Through the Eyes of a Reindeer

A visitor recently asked me why reindeer have horizontal pupils. The question had me thinking about the importance of vision to reindeer. It is easy to forget that many animals don’t see the world the same way we do, and reindeer eyes are in many ways different from our own.

Reindeer live in arctic and sub arctic regions that experience hugely variable light levels. Above the arctic circle, they’ll experience 24 hour darkness in winter and 24 hour sun in summer, and no matter how dark it is, they still need to be able to find their food and see predators coming. So how do reindeer eyes cope with their environment?

Origami – Photo by Kate Brown

When you look at a reindeer, the most obvious thing about their eyes is that they are placed on the side of their heads. This is common in prey animals, and it gives them a wide field of vision that means they can see danger coming from almost any direction. The placement of their eyes does mean they have a blind spot right in front of their noses, but aside from this it’s very hard to sneak up on a reindeer!

Ladybird – Photo by Kate Brown

If you look a bit closer, the next thing you might notice – aside from their beautiful eyelashes – is that they have long,  horizontal pupils. This adaption helps focus a reindeer’s sense of sight at the ground level and the horizon – where their food and their predator’s are found. These horizontal pupils also help compensate for the placement of their eyes and reduce the size of that blind spot in front of a reindeer, so that they can see forwards – vital for finding an escape route when on the run from a predator.

Dr. Seuss – Photo by Kate Brown

If you looked deeper into a reindeer’s eyes, you would find even more amazing adaptions. In 2013 scientists discovered that reindeer eyes actually change colour with the seasons! A layer of tissue in the retina changes from a golden colour in summer to a deep blue in winter. This change means that less light is reflected back out of the eye, helping reindeer keep their vision sharp even in the long dark winter months. This is the first time this kind of colour change in a mammalian eye has been found. It is thought the colour change might be caused by increased pressure in the eye in winter. The pupil is permanently dilated in the dark, and this reduces the space in the collagen structure of the retina, which in turn changes the reflectiveness of the retina and shortens the wavelengths of light being reflected.

Two dissected reindeer eyes, the left taken from a reindeer that died in winter, the right from one that died in summer. Photo by Glen Jeffrey (The Independent).

Reindeer vision is made even more interesting by the fact that they can see into the UV range. There are high levels of UV light present in the polar regions of the world due to the reflections from snow and ice. Many lichens – the favoured winter food of reindeer – are very effective at absorbing UV. Wolf fur as well is also shown to absorb UV, so being able to see UV wavelengths helps reindeer pick out both their food and their predators in the snow (see here for a previous blog on UV vision).

Reindeer seem to view the world quite differently to us, and it’s clear their eyes have had to adapt to many challenges. They’ve had to adapt to pick out predators and hard to find food, to cope with extreme seasonal changes in light, and to deal with the large amount of UV present in the arctic. Their eyes are just another example of how well adapted reindeer are for their environments.

Kate

Sources:

Why do animal eyes have pupils of different shapes? | Science Advances (sciencemag.org)

Arctic reindeer extend their visual range into the ultraviolet | Journal of Experimental Biology | The Company of Biologists

Shifting mirrors: adaptive changes in retinal reflections to winter darkness in Arctic reindeer | Proceedings of the Royal Society B: Biological Sciences (royalsocietypublishing.org)

Keeping Warm

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Reindeer facing resolutely into the weather.

Reindeer are the past masters at keeping warm. When you evolve to live in temperatures below minus 30 Celsius then you need all your wits about you to keep warm.

Duke snow
Some of the boys toughing out a blizzard, ice plastered to their foreheads, including big Magnus (centre) who even has snow layering his antlers.

To begin with they have an extremely dense winter coat, 2,000 hairs to the square centimetre although I have to confess to not having confirmed that by counting them myself! Secondly, each individual hair is hollow for the same reason that we have holofil in our duvets. Air is a very good insulator so the combination of air in each hair and around each hair increases the insulation factor. In fact, reindeer are like a mobile thermos flask, neither allowing the cold in or indeed the warmth out. A bed of snow is a comfortable spot for a reindeer, and they can lie on it without even melting it.

Caddis
Caddis and her young calf Mozzarella (in 2013) – calves have even thicker coats than adults so they can stay snug in late snowfall.

You may have noticed that cows or horses out in fields in windy weather tend to put their backs to the prevailing winds (as well as look pretty miserable). That is actually not a very clever thing to do, as the wind lifts the hairs and takes away more of their body heat. They need to copy reindeer who face a blizzard. By doing this they keep the hair across their body flat and so do not lose heat. The only disadvantage to this is you end up with a ice pack on your face. Lucky that reindeer have hairy foreheads.

Jenga
Jenga in just a wee bit of snow, but not cold!

Actually if you study a reindeer closely from nose to tail you will find no bare skin anywhere and that even applies to the bottom of their feet. Yet another fine adaption to the cold, and with the added bonus of improving their grip on the ice and snow.

Finally, if you measure the temperature of the blood of a reindeer at its extremities you will find it is cooler. Once again this is to prevent heat loss. To do this a counter current system has evolved whereby the warm blood exiting from the main part of the body passes close to the colder blood coming back from the extremities. The net effect is the cold blood is warmed and the warm blood is cooled and the heat remains in the body.

sam_5358
A reindeer’s perfectly designed coat beats all of the artificial layers us herders have to wear – no contest! Look at the snow sitting unmelted on top of the fur.

There are other heat saving aspects to reindeer but I think that’s enough for now. Food for thought however it does mean that they can get awfully hot in the warmer weather. I reckon reindeer would be the first to sign up to a programme to slow down global warming!

Tilly

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