Reindeer Ultraviolet Vision

June 2, 2011

Snow-covered landscapes are nearly featureless, at least in the visible spectrum. It makes sense that being able to see things outside the normal vision range, either in the infrared or ultraviolet, would have an evolutionary advantage for animals that live in snow. This appears to be the case for reindeer, known also as caribou in North America. A diverse and international team of scientists from London, England, Tromsø, Norway, and Perth, Australia, have found that reindeer can see in the ultraviolet.[1-4]

Because of Earth's axial tilt, the Arctic has average daily light levels that change considerably over the course of a year. In the arctic winter, the Sun barely rises at mid-day, so most of the terrain illumination is via scattered light. The arctic is also more UV-rich than other terrain because snow and ice scatter short wavelength light quite effectively. Snow is nearly 100% reflective at ultraviolet wavelengths.[2]

The scientific team, which published its results in a recent issue of the Journal of Experimental Biology, found that the cornea and lens of the eyes of arctic reindeer are transmissive at ultraviolet wavelengths, reindeer respond to UV stimulation, and both the rods and cones of reindeer eyes respond to low-intensity ultraviolet light. In this very thorough study, it was found that there is no ultraviolet-specific photoreceptor in reindeer.

Human vision extends through a visible spectrum from violet at a wavelength of about 400 nm to red (700 nm). The scientific team found that reindeer vision extends into the 320-350 nm range of ultraviolet.[3] Part of the study involved measurement of the retinal response of anesthetized reindeer using electroretinography.[3,4] In this technique, electrical signals are recorded from a small piece of gold foil placed inside the eyelid.

One evolutionary advantage comes from the differential ultraviolet reflections from arctic vegetation; that is, reindeer can find more food. Explains Glen Jeffery, lead author of the article, "When we used cameras that could pick up UV, we noticed that there are some very important things that absorb UV light and therefore appear black, contrasting strongly with the snow. This includes urine – a sign of predators or competitors; lichens – a major food source in winter; and fur, making predators such as wolves very easy to see despite being camouflaged to other animals that can't see UV."[2]

Albino barren ground caribou; a.k.a., a reindeer. (Photo via Wikimedia Commons).

Evolution couldn't stop just there, since ultraviolet damages photoreceptors. It's conjectured that there co-evolved mechanisms that protect reindeer eyes from the extreme ultraviolet that exists during daylight in late winter when the snow-cover is extreme. Jeffrey comments that "There's no evidence that arctic foxes or polar bears suffer from snow blindness, so I bet you that most of the arctic animals up there are seeing into UV."[4] The reindeer research was supported by the British Biotechnological and Biological Sciences Research Council.[1]

Reindeer might have an easier time of seeing humans, but it would be nice if humans were better able to see reindeer.[5] About 200,000 reindeer live in Norway, and there are about 500 automobile-reindeer collisions annually.[6] Reflective yellow collars have been used to make domesticated reindeer more visible to drivers,[6] but there is a technological variation of the reindeer's enhanced vision that could offer help. This is multispectral or hyperspectral image fusion.

In hyperspectral image fusion, the image presented to a viewer (in the reindeer avoidance case, the automobile driver) is not just a visible light image, but an image overlaid with false color information from other spectral bands. This could be electromagnetic radiation in other light bands, such as infrared or ultraviolet; or, it could be something in the radio bands, such as radar. Just as the reindeer can distinguish objects better using out-of-band imaging, humans can distinguish subtle terrain details this way.

I can't leave the combined topic of the Arctic and ultraviolet light without recalling an interesting, but since discredited, hypothesis about polar bears. In the mid-70s, it was discovered that polar bears reflect very little ultraviolet. As we see now, that's a very good thing for Santa's reindeer; but, if the radiation isn't reflected, then it must be absorbed. An untested theory that explained this was that the hairs in polar bear fur, which are transparent in visible light, act as optical fibers that capture the light and direct it into the bear.

No experiments were done, but it was a cute idea, and it got press coverage in The New York Times, the Christian Science Monitor and Time magazine. Daniel W. Koon, a professor of physics at St. Lawrence University (Canton, New York), got some hair from a polar bear in a local zoo, and he found that the hair transmits essentially no ultraviolet light.[7-9] The reason for ultraviolet-black polar bears is just that the protein from which the hairs are composed, keratin, absorbs ultraviolet light.

World renown expert in reindeer husbandry. Father Christmas (Pere Noel), from Canadian Illustrated News, vol.XII, no. 26 (1875), p. 401. (Photo via Wikimedia Commons).

References:

1. Christopher Hogg, Magella Neveu, Karl-Arne Stokkan, Lars Folkow, Phillippa Cottrill, Ronald Douglas, David M. Hunt and Glen Jeffery, "Arctic reindeer extend their visual range into the ultraviolet," J Exp Biol, vol. 214, no. 12 (June 15, 2011), pp.2014-2019.
2. Reindeer use UV light to survive in the wild, Press Release, University of Central London, May 26, 2011.
3. Reindeer Use Ultraviolet Light Vision For Survival, RedOrbit, Inc., May 27, 2011.
4. Neil Bowdler, "Tests show Arctic reindeer 'see in UV'," BBC News, May 26, 2011.
5. Andreas Seiler, "The toll of the automobile: Wildlife and roads in Sweden," Doctoral thesis, Department of Conservation Biology, Swedish University of Agricultural Sciences, Uppsala, Sweden, 2003.
6. Norway's reindeer fitted with reflectors to stop car crashes, Telegraph (UK), December 22, 2010.
7. Daniel W. Koon, "Is polar bear hair fiber optic?", Applied Optics, vol. 37, no. 15, (May 20, 1998), pp. 3198-3200.
8. Daniel W. Koon, "Is polar bear hair fiber optic?", St. Lawrence University Web Site.
9. Ned Rozell, "Debunking the Myth of Polar Bear Hair," Alaska Science Forum, Article 1390, May 28, 1998.