How science came to see ultraviolet light in animals

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

In the 1880s, John Lubbock – banker, archaeologist, polymath – split a beam of light with a prism and shone the resulting rainbow on ants. The ants rushed away from the light. But Lubbock noticed that they were also fleeing from a region just beyond the purple end of the rainbow, which looked dark to his eyes. This area was not dark for the ants, however. It was bathed in ultraviolet light, 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 ants as a distinct and separate color (of which we can form no idea),” Lubbock wrote foresight. “It would seem that the colors of objects and the general appearance of nature must present a very different aspect to them than what it does to us.”

At the time, some scientists thought the animals were color blind or saw the same spectrum as us. Lubbock showed that ants are exceptional. Half a century later, bees and minnows were also found to see ultraviolet. The narrative has changed: some animals can see colors that 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 had UV-sensitive photoreceptors. The narrative has 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 short cones that are UV-tuned. Okay, okay, mammals can have UV vision, but only small ones like rodents and bats. Not so: In the 2010s, Glen Jeffery discovered 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 distinct color, but they can still feel it. Some humans can too.

Our lenses generally block UV, but people who have lost their lenses through surgery or accidents may perceive UV as whitish blue. This is what happened to the painter Claude Monet, who lost his left lens at the age of 82. He started seeing the UV light reflecting off the water lilies and started painting them whitish blue instead of white. Monet aside, most people can’t see UV, which is probably why scientists were so keen to believe 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’re the weirdos.

Ultraviolet vision is then omnipresent this a lot of nature to have to look different from most other animals.* The water scatters UV light, creating an ambient ultraviolet fog, against which fish can more easily see the tiny UV-absorbing plankton. Rodents can easily see the dark silhouettes of birds against the UV-rich sky. Reindeer can quickly distinguish mosses and lichens, which reflect little UV, on a slope covered with UV-reflecting snow. I could go on.

I will continue. The flowers use spectacular UV patterns to advertise their wares to pollinators. Sunflowers, marigolds and black-eyed Susans all appear uniformly colored to human eyes, but bees can see the UV spots at the base of their petals, which form vivid bubbles. Usually these shapes are guides that indicate the position of the nectar. Sometimes they are traps. Crab spiders hide on flowers to ambush pollinators. To us, these spiders seem to match the colors of their chosen flowers, and they’ve long been treated as masters of camouflage. But they reflect so much UV that they are very visible to a bee, which makes the flowers they sit on much more attractive. 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 reflected a lot of UV; as one wrote, “blue tits are ultraviolet tits”. To humans, these birds all look alike. But thanks to their UV patterns, males and females are 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. Because UV light is strongly scattered by water, predatory fish that need to spot their prey from a distance are often insensitive to it. Their prey, in turn, exploited this weakness. Billfish from Central American rivers look dull to us, but as Molly Cummings and Gil Rosenthal have shown, males of some species have strong UV stripes along their sides and tails. These markings attract females, but they are invisible to the swordtail’s main predators. And in places where these predators are more common, swordtails have more vivid UV markings. “They could get away with being super flamboyant” without drawing danger, Cummings says. Similar secret codes exist in Australia’s Great Barrier Reef, home to the ambon damsel. In human eyes, it looks like a lemon with fins and resembles other closely related species. But Ulrike Siebeck discovered that her head was actually streaked with UV streaks, as if invisible mascara had run down her face. Predators cannot see these markings, but the ambons themselves use them to distinguish their own species from other damselflies.

For us, UV is enigmatic and intoxicating. It’s an invisible hue located just at the edge of our vision – a perceptual void that our imagination is keen to fill. Scientists have often assigned it a special or secret meaning, treating it as a secret communication channel. But aside from the Ambon Damsels and the Swordtails, most of those claims have sunk. * 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 could 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 might call “ultrayellow.” They might first claim that other creatures can’t see ultra-yellow, then later wonder why so many of them do. They might ask if it’s special. They could photograph roses through ultra-yellow cameras and rave about how different they are. They might wonder if the large bipedal animals that see this color exchange secret messages through their red cheeks. They might eventually realize that it’s just another color, special mostly in its absence from their vision. And they might wonder what it would be like to add it to their Umwelt, bolstering their three color dimensions with a fourth.

Extract of A huge world © 2022 by Ed Yong. Used with permission from 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 the written permission of the publisher.