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It is easy to pretend to be a caribou in summer.
We stand at about the same height and we both see the world in colour – or rather – in wavelengths. We can both see the tiny little violet wavelengths 380–450 nanometres long, up to the red wavelengths that roam along at 620–750 nanometres.
As the earth tilts and the sun dips below the arctic circle however, the 100 million years between ours and the caribou’s last common ancestor brings an end to this reverie.
Caribou and Reindeer (or Rangifer tarundus as they are known in the scientific world) have lived in the high arctic for 1.8 million years. In this great expanse of time, they have developed and adapted in tune with their environment.
In the long months of winter darkness, the caribou’s pupils grow from a horizontal elongated shape to a wide, round opening over 12 times the size. This allows more light to enter their eyes, but this – like so many stories in evolution – is nothing compared to what follows:
The opening of the pupil causes the coloured iris to fold in and increases the pressure within the eye. The increasing pressure causes fibrous collagen to become more closely packed together, and this fibrous collagen lies in a special region of the eye called the tapetum lucidum.
The tapetum lucidum sits immediately behind the retina and its reflective surface sends visible light directly back through the eye a second time, thus enhancing the image. It is a common adaptation in nocturnal animals and is also the cause of eyeshine in flash photographs of animals at night.
As the fibrous collagen is squeezed together, the caribou’s tapetum lucidum transforms from iridescent golden brown to deep blue. The tightly spaced fibres cause the light to be scattered among photoreceptor cells (rather than being reflected) and allows shorter wavelengths of light to be captured.
Ultraviolet Radiation, or UV, is a wavelength of 100–400 nanometres long and sits just below the rainbows you and I can see. In mid-winter, caribou can detect UV approaching these wavelengths, but why on earth would they have evolved to see bigger rainbows?
Well, it’s more likely that this new sensory superpower spread through caribou populations because it increased their chances of survival.
At higher latitudes there is more UV in the environment, which in winter is strongly reflected and scattered by snow. By tuning their photoreceptor cells to perceive UV radiation, caribou can distinguish darker areas where UV is being absorbed.
Things that would be invisible to you and I in the colourless arctic winter appear dark against the snow; things like delicious lichen and moss, the urine patches of potential mates, and the bodies of stalking predators.
Misha is a whitehorse-based artist and science communicator. Please contact her with your ideas for future stories and comics!
Journal articles referenced in the creation of this piece:
M. Stevens (2013) Sensory Ecology, Behaviour, and Evolution, Oxford University Press, pp: 170-173
R. H. Douglas, G. Jeffery (2014) The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among mammals, Proceedings of the Royal Society B, Vol. 281, No. 1780
N. Tyler, K. Stokkan, C. Hogg, C. Nellemann, A. Vistnes, and G. Jeffery, (2014), Ultraviolet Vision and Avoidance of Power Lines in Birds and Mammals, Conservation Biology, Vol. 28, No. 3, pp. 630–631
N.J.C. Tyler, G. Jeffery, C.R. Hogg, K.-A. Stokkan (2014) Ultraviolet Vision May Enhance the Ability of Reindeer to Discriminate Plants in Snow, Arctic Institute of North America, Vol. 67, No.
K. Stokkan, L. Folkow, J. Dukes, M. Neveu, C. Hogg, S. Siefken, S. C. Dakin, and G. Jeffery (2013) Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer, Proceedings of the Royal Society B, Vol. 280, No., 1773
D. M. Hunt and L. Peichl (2014) S cones: Evolution, retinal distribution, development, and spectral sensitivity, Visual Neuroscience, Vol. 31, No. 02, pp 115-138
A. Skarin, C. Nellemann, L. Ro¨nnega˚rd, P. Sandstro¨m, H. Lundqvist (2015) Wind farm construction impacts reindeer migration and movement corridors, Landscape Ecology, Vol. 30, pp:1527–154
T. Malmström and R. H. H. Kröger (2006) Pupil shapes and lens optics in the eyes of terrestrial vertebrates, Journal of Experimental Biology, Vol. 209, pp:18-25