Santa’s Companions Have UV Vision

You may have heard of infrared (IR) vision, an ability possessed by many snake species, but have you heard of ultraviolet (UV) vision? With minute changes in the visual systems of animals, such abilities are possible. In some cases, these abilities are highly advantageous in the context of the animal’s surroundings. One UV-sensing mammal is Santa Claus’ preferred vehicle, the formidable reindeer (Rangifer tarandus) (Figure 1).

With incredible scattering of shorter wavelengths reaching the Earth from the Sun and their reflection from snow and ice, it is not hard to believe that the Arctic is so UV-rich (Hogg et al., 2011). Moreover, it has been observed in Arctic reindeer that the cornea and lens do not block all UV light, as they do in humans. Where the cornea and lens of human visual systems would not allow wavelengths shorter than 400 nm (UV) to enter the eye, both the cornea and lens structures of reindeer transmit UV wavelengths well until ~300 nm (Hogg et al., 2011) (Figure 2). To further validate UV vision in reindeer, Hogg et al. (2011) showed that the retina responds electrophysiologically to UV wavelengths; they even demonstrated that both rod and cone photoreceptors in the reindeer visual system respond to UV stimulation. Therefore, UV does not simply enter the eyes of reindeer; once inside, it triggers electrical signals that enable understanding of the visual field.

Reindeer, subject to interactions with other organisms present in the Arctic, must partake in the Darwinian struggle for existence. Luckily, they have evolved the ability to detect differences in UV light, allowing them to better identify predators and prey in their environment. The white fur of Arctic wolves (Canis lupus arctos) generally exhibits high UV absorption relative to other visible wavelengths (Lavigne and Oritsland, 1974). In addition, lichens (Cladonia rangiferina), a mutualistic composite of organisms that dominate the Arctic as the major primary producer, also display strong absorption in the UV (Petzold and Goward, 1988). The benefit of these properties to reindeer is enormous. As both lichens, the preferred feed of the reindeer, and wolves, their main predators, appear in high contrast against most UV-reflecting backgrounds, reindeer can easily scavenge for food and exhibit predator avoidance. Furthermore, variations in the UV albedo of Arctic snow give snow a surface quality-dependent UV sensitivity (Meinander et al., 2008). In visible wavelengths, what could appear a monotonous white now has a UV gradient, aiding the reindeer in traversing their local environment. Thus, the ability to discern their visual fields in UV is a major advantage for Arctic reindeer.

All in all, special differences in the cornea and lens structures of reindeer allow them to “see” UV wavelengths. As UV is known to cause DNA damage, further research with reindeer should elucidate potential UV-protection mechanisms that they have evolved in something as delicate as the eye. And although human ability to perceive UV wavelengths is yet to become feasible, understanding UV-protection mechanisms may at least eventually help prevent UV-mediated mutagenesis. As of now, it seems simply that sensing UV in addition to wavelengths of our visible spectrum puts reindeer at an advantage in their environment. With not only an elevated ability to perceive predators and prey, but also the ability to differentiate between gradients of white snow, it’s no wonder why Santa’s sleigh is led by the wonderful Arctic reindeer!

References

Hogg, C., Neveu, M., Stokkan, K.-A., Folkow, L., Cottrill, P., Douglas, R., Hunt, D.M. and Jeffery, G., 2011. Arctic reindeer extend their visual range into the ultraviolet. Journal of Experimental Biology, 214(12), pp.2014–2019.

Lavigne, D.M. and Oritsland, N.A., 1974. Ultraviolet photography: a new application for remote sensing of mammals. Canadian Journal of Zoology, 52(7), pp.939–941.

Meinander, O., Kontu, A., Lakkala, K., Heikkilä, A., Ylianttila, L. and Toikka, M., 2008. Diurnal variations in the UV albedo of arctic snow. Atmospheric Chemistry and Physics, 8(21), pp.6551–6563.

Petzold, D.E. and Goward, S.N., 1988. Reflectance spectra of subarctic lichens. Remote Sensing of Environment, 24(3), pp.481–492.

Stockholm Environment Institute, 2015. Arctic reindeer, Rangifer tarandus. Available at: <https://www.sei.org/publications/governance-of-arctic-ecosystem-services/arctic-reindeer/> [Accessed 24 Nov. 2018].