The Amazing World Of UV Vision, As Seen Through The Eyes Of…. Animals

What does the world look like through the UV-sensitive eyes of various creatures from insects, birds, animals and fish? Welcome to the first post of a four part series looking into the wonderful ultraviolet world of various species across the planet. In this post I find out which animal has the strongest UV vision, why they have evolved to see this way, and illustrate how they may visually perceive the world. I also find out how much of the UV world our beloved pet dogs and cats can see compared to other animals.

L: Human vision. R: Mouse vision. Mice can't see red, so see the world in green and blue hues, as well as ultraviolet.

The world is brimming with colour - and not just your usual rainbow of colours most humans perceive. A large variety of creatures have taken their colour vision to the next level, evolving extra photoreceptors to see well into a spectrum largely invisible to humans. The mysterious ultraviolet (UV) spectrum sits just outside the 'visible' spectrum (light wavelengths from red to purple) which humans see.

It's worth pointing out that fully functioning human vision is a marvel of evolution in its own right. We are one of only a few species with trichromatic vision (red, green and blue photo-receptor pigments) that includes the red cone receptor. Apart from our close primate relatives, most mammals are dichromatic, seeing in mainly just green and blue hues. Some however, have evolved extra photoreceptors sensitive to different energy wavelengths, giving them the ability to see into the ultraviolet spectrum.

Animals: From Pets to Pests – which one can see UV the best?

In 2014, London-based academics Ron Douglas and Glen Jeffery published the findings of their experiment into the visual UV sensitivity of mammals. They conducted experiments on the lenses of 38 different mammalian species, ranging from familiar animals such as sheep, cows, cats and dogs, to the more exotic okapis, red pandas and Arabian orynx.

Their findings concluded that UV sensitivity was widespread among mammals, and also helped solve a mystery that had baffled scientists for years.

Scientists had observed how many animals actively avoided going near power lines despite them not being impassible barriers. The results of of the 2014 experiment proved that most animals can actually see the random and terrifyingly intense flashes of corona discharges produced by power lines. Filmed using special UV cameras, this footage shows how the humming and crackles of electricity, actually appear as bright balls of flickering UV light. Electrical wires may be creating just as much, if not more, geographical disruption to wildlife movement than roads.

Data from the Douglas and Jeffery experiment concluded that the creature able to see furthest into the UV spectrum was the humble mouse. Mice have dichromatic vision, seeing in green and blues. Unlike humans they do not have a red photoreceptor but see the world in bluey-green hues as well as UV, which adds a violet glow to any object that reflects ultraviolet.

As a curious side note, over a decade ago mice were scientifically bred to have human trichromatic vision, in an experiment to determine how adaptable the mammalian brain is. The experiments successfully tweaked their genes to include the red photoreceptor. These super mice were proven to able to see the world the way humans do, although is not known how much this would have affected the mouses ability to see UV.

Since mice and other rodents can see the UV spectrum so well, it's no coincidence then that the urine of many rodent species strongly reflects ultraviolet light. Although scientists were aware of the importance of communicating through urine, it was initially considered to be scent marking. Now we know it's also a visual cue for helping these little creatures find their way. According to the National Geographic 'fresh [rodent] urine reflects more strongly than old'.... showing 'how fresh a path is, what came past, when and what sex the animal was.'

The down side is that birds of prey also have UV vision and have improved their hunting success by following these rodent urine trails – the brighter ones being more likely to provide a fresh meal.

What about our favourite domestic critters?

Finally, if you were wondering whether your beloved pet cat or dog has special UV vision powers, the research by Douglas and Jeffery put them squarely in the top 10 – check out the infographic below. Interestingly, the reindeer, who's famously impressive UV vision was mentioned in my post 'What Exactly Is UV Or 'Black' Light? And Why We Humans Are All Colour-blind....' ranked a mere 15th.

Below is a round up of the 10 animals in the experiment most sensitive to UV. For more information on the other 28 species examined check out this article by Douglas and Jeffrey:

Mice see furthest into the UV spectrum, though Fruit Bats see the largest range within the UV spectrum.

Why do animals see differently from humans?

Most animals’ vision is highly tuned to their environments and the times of day they are awake. In the case of reindeer, it helps them spot preditors such as wolves, that would otherwise be totally camouflaged when viewed only in visible light, especially during long, snowbound winter nights.

Why humans have lenses that block UV and many animals don't, remains a mystery. A correlation between high visual accuity and cornias that block UV has recently been identified. This helps us to answer the question why humans don't see into the UV spectrum rather than speculate on why most animals do.

Human colour vision evolved as we became more active during daylight hours (diurnal) and needed to distinguish colours better during the day. Our vision enabled us to spot the difference between unripe, potentially hard to digest fruits, and the ripe ones. This may also explain why some marsupials who mainly live on a diet of foliage and berries also share our trichromatic vision. Humans are more heavily reliant on sight than any other sense when compared to other animal species. Although many diurnal animals are dichromatic, they support their 'poorer' vision with other stronger senses, usually hearing and smell, as well as seeing in ultraviolet.

How the 10 Animals with the best UV vision compare to the human vision spectrum (visible light).

Mammals are not the only creatures to see into the ultraviolet spectrum. Our planet is populated with countless species of insects, birds and fish all with some degree of UV vision. Over the course of 4 posts, I will be taking a closer look at each category to find out what they see, why they evolved this form of vision, and what that looks like to us. So why not check out my other posts, for the full picture:

'The Amazing World Of UV Vision, As Seen Through The Eyes Of ….Birds'

'The Amazing World Of UV Vision, As Seen Through The Eyes Of ….Insects'

'The Amazing World Of UV Vision, As Seen Through The Eyes Of ….Fish'