Given their sleek ways, cats have long been a symbol of mysticism, and now the reputation gets a boost from new research that finds common house cats see things that are invisible to us.
Everything from psychedelic stripes on flowers to flashy patterned feathers on birds are likely detectable by cats and certain other animals, while humans remain oblivious to such things. We are also -- perhaps luckily -- missing out on seeing a whole world of urine markers blanketing the landscape.
The secret behind the feline vision "superpower" is ultraviolet light (UV) detection. A new paper, published in the Proceedings of the Royal Society B, found that cats, dogs and certain other animals see this form of light that is usually invisible to humans.
"There are many examples of things that reflect UV, which UV sensitive animals could see that humans can't," co-author Ronald Douglas told Discovery News. "Examples are patterns on flowers that indicate where nectar is, urine trails that lead to prey, and reindeer could see polar bears as snow reflects UV, but white fur does not."
A reindeer, a cat and a dog could therefore probably see a white-furred animal, such as a bunny, hopping through a snow blizzard, while most people would just see a blur of all white.
Douglas, a professor of biology at City University London specializing in the visual system, and co-author Glen Jeffery, a professor of neuroscience at University College London, determined that cats, dogs, rodents, hedgehogs, bats, ferrets and okapis all detect substantial levels of UV.
"It has been known for nearly a hundred years that many invertebrates, such as bees, see UV," Douglas said, adding that birds, fish, and some reptiles and amphibians were added to the list in more recent decades.
"However," he added, "it was assumed that most mammals do not see UV because they have no visual pigment maximally sensitive in the UV and (instead possess) lenses like those of man, that prevent UV reaching the retina."
He explained that visual pigments are the substances that absorb light and turn it into the electrical activity that nerve cells transmit. They turn out not to be always necessary for UV sensitivity. Instead, the "ocular media" (transparent parts of the eye like the cornea and crystalline lens) in certain animals transmits UV wavelengths.
The ability allows more light to reach the retina, "which would be good for a nocturnal cat," Douglas said.
The skill might also help to explain why cats become obsessed with unusual objects, like sheets of paper.
Man-made optical brighteners are sometimes added to paper, fabrics, laundry detergents, cosmetics and shampoos to make them appear brighter. Since optical brighteners absorb light in the UV spectrum, they might appear different, or stand out more, to UV-sensitive animals.
Certain people, such as those who have had their lenses replaced during cataract surgery, can see at least some UV, but most humans cannot.
"We do all assume that it (UV) may be harmful," Jeffery told Discovery News. "I work a lot in the Arctic where UV levels can be very high, particularly in spring and early summer when there is still a lot of snow and ice. These surfaces reflect 90 percent of the UV, so the animals are exposed from above and below. If you do not have snow goggles on, your eyes hurt within 15 minutes."
Studies on reindeer, however, find that repeated exposure to UV doesn't bother them at all.
Cats, reindeer and other animals built to detect UV might be protected from this visual damage in some way. It's also thought that UV light tends to create more blurry images.
"Now, if there is one thing humans are good at, it's seeing detail," Douglas said. "Perhaps that's why they have a lens that removes the UV. If they didn't, the world would appear more blurred."