“The eyes are the mirror of the soul,” says a proverb. If you looked into the eyes of a reindeer, you would see practically nothing but darkness. For the eyes of the Arctiartiodactyls seem as black as night. But in fact, although the eyes of animals are not the mirror of the soul, they are nevertheless enormously fascinating and adaptable, as a study by two researchers from University College in London (UK) shows. They undergo very special adaptations.
Reindeer have the ability to find food in the twilight and darkness of the polar night and, at least in most cases, to spot enemies such as wolves. Researchers Dr. Robert Fosbury and Dr. Glen Jeffrey of the Institute of Ophthalmology at the UK’s University College in London have now discovered that the eyes undergo special adaptations in summer and winter to provide reindeer with optimal vision.
The so-called tapetum lucidum that sits behind the retina of reindeer (and other animals such as cats and dogs) and acts as a residual light amplifier, undergoes structural remodeling from summer to winter, allowing the animals to see lichen and the shiny fur of wolves even with little light. The results of the two researchers’ study were published in the journal Proceedings of the Royal Society B.
In many animals, the tapetum lucidum functions in such a way that incoming light, when it does not land on the retina, is reflected back by the tapetum, allowing more light to reach the retina. Structurally, according to the researchers, this skin in reindeer consists of bundles of tiny collagen fibers grouped together in a hexagonal pattern. The spaces between the bundles are filled with water, the amount of which adjusts according to the season. In summer, the amount of water is higher than in winter and the bundle boxes are larger with more spaces between them. As a result, the tapetum reflects more of the yellow to green light back to the retina.
In winter, on the other hand, the amount of water and the pressure in the eye decrease, and the bundles become smaller and narrower. As a result, the two scientists say, more blue light is cast onto the retina, allowing reindeer to see their surroundings more accurately despite the “blue phase” of the steadily increasing polar night. “The reindeer see an image that is brighter but slightly fuzzier because the mirror scatters some light sideways, a bit like a misted glass,” the two researchers explain in an article in “The Conversation”.
This adaptation helps reindeer in their Arctic environment. This is because when the polar summer with its long bright phases slowly turns into the dark winter season, the proportion of blue light becomes higher and higher, as the ozone layer filters out the remaining color spectra when the sun is below the horizon, creating the so-called blue phase. Lichens, however, reflect blue light only slightly, so they are almost dark. But due to the adjustment of the eye, the contrast is increased (with increased image noise at the same time) and the sharpness decreases a bit. This is not a problem for reindeer, however, as they rely more on contrast than sharpness. Also, the fur of wolves and other predators actually reflects less blue light, appearing as black and therefore barely visible if the reindeer did not have the contrast increase.
But not everywhere reindeer are endangered by wolves. There are no wolves on Svalbard and the reindeer there, a subspecies of the European reindeer, could actually take it easy. But meanwhile, it seems as if polar bears on the archipelago have the nimble artiodactyls in their sights as prey. A study on whether this is actually the case has just started. If this is the case, the Svalbard reindeer will be glad to have their dynamic eyes. Whether they help against the king of the Arctic, however, remains to be seen.
Dr. Michael Wenger, PolarJournal
Link to the study: Fosbury R. A. E. and Jeffery G. (2022) Reindeer eyes seasonally adapt to ozone-blue Arctic twilight by tuning a photonic tapetum lucidum Proc. R. Soc. B.289: 2022100220221002