, the researchers hypothesize. Reindeer eat lichen, which absorb UV light, and they try to avoid being eaten by wolves, whose fur also absorbs the UV spectrum. Being able to see ultraviolet wavelengths, then, may give the animals the ability to see contrasts, helping them find food and avoid predators when surrounded by reflective snow and ice.
UV vision might also help reindeer distinguish subtle contours in what might look like a flat landscape of snow, which would help them navigate safely.
The animal kingdom contains a wide variety of designs for eyes and visual systems that can be wildly different form how humans see, Banks said. But research on animal eyes often helps scientists better understand the ways that vision works in people.
Work on chickens, for example, has led to a theory that the growing use of computers, books and cell phones might explain a rapid rise in nearsightedness over the past 60 years or so. Researchers have also recently discovered a new kind of ultraviolet light-sensing receptor in many animals that seems to play a role in regulating circadian rhythms.
Figuring out how reindeer manage to let ultraviolet rays into their eyes without going blind might end up having applications for mountaineers and others.
"How does a reindeer get away with it and not have the health consequences that we have if we're exposed to a lot of UV light?" Banks said. "For humans, the sum total of UV light exposure over a lifetime is predictive of a lot of bad things that we don't want to have. That might help us understand how to protect people more."