Tests show Arctic reindeer ‘see in UV’
By Neil Bowdler / 26 May 2011
Arctic reindeer can see beyond the “visible” light spectrum into the ultra-violet region, according to new research by an international team. They say tests on reindeer showed that the animal does respond to UV stimuli, unlike humans. The ability might enable them to pick out food and predators in the “UV-rich” Arctic atmosphere, and to retain visibility in low light. Details are published in the The Journal of Experimental Biology.
UV light is invisible to humans. It has a wavelength which is shorter (and more energised) than “visible” light, ranging from 400 nanometres down to 10nm in wavelength. The researchers first established that UV light was able to pass through the lens and cornea of the reindeer eye by firing light through a dissected sample. The tests showed that light down to a wavelength of about 350nm passed into the eye. They then sought to prove that the animals could “see” the light, by testing the electrical response of the retina of anaesthetised reindeer to UV light. “We used what is called an ERG (electroretinography), whereby we record the electrical response to light by the retina by putting a little piece of gold foil on the inside of the eyelid,” co-author Professor Glen Jeffery of University College London told BBC News.
The tests showed that photoreceptor cells or “cones” in the retina did respond to UV light. “If you’re a bumblebee, you wouldn’t think much of what this animal is doing because it’s seeing in what’s called ‘near UV’ (about 320 to 400nm), but that’s still very high energy stuff.” The researchers believe UV vision could enable the reindeer to distinguish food and predators in the “white-out” of the Arctic winter and the twilight of spring and autumn.
Lichen, on which the animal feeds, would appear black to reindeer eyes, they say, because it absorbs UV light. The animal’s traditional predator, wolves, would also appear darker against the snow, as their fur absorbs UV light. Urine in the snow would also be more discernable in UV vision, which might alert reindeer to the scent of predators or other reindeer. Neither did the animal appear to suffer any damage as a result of seeing in UV, say the researchers, or suffer the “snow blindness” humans can experience in the UV-rich Arctic environment.
Professor Lars Chittka of Queen Mary University London, who has explored the UV capabilities of bees, said the study showed what we call the “visible” spectrum did not apply to most of the animal kingdom. “It’s further evidence that UV sensitivity across animals is the rule rather than the exception, and that humans and some other mammals are actually a minority in not having UV sensitivity,” he said. Professor Chittka was not surprised the UV light appeared to do no damage to the reindeer retina. He said the tests suggested the eye would only admit lower-frequency UV light (“UV-A light”) rather than more damaging higher-frequency light (“UV-B”). Further modelling and behavioural tests would also be needed to verify that reindeer’s apparent capacity to detect UV light really did result in “better detection of predators and arctic lichens”, he said.
The same research team which conducted the reindeer tests will soon repeat the same experiments on seals to see whether they can see into the UV region. Professor Jeffery believes many Arctic animals are likely to have the capacity. “There’s no evidence that Arctic foxes or polar bears suffer from snow blindness, so I bet you that most of the Arctic animals up there are seeing into UV.”
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“The Arctic has extreme seasonal changes in light levels and is proportionally UV-rich because of scattering of the shorter wavelengths and their reflection from snow and ice. Here we show that the cornea and lens in Arctic reindeer do not block all UV and that the retina responds electrophysiologically to these wavelengths. Both rod and cone photoreceptors respond to UV at low-intensity stimulation. Retinal RNA extraction and in vitro opsin expression show that the response to UV is not mediated by a specific UV photoreceptor mechanism. Reindeer thus extend their visual range into the short wavelengths characteristic of the winter environment and periods of extended twilight present in spring and autumn. A specific advantage of this short-wavelength vision is the use of potential information caused by differential UV reflections known to occur in both Arctic vegetation and different types of snow. UV is normally highly damaging to the retina, resulting in photoreceptor degeneration. Because such damage appears not to occur in these animals, they may have evolved retinal mechanisms protecting against extreme UV exposure present in the daylight found in the snow-covered late winter environment.”
Reindeer see a weird and wonderful world of ultraviolet light / May 31, 2011
Researchers have discovered that the ultraviolet (UV) light that causes the temporary but painful condition of snow blindness in humans is life-saving for reindeer in the arctic. A BBSRC-funded team at UCL has published a paper in the Journal of Experimental Biology that shows that this remarkable visual ability is part of the reindeer’s unique adaptation to the extreme arctic environment where they live. It allows them to take in live-saving information in conditions where normal mammalian vision would make them vulnerable to starvation, predators and territorial conflict. It also raises the question of how reindeer protect their eyes from being damaged by UV, which is thought to be harmful to human vision.
Lead researcher Professor Glen Jeffery said “We discovered that reindeer can not only see ultraviolet light but they can also make sense of the image to find food and stay safe. Humans and almost all other mammals could never do this as our lenses just don’t let UV through into the eye. “In conditions where there is a lot of UV – when surrounded by snow, for example – it can be damaging to our eyes. In the process of blocking UV light from reaching the retina, our cornea and lens absorb its damaging energy and can be temporarily burned. The front of the eye becomes cloudy and so we call this snow blindness. Although this is normally reversible and plays a vital role to protect our sensitive retinas from potential damage, it is very painful.”
Human beings are able to see light with wavelengths ranging from around 700nm, which corresponds to the colour red, right through all the colours of the rainbow in sequence to 400nm, which corresponds to violet. Professor Jeffery and his team tested the reindeer’s vision to see what wavelengths they could see and found that they can handle wavelengths down to around 350-320nm, which is termed ultraviolet, or UV, because it exceeds the extreme of the so-called visible spectrum of colours.
The winter conditions in the arctic are very severe; the ground is covered in snow and the sun is very low on the horizon. At times the sun barely rises in the middle of the day, making it dark for most of the time. Under these conditions light is scattered such that the majority of light that reaches objects is blue or UV. In addition to this, snow can reflect up to 90% of the UV light that falls on it. Professor Jeffery continued “When we used cameras that could pick up UV, we noticed that there are some very important things that absorb UV light and therefore appear black, contrasting strongly with the snow. This includes urine – a sign of predators or competitors; lichens – a major food source in winter; and fur, making predators such as wolves very easy to see despite being camouflaged to other animals that can’t see UV.”
This research raises some interesting questions about the effect of UV on eye health. It had always been assumed that human eyes don’t let UV in because of the potential that it will cause damage, just as it does to our skin. In our eyes, UV could damage our sensitive photoreceptors that cannot be replaced. This would lead to irreversible damage to our vision. Arctic reindeer are able to let UV into their eyes and use the information effectively in their environment without suffering any consequences. Professor Jeffery added “The question remains as to why the reindeer’s eyes don’t seem to be damaged by UV. Perhaps it’s not as bad for eyes as we first thought? Or maybe they have a unique way of protecting themselves, which we could learn from and perhaps develop new strategies to prevent or treat the damage the UV can cause to humans.”
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.”