Ever walked into a room to see your cat (or dog for that matter) just staring at the wall? The internet is full of ‘my cat sees dead people’ type stories. It’s possible that they’re seeing something that you can’t, and that’s ultraviolet (UV) light.
Light, as we see it, is waves of electromagnetic (EM) radiation. The distance from one wave to the next (the wavelength) determines what colour we see the light as. The light that we see has a range of wavelengths from about 400nm (blue) to 700nm (red). EM radiation with a slightly shorter wavelength is known as UV light. Usually we can’t see UV light. Despite this, if you go outdoors you’ll be bathed in it, especially if you’re in direct sunlight. So UV light is everywhere.
Side step to some ocular anatomy and physiology:
The eye is a pin-hole camera. Light enters the globe through the hole in the iris, is focused by the lens, passes through some eye jelly and then hits the retina, which lines the back of the eye. The retina is covered in photoreceptors (photo=light, receptor=receiver). We have different photoreceptors for different things; some of our photoreceptors are specialised to deal with interpreting colour.
The colour photoreceptors (known as ‘cones’; they look like cones) contain pigments that are sensitive to specific wavelengths of light. Humans express three pigments, which have peak sensitivities to blue, green and red light. Mixtures of these three give us every colour that we see every day. Remember squinting at old TV’s that had chunky pixels? Each pixel is made of a red/blue/green light. I’m struggling to do this with 1080p HD.
At this point it’s tempting to talk about the Mantis Shrimp. Tempting because it’s relevant, and because they’re SO COOL! But I won’t. People far better qualified and/or wittier have written posts them (see the Oatmeal). I’ll restrain myself to say that our three visual pigments are fairly modest compared to some other animals.
So our cones are maximally sensitive to blues, greens and reds. But – they are also mildly sensitive to wavelengths just shorter than the peak sensitivity. Does this mean we can see short wave light, like UV? Well no, not usually.. But! Something weird happens if you remove a person’s lens, as was done in old fashioned cataract treatment. People reported seeing blues that seemed strange and vivid. Their ‘blue’ cone pigment was reacting to UV. Why not before? It tells us that the lens is filtering UV light.
This question of, ‘How much UV light does the lens block’ was recently addressed by a cool study that looked at lenses across mammals. This, along with the research group’s other work comes up with some illuminating findings (ha! illuminating!). So they took a load of mammalian lenses from 144 eyes, representing 38 different mammal species. What I like is the variety present here. Amongst others, they have cats, dogs, mice, meerkats, a red-faced spider monkey, a tree-shrew and my favourite, an okapi. All of these animals died for reasons unrelated to this study. See here for the list.
These animals all come from different parts of the world, have massively different habitats and behave very differently from each other. This makes comparing the results between them really fun.
They shone light of varying wavelength through the lenses, and recorded what came through the other side. They also checked out the cornea and the vitreous humour (the eye jelly we spoke about before), but these structures don’t impact upon light absorption.
The results are as varied as the species studied. It was already known that murid rodents (mice, rats) let in a lot of UV light, and that primates block most of it. It’s the inbetweeners which we didn’t know much of before. Cats and dogs let in loads of UV light. Does that mean they see into the UV spectrum? This bit we don’t know. Having watched Yashi (Scatterfeed’s cat) stare at the ceiling for unhealthy lengths of time, I do wonder. It’s possible she’s just being weird, although UV sensitivity and weirdness are not mutually exclusive phenomena.
What else did we find out? Meerkats are very good at blocking UV. In fact they’re so good, they actually block out some of the light in the blue wavelength, so probably things appear yellowish. Their lens even looks yellowish. To a meerkat, life looks like one continuous flashback from Breaking Bad. This might be really useful if you’re a meerkat, living in the exposed sunlight of the Kalahari desert. UV rays are harmful to the eye over time, so having a lens that blocks these rays out is the biological equivalent to sunglasses. Yep, meerkats are cool.
Reindeer are really strange. If you’ve ever been skiing, you’ll know that sunglasses are a must in a snow abundant environment. However, reindeer for some reason let in a reasonable amount of UV light. Not only that, but we know from previous work that they do respond to UV light, so these guys are definitely seeing the ultraviolet stuff. Seems like a poor plan if you live on the Arctic tundra. But – when you imagine how a reindeer views their world, one benefit leaps out. Look at the picture below. This is how we see a polar bear in a snowy landscape.
This is how a reindeer might see the same polar bear. Note – my brain has never seen UV light and has no clue as to how reindeer see the world. Hence, I’ve chosen to colour the polar bear pink.
Ok… so there’s a bit of creative interpretation going on. There’s science here somewhere. Snow is highly reflective of all wavelengths of light, but white fur tends to absorb certain wavelengths, like UV light. Reindeers’ ability to see UV light enables them to contrast predators with white fur from their white surroundings. Useful, and fabulous. Despite their longterm (over about 20years) UV exposure, reindeer eyes don’t seem to suffer from the deleterious effects of UV light. This is pretty interesting, as it hints that they have some other kind of protective mechanism.
It’s tempting to wonder why some mammals (plus a ton of non-mammals) can let in UV light. UV is nothing special, it’s just short wavelength light; no big deal. A better question would be: why do humans block it? If we assumed that letting UV light through the lens is an ancestral state to mammals (a distinct possibility), then something has driven selected species (like humans) to block this ability over time. The benefits of blocking UV light could be the meerkat’s sunglasses effect, or the improvement in visual acuity (UV light tends to scatter more, giving slightly fuzzier images).
Of course a lot of this is conjecture; I know that Yashi’s lens is letting through UV light, but I don’t know if she’s seeing it. And there’s no argument that says ‘If her lens lets through UV, surely it makes sense that she will be able to see it.’ Characteristics found in modern day animals don’t always have a current utility. So the possibility remains that she’s merely being weird, or maybe is actually staring at something obvious.
References and links:
- Douglas RH and Jeffrey G, The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among animals, Proc.R.Soc. B, 7th April 2014, vol 281 – open access
- Hogg C et al, Arctic reindeer extend their visual range into the ultraviolet, J Exp Biol 15th June 2011, vol 214 – open access
- Hunt DM et al, Spectral tuning of shortwave-sensitive visual pigments in vertebrates, Photochem Photobiol 2007 83(2) – abstract only
- The Oatmeal, Why mantis shrimp are my new favourite animal