Unlike humans and most other mammals, birds can see ultraviolet light. But not all avian eyes are created equal, according to new research. Apparently, birds with smaller eyes are better able to perceive the UV spectrum.
Many animals can see ultraviolet light, including some amphibians, reptiles and fish. Even a few select mammals can see UV light, such as rats, bats and, surprisingly, reindeer. In general, this ability helps animals attract mates, find food and detect predators.
In the 1970s, scientists first learned that birds also see ultraviolet light, when they discovered UV-sensitive photoreceptors in the retina of certain birds. "[The researchers] found those receptors and saw certain behavioral responses to ultraviolet light," explained Olle Lind, a biologist at Lund University in Sweden. Today, scientists don't even need to look at an animal's eyes to see if it has the photoreceptors necessary to see UV light — they can figure it out by looking at the genome.
But possessing UV-sensitive photoreceptors is only half the equation. "If you want to see something, you have to get the light through the eye and to the retina," Lind told io9. That is to say, even if you have the right receptors, you will only see UV light if your ocular media — which consists of the cornea, aqueous humour, lens and vitreous humour — is transparent to the light. Your UV vision will be dampened if parts of your eyes are absorbing or scattering some of the UV light before it gets to your retina.
Surprisingly, very few studies on animal UV vision have actually taken into account this so-called ocular media transmittance (OMT). Recently, Lind and his colleagues looked at the OMT of a few raptor species, and found that the birds' eyes didn't actually allow much UV light to get to the retina. So they decided to broaden their tests and look at other birds, too.
For their new study, just published in the journal Proceedings of the Royal Society B, Lind and his team looked at the OMT of 38 species of birds, including ostriches, common swifts, mallards, tawny owls, Bourke's parrots, Eurasian sparrowhawks and great tits, among others. Instead of just analyzing the genome, as they would have done if they were surveying photoreceptors, the team had to acquire and experiment with the eyes of some of the bird species.
"You can't say anything about the ocular media just from the genome," Lind said. To get the eyes that they required, the researchers teamed up with a bird rescue service in southern Sweden. Whenever program members came across a bird they couldn't save, they'd call the researchers and rush the animals to Lund University.
From here, the scientists performed a simple procedure that involved removing the eyes from a freshly dead bird, shining a light through the eyes and measuring how much light is transmitted to the retinas. "You have to do it properly, but it's very straightforward," Lind said. They then found OMT data for various other bird species in the scientific literature. They also measured the size of the birds' eyes and modeled their color vision.
The scientists found that the eyes of smaller birds were more UV transparent than the eyes of larger birds. This means that small birds, such as songbirds, can take full advantage of ambient UV light, while larger birds, such as swifts and raptors, block a lot of the UV light from reaching the retina.
"If you want to be highly UV sensitive — be a UV specialist — you have to be small," Lind said. "For me, that is quite thrilling because it means that your perception of the world is dependent on your physical size."
It's not clear why large eyes block more UV light than small eyes, but Lind suggests that it may be difficult to make biological material transparent to ultraviolet light (and it's therefore easier to do this on a small scale).
On the other hand, having eyes with low UV transmittance may be a kind of adaptation for raptors and other birds that hunt out in the open air. These birds have the highest visual acuity of all animals, so blocking out most UV light may be a way for them to keep their sharp vision. Ultraviolet light is also very damaging to the retina — blocking this light likely protects their eyes while they hunt in broad sunlight.
So what does this all mean for giving people UV vision?
Well, humans actually have pigments that are sensitive to UV light, but we fail where the raptors fail. That is, our lens contains pigments that block some 95 percent of the incident light at wavelengths shorter than 400 nanometers (ultraviolet light is between 10 and 400 nm). So naturally, removing our lenses would give us ultraviolet vision, and this is actually what people did in the past to deal with cataracts, Lind said.
The most famous example of this procedure involves Claude Monet. In his 80s, Monet had a cataract in one of his eyes, which caused colors to look muddy. After many failed treatments, he finally had the lens of that eye removed, allowing him to see ultraviolet light, Lind explained.
People who don't have lenses often report seeing ultraviolet light that looks whitish blue or whitish violet. This happens because our color receptors (red, green and blue) are all sensitive to ultraviolet light, so the light comes in as a mixture of the three receptors, with a slight nod to blue side of the spectrum.
Birds, however, have a fourth color receptor for ultraviolet light. What birds see — even those with large eyes that block a lot of UV light — is nothing like what you would see if you had your lenses removed. So while you may get some UV vision if you had your lenses removed, it won't be "true" UV vision.
"It's impossible to imagine that," Lind said. "Think about a new color, think about something you've never seen. We don't even have a word for that — it's mind bending."
Check out the study over in the Proceedings of the Royal Society