First published October 14, 2017.
joncarling
Yes, there is good evidence that the eyes of cats, dogs and many other mammals let through ultraviolet (UV) light that the human eye filters out. The key study, published in 2014 in Proceedings of the Royal Society B by Ronald Douglas of City University London and Glen Jeffery of University College London, measured how much short-wavelength light passes through the lenses of 38 mammal species. Dogs, cats, ferrets, hedgehogs and several rodents transmitted a large share of UVA light, while humans and other daytime primates blocked almost all of it. That suggests these animals are probably sensitive to some UV. It does not mean they see ghosts, spirits or "frequencies" of another world, a claim that was often attached to this research online.
What ultraviolet light is and why humans cannot see it
Light is a band of electromagnetic radiation sorted by wavelength. Human vision covers roughly 400 to 700 nanometres, from violet to red. Ultraviolet sits just beyond violet, at shorter wavelengths; the part closest to visible light, UVA, runs from about 315 to 400 nanometres.
Interestingly, the light-sensitive cells in the human retina can respond to some UVA. The barrier is the lens at the front of the eye, which absorbs most near-UV before it reaches the retina, while the cornea blocks even shorter wavelengths. People who have lost their natural lens, a condition called aphakia, sometimes report seeing UVA as a whitish blue or violet glow. Modern replacement lenses used in cataract surgery are generally designed to block UV as well.
How the 2014 lens study worked
Douglas and Jeffery examined lenses from 38 species in 25 families and nine orders, taken from animals after death. They placed each intact lens in a spectrophotometer and measured how much light passed through it at every wavelength from 300 to 700 nanometres.
For each species they recorded two numbers: the wavelength at which the lens let through half the light, and the percentage of UVA (315 to 400 nm) that made it through. This is a measurement of the eye's optics, not a behavioural test of what the animal experiences.
Which animals let UV through
The results showed a wide spread:
- High UVA transmission: mice and black rats (around 80 percent), hedgehogs (about 66 percent), dogs (about 61 percent), cats (about 59 percent), ferrets (about 56 percent) and the okapi (about 53 percent).
- Almost no UV transmission: daytime primates, including humans, as well as grey squirrels, prairie dogs, tree shrews and meerkats. Their lenses are slightly yellow and contain a pigment that absorbs most strongly around 360 to 370 nanometres.
- In between: many larger animals, such as horses, cattle and alpacas, whose bigger lenses filter out much of the UV without looking yellow.
The authors also found that older animals of some species, such as rats and mice, transmitted less short-wavelength light, showing that lenses change with age, much as human lenses yellow over a lifetime.
Why some eyes block UV and others do not
A broad pattern emerged. Species that are active at night or at dawn and dusk tend to have UV-transmitting lenses, while strongly daytime species tend to block UV. The pattern is not perfect: the okapi is active by day yet transmits UV, and a nocturnal flying squirrel had a clear lens.
The researchers linked UV-blocking lenses to retinas built for sharp detail, with many cone cells. Short-wavelength light scatters more and focuses at a slightly different point from longer wavelengths, so filtering it out can make images crisper. Night-active animals gain more from catching every bit of available light than from extra sharpness, so letting UV through is useful to them. Blocking UV may also help protect the retina, though reindeer, which live in UV-rich snowy landscapes, seem to cope well with UV reaching their eyes.
Does a clear lens mean the animal truly sees UV?
Not automatically, and the authors were careful about this. A lens that passes UV makes UV sensitivity possible, because all visual pigments absorb some UV. Whether the brain then uses that signal depends on the photoreceptors and on neural processing that is still not fully understood.
Dogs and cats do not have a cone tuned specifically to UV, as many birds and insects do. So they probably cannot perceive UV as a separate colour. It is more likely that UV adds a little extra brightness or contrast to what they already see. The paper suggested possible uses, such as reindeer picking out predators against snow, or rodents following UV-reflective urine trails, but it did not prove any specific function in pets.
The study also had limits: some species were represented by only one or two lenses, and it measured eyes rather than behaviour.
Myths: spirits, ghosts and "seeing other frequencies"
When the research spread online, many sites used it to suggest that pets can see spirits or invisible beings. Nothing in the study supports that. It shows only that some animals' lenses allow ordinary sunlight of slightly shorter wavelengths to reach the retina. UV is a normal part of daylight, and it can be measured with simple instruments.
If your cat stares intently at an empty corner, more everyday explanations are likely: cats hear higher-pitched sounds than people, notice tiny movements such as insects or dust, and react to reflections and shadows. Their sensitivity to UV, if it plays any role, would be one more small difference in how they experience the same room.
The real takeaway is more interesting than a ghost story. Each species lives in a slightly different sensory world, shaped by when it is active and what it needs to see. Your dog or cat shares your home but not quite your view of it.
Sources: Douglas & Jeffery (2014), The spectral transmission of ocular media suggests ultraviolet sensitivity is widespread among mammals, Proc. R. Soc. B; Wikipedia: Ultraviolet. Header image credit: joncarling.
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