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  • Writer's pictureBecky Faulks

Reindeer Eyes Change Colour During Arctic Winter

Professor Glen Jeffery shares updates on his research into the field

 




In an incredible piece of research, scientists have discovered that reindeer can change their eye colour to adapt to the dark arctic winter. This change occurs in a reflective layer called the tapetum lucidum, which gives the eyes a golden hue during summer and a blue one during winter.


According to a 2022 paper by ophthalmologist Robert A.E. Fosbury and neuroscientist Glen Jefferey of University College London, while the human pupils may dilate temporarily to let in more light when it’s dark, the constant dark conditions in the arctic cause reindeer eyes to dilate for months on end. The result of this is that pressure builds up inside the eyeball and the tapetum changes as a result. The tapetum layer is made up of rows of collagen, which become more tightly packed due to the pressure from the dilated pupil. The spacing between these collagen rows affects the type of light reflected by the eye – spaced normally, they reflect yellow light; spaced tightly, they reflect blue. All this makes the retina of blue eyes at least 1,000 times more sensitive to light than the golden ones, which is ideal during winter.





These adaptations not only ensure that reindeer are not wasting excess energy hunting through the snow for food, but also that they are alerted to predators in a timely fashion. Research published in the Journal of Experimental Biology in 2011 showed that reindeer responded to UV light, which is high in the arctic even while yellow light is lacking. UV light is invisible to humans, which means that while humans experience ‘snow blindness’ in the white out of the arctic, reindeer are able to distinguish between food, predators and even urine in the snow. Pale lichen, a reindeer’s main food source, would appear black against the snow as it absorbs UV, as would a wolf, the reindeer’s main predator.


According to Professor Lars Chittka of Queen Mary University London, who has explored the UV capabilities of bees, it is unsurprising that UV light appears to cause no damage to the eyes of reindeer. Tests suggest that only low frequency (UV-A) light is admitted into the eye, while harmful, higher frequency UV-B rays are kept out. This is why UV light does not appear to damage the retina.





With this in mind, the question appears to be not why reindeer eyes do change colour during winter, but why the eyes of other species don’t. Professor Jeffery explains:


'You need a mirror behind your eye (tapetum) for it to change colour and not all animals have one of these. Some of the animals that live in the dark (e.g. rodents) don’t have one. However, the mechanism can be seen in these animals, including humans, because this depends on changes in eye pressure. In reindeer, the pupil dilates in winter darkness. This increases the pressure in the eye and the colour changes. Humans don’t have a tapetum, but when we sleep (with our eyes closed) the pressure in the eye goes up in the same way as it does in reindeer. So if we had a tapetum, the colour of the reflection would shift. There are other mammals in the Artic – foxes and polar bears. We do not think they change their tapetums because the structure of the mirror is different and highly inflexible, so cannot change.'


Professor Jefferey adds that night vision is probably similar in most diurnal mammals, i.e. those who are active during the day and sleep at night.

   

'We speculate on whether a horse or cow could do the colour change – they have the same type of mirror as a reindeer. UV vision is a totally separate issue – I think most mammals other than primates see UV to a greater or lesser extent. It just significantly greater in reindeer.'

 




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