How Science Came To See Ultraviolet Light In Animals

The following is an excerpt from An Immense World: How Animal Senses Reveal the Hidden Realms Around Us by Ed Yong.

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In the 1880s, John Lubbock—banker, archeologist, polymath—split a beam of light with a prism and shone the resulting rainbow onto ants. The ants scurried away from the light. But Lubbock noticed that they also fled from a region just beyond the rainbow’s violet end, which looked dark to his eyes. This area wasn’t dark to the ants, though. It was bathed in ultraviolet—literally “beyond violet” in Latin. Ultraviolet (or UV) light has wavelengths ranging from 10 to 400 nanometers. It is largely invisible to humans, but must be “apparent to the ants as a distinct and separate color (of which we can form no idea),” Lubbock presciently wrote. “It would appear that the colors of objects and the general aspect of nature must present to them a very different appearance from what it does to us.”

At the time, some scientists believed that animals either are colorblind or see the same spectrum that we do. Lubbock showed that ants are exceptional. Half a century later, bees and minnows turned out to see ultraviolet, too. The narrative shifted: Some animals can see colors we can’t, but the skill must be very rare. But after another half century, in the 1980s, researchers showed that many birds, reptiles, fish, and insects have UV-sensitive photoreceptors. The narrative changed again: UV vision exists in many groups of animals, but not in mammals. Still wrong: In 1991, Gerald Jacobs and Jay Neitz showed that mice, rats, and gerbils have a short cone that is tuned to UV. Okay, fine, mammals can have UV vision, but only small ones like rodents and bats. Not so: In the 2010s, Glen Jeffery found that reindeer, dogs, cats, pigs, cows, ferrets, and many other mammals can detect UV with their short blue cones. They probably perceive UV as a deep shade of blue rather than a separate color, but they can sense it nonetheless. So can some humans.

Our lenses typically block out UV, but people who have lost their lenses to surgeries or accidents can perceive UV as whitish blue. This happened to the painter Claude Monet, who lost his left lens at the age of 82. He began seeing the UV light that reflects off water lilies, and started painting them as whitish blue instead of white. Monet aside, most people can’t see UV, which probably explains why scientists were so eager to believe that the ability was rare. In fact, the opposite is true. Most animals that can see color can see UV. It’s the norm, and we are the weirdos.

Ultraviolet vision is so ubiquitous that much of nature must look different to most other animals.* Water scatters UV light, creating an ambient ultraviolet fog, against which fish can more easily see tiny UV-absorbing plankton. Rodents can easily see the dark silhouettes of birds against the UV-rich sky. Reindeer can quickly make out mosses and lichens, which reflect little UV, on a hillside blanketed by UV-reflective snow. I could go on.

I’m going to go on. Flowers use dramatic UV patterns to advertise their wares to pollinators. Sunflowers, marigolds, and black-eyed Susans all look uniformly colored to human eyes, but bees can see the UV patches at the bases of their petals, which form vivid bullseyes. Usually, these shapes are guides that indicate the position of nectar. Occasionally, they are traps. Crab spiders lurk on flowers to ambush pollinators. To us, these spiders seem to match the colors of their chosen blooms, and they’ve long been treated as masters of camouflage. But they reflect so much UV that they are highly conspicuous to a bee, which makes the flowers they sit upon that much more alluring. Rather than blending in, some of them attract their UV-sensitive prey by standing out.

Many birds also have UV patterns in their feathers. In 1998, two independent teams realized that much of the “blue” plumage of blue tits actually reflects a lot of UV; as one of them wrote, “Blue tits are ultraviolet tits.” To humans, these birds all look much the same. But thanks to their UV patterns, males and females look very different from each other. The same is true for more than 90% of songbirds whose sexes are indistinguishable to us, including barn swallows and mockingbirds.

It’s not just humans who can’t see UV patterns. Since UV light is heavily scattered by water, predatory fish that have to spot prey at a distance are often insensitive to it. Their prey, in turn, have exploited this weakness. The swordtail fish of Central American rivers look drab to us, but as Molly Cummings and Gil Rosenthal showed, males of some species have strong UV stripes along their flanks and tails. These markings are alluring to females, but they’re invisible to the swordtails’ main predators. And in places where those predators are more common, swordtails have more vivid UV markings. “They could get away with being super-flamboyant” without attracting danger, Cummings says. Similar secret codes exist in Australia’s Great Barrier Reef, home to the ambon damselfish. To human eyes, it resembles a lemon with fins, and looks identical to other closely related species. But Ulrike Siebeck found that its head is actually streaked with UV stripes, as if invisible mascara had run all over its face. Predators can’t see these markings, but the ambons themselves use them to distinguish their own kind from other damselfish.

For us, UV feels enigmatic and intoxicating. It’s an invisible hue lying just on the edge of our vision—a perceptual void that our imaginations are keen to fill. Scientists have often attributed special or secret significance to it, treating it as a channel for covert communication. But aside from the ambon damselfish and swordtails, most such claims have foundered.* The reality is that UV vision and UV signals are extremely common. “My personal view is that it’s just another color,” Innes Cuthill, who studies color vision, tells me.

Imagine what a bee might say. They are trichromats, with opsins that are most sensitive to green, blue, and ultraviolet. If bees were scientists, they might marvel at the color we know as red, which they cannot see and which they might call “ultrayellow.” They might assert at first that other creatures can’t see ultrayellow, and then later wonder why so many do. They might ask if it is special. They might photograph roses through ultrayellow cameras and rhapsodize about how different they look. They might wonder whether the large bipedal animals that see this color exchange secret messages through their flushed cheeks. They might eventually realize that it is just another color, special mainly in its absence from their vision. And they might wonder what it would be like to add it to their Umwelt, bolstering their three dimensions of color with a fourth.

Excerpted from An Immense World © 2022 by Ed Yong. Used by permission of Random House, an imprint of Random House, a division of Penguin Random House LLC, New York. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.