Reindeer - The Eyes Have It
As always happens in the UK in December, Christmas takes over everything. You can’t go anywhere without hearing Wham or Slade, decorations fill highstreets and cover homes, and suddenly all our wine and cider are hot with a tot of rum in it. It’s always about now that people are also more interested in Robins and Reindeer. I’m not apart from this if anything I play an active part in blurting out a slew of information about how Robins are regularly mistaken for their close relatives, Nightingales because they sing at night-time under artificial Christmas lights.
This year though I went down a research rabbit hole when my mum told me that she had heard about Reindeers eyes changing colours and wanted to know why. I read about their feet, their antlers, their noisy tendons. I learnt about their subspecies variation, their eating habits, reintroductions in Scotland. There was a lot, but I’ve picked a couple of my favourites. So, strap in for some of the reasons why I think reindeer are awesome.
Reindeer, called caribou in northern America, are native to the arctic (Skarin et al., 2020) and show what is known as circumpolar distribution, appearing across Siberia, Northern Europe and Northern America. These parts are very cold, and in many cases exist as tundra. For reindeer to survive these conditions, they have a number of adaptions which help them to thermoregulate. Most obviously is their fur. Reindeer fur constitutes of two layers, woollen hairs which are dense short underfur, and a longer shaggier layer of thick, hollow guard hairs (Timisjärvi et al., 1984). This hollow outer layer is similar to that found in polar bears. It provides greater volumes of air trapped to insulate against heat loss.
Another thermoregulation adaption found in reindeer is their noses. We all know Rudolph had a red nose, but it turns out that he also had a mass of complex turbinals in his nostrils covered in a thin layer of mucosa. That probably wouldn’t have fit so well in the song. Magnanelli et al. (2017) describe the heat and water retentive properties of nasal turbinates in both experimental and modelling terms. By providing a complex placement of large wet areas for air to move over as it travels both into and out of the body; reindeer are able to warm the air as it heads to the lungs, and cool the air to retain the heat as air exits the body.
It is not only the cold which reindeer needs to combat but also the cyclical nature of light availability, from the lack of dark in the summer to the lack of daylight in the winter. One way that reindeer can cope with these variations in light availability is that they are able to perceive ultraviolet light (Hogg et al., 2011 and Tyler et al., 2014). At Arctic latitudes, the way low wavelength light is scattered and reflected by snow and ice makes the area UV rich. Predators such as wolves absorb more UV light than the surrounding environment. The reindeer’s ability to see these dips in UV negates any camouflage the wolf may have alerting the reindeers to their presence.
Furthermore, as the light availability changes over the year, reindeer's eyes follow suit. During the summer, the reindeer have golden coloured eyes and overwinter their eyes become an icy blue (Stokkan et al., 2013). This colour observed is a reflection of light entering the eye by the tapetum lucidum (Ollivier et al., 2004) a layer of tissue behind the retina in the eye of many diurnal vertebrates, which reflects certain wavelengths of light resulting in specific colours being observed. In the summer months, brown is observed as most light is reflected back through the retina, reducing the amount of light entering the optic nerve, useful in months of near-constant daylight.
Due to the lack of light in the winter, the pupils of the reindeer are permanently dilated. This state of permanent dilation results reduced ocular drainage, which in turn created a greater ocular pressure. The increased pressure inside the eyes is believed to knit the tapetum lucidum structure closer together, and therefore reflect less and shorter wavelengths of light increasing signals to the optic nerve and producing their winter blue eyes. Now, I don’t know about you, but I think that’s pretty cool.
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.
Magnanelli, E., Wilhelmsen, Ø., Acquarone, M., Folkow, L.P. and Kjelstrup, S., 2017. The nasal geometry of the reindeer gives energy-efficient respiration. Journal of Non-Equilibrium Thermodynamics, 42(1), pp.59-78.
Ollivier, F.J., Samuelson, D.A., Brooks, D.E., Lewis, P.A., Kallberg, M.E. and Komáromy, A.M., 2004. Comparative morphology of the tapetum lucidum (among selected species). Veterinary Ophthalmology, 7(1), pp.11-22.
Skarin, A., Verdonen, M., Kumpula, T., Macias-Fauria, M., Alam, M., Kerby, J.T. and Forbes, B.C., 2020. Reindeer use of low Arctic tundra correlates with landscape structure. Environmental Research Letters.
Stokkan, K.A., Folkow, L., Dukes, J., Neveu, M., Hogg, C., Siefken, S., Dakin, S.C. and Jeffery, G., 2013. Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer. Proceedings of the Royal Society B: Biological Sciences, 280(1773), p.20132451.
Timisjärvi, J., Nieminen, M. and Sippola, A.L., 1984. The structure and insulation properties of the reindeer fur. Comparative biochemistry and physiology. A, Comparative physiology, 79(4), p.601.
Tyler, N.J.C., Jeffery, G., Hogg, C.R., Stokkan, K.A. and Giguère, N., 2014. Ultraviolet vision may enhance the ability of reindeer to discriminate plants in snow. Arctic, pp.159-166.
Written by Libby Bowles
I’m a PhD Researcher at Prifysgol Aberystwyth, studying the perenniality syndrome in Grasses. I’m passionate about public engagement in science and the sharing of knowledge regarding the world we live in, especially the weird and wonderful.