Rudolph sees blue

In nearly all mammals, including humans, the cornea and lens block UV light from entering the eye, but not in the arctic reindeer; not only does UV light reach the back of a reindeer’s eye, but it forms an image that could be a life-saver in the depths of the arctic winter. During the winter, reindeer living in the northern-most regions of Norway experience three months of almost total darkness. The sun is low on the horizon and as a result, due to the Rayleigh scattering phenomenon, light is bent towards the blue end of the spectrum. This plunges reindeer into an environment that, to human vision, takes on a deep blue hue for weeks at a time. Finding food, avoiding predators, and identifying mates and adversaries could be extremely difficult.

It seems that whilst most mammals have evolved to block UV – presumably a protective strategy due to its high energy and potential to cause damage – reindeer have developed the ability to use UV to their advantage in a very extreme environment. Snow reflects almost 90% of UV light that falls on it and, in contrast, most UV is absorbed by lichens (a main winter food source), fur, and urine. If you were a reindeer, you would definitely need to spot these things in the vast expanse of snow and darkness.

Dr Glen Jeffery, a researcher at UCL, was funded by the Biotechnology and Biological Sciences Research Council to investigate vision in extreme environments. He and his team travelled to the University of Tromso in northern Norway to examine the reindeer and their environment.

“It really is extreme up there,” he said. “We had to borrow a go-anywhere vehicle from the Norwegian army just to get around; it’s very cold indeed; and the darkness gets pretty depressing after a few days. If you’re trying to survive in that environment you really want to get as much visual information as you possibly can and it seems that in physiological terms reindeer may be going to extraordinary lengths to do so”.

Jeffery continued: “The reason for the trip was two-fold – we wanted to measure the wavelengths that reindeer are seeing, and also get an idea of what the world looks like to them.

“We used the facilities at Tromso to examine some captive reindeer and we were able to register visual responses at wavelengths down to about 350-320nm – just over the boundary into ultraviolet. So we know that they are definitely seeing something in the ultraviolet range.”

So with this knowledge they travelled out in their borrowed Norwegian army vehicle with two cameras in tow – one standard digital camera, and an adapted camera that is sensitive to UV.

“We noticed smears on the snow and quite quickly realised that it was reindeer urine. In fact, we even tested our own urine and found exactly the same effect”

“As soon as we took images with the cameras side-by-side, we started to notice some really important features in the environment,” said Jeffery. “Certain things were appearing black in stark contrast to the snow when viewed with the UV sensitive camera, but the same items were really just a grey smudge when viewed through the ordinary one. “We noticed smears on the snow and quite quickly realised that it was reindeer urine. In fact, we even tested our own urine and found exactly the same effect. It seems likely then that they are not only seeing urine from potential mates and adversaries in their own species, but they may well be noticing urine from predators as well.” In addition to spotting the urine of predators, it is notable that fur absorbs a lot of UV. Indeed, the thicker the fur, the more it absorbs. The traditional predator of the arctic reindeer is the grey wolf, which has very dense fur and will, to them, show up very black against the white snowscape. Under such cold conditions, finding food is imperative and one of the main sources during the winter months comes from the various lichens that grown on rocks and trees. “Reindeer will do virtually anything for a certain type of lichen – they love it,” said Jeffery. “Often the lichens are under the snow, perhaps with just a little bit of pale green poking up. But with the UV camera lichens showed up really dark against the snow and were much easier to spot.” So, this ability of reindeer to not only see ultraviolet light, but also make sense of an image, helps them to stay safe and fed and also plays a role in their social interactions.

It’s perhaps sobering to think that other animals do not see the world in the way that we do. In the mammalian kingdom only monkeys see the world in the same way as humans – perhaps not surprising given our closeness in evolutionary terms. Reindeer are not completely alone in seeing UV light, but so far they are the only mammal that does it in the way they do.

“Some rodents see in the UV as well,” said Jeffery. “And, like reindeer, that’s probably so they can spot urine, go over to it, smell it, and get some information about the other creatures around them. But rodents have a really weird retina, which doesn’t look anything like the reindeer retina. Rodents aren’t generally dealing with these extremes of light and dark and so they have probably evolved their UV vision due to different sorts of pressures and hence the physiological solution is different.”

Dr Jeffery’s team is now beginning to look for this ability in other arctic mammals, beginning with the arctic seal: “Our initial results from working with the arctic seal, suggest that it is probably doing something very similar to the reindeer, which is really exciting – we might be looking at a physiological adaptation that is peculiar to arctic mammals…a whole new evolutionary niche!”

This research also raises some interesting questions about the effect of UV on eye health. Human eyes are clearly sensitive to UV and our eyes can be damaged if over exposed. Snow blindness is a temporary but painful condition experienced by people going into very bright snowy environments where the UV reflected off the snow causes a burn the front of the eye. It had always been assumed that UV could also damage our sensitive photoreceptors, which cannot be replaced – this would lead to irreversible visual impairment or blindness. But if reindeer can let UV in past their cornea and lens without suffering any such consequences, perhaps there is something we can learn about protecting ourselves from UV. Or perhaps it isn’t as bad for human eyes as we think?

Professor Douglas Kell, Chief Executive, BBSRC said “We can learn a lot from studying the fundamental biology of animals and other organisms that live in extreme environments. Understanding their cell and molecular biology, neuroscience, and other aspects of how they work can uncover the biological mechanism that meant they can cope with severe conditions. This knowledge can have an impact on animal welfare and has the potential to be taken forward to new developments that underpin human health and wellbeing.”