How science discovered 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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An immense world: how animal senses reveal the hidden realms around us

In the 1880s, John Lubbock—banker, archaeologist, scholar—split a beam of light with a prism and shone the resulting rainbow on ants. The ants ran away from the light. But Lubbock noted that they also fled from an area just beyond the violet end of the rainbow, which looked dark to his eyes. However, this area was not dark to the ants. It was bathed in ultraviolet – literally “beyond violet” in Latin. Ultraviolet (or UV) light has wavelengths from 10 to 400 nanometers. It is largely invisible to humans, but should be “visible to the ants as a distinct and distinct color (of which we can form no idea),” Lubbock wrote presciently. “It seems that the colors of objects and the general aspect of nature must have a very different appearance to them than what it does to us.”

At the time, some scientists believed that animals are color blind or see the same spectrum as we do. Lubbock showed that ants are exceptional. Half a century later, bees and whitefish were also found to see ultraviolet. The story changed: some animals can see colors that we can’t, but the ability 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 story 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 attuned to UV. Okay, fine, 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 see UV as a deep shade of blue rather than a distinct color, but they can still feel it. Some people can do that too.

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Our lenses usually block UV, but people who have lost their lenses through surgery or accidents may perceive UV as whitish blue. This happened to the painter Claude Monet, who lost his left lens at the age of 82. He started seeing the UV light reflecting off water lilies and started painting them whitish blue instead of white. Other than Monet, most people can’t see UV, which probably explains why scientists were so eager 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 crazy ones.

Ultraviolet vision is So omnipresent That a lot from nature should They look different from most other animals.* Water scatters UV light, creating an ultraviolet fog in the environment that makes it easier for fish to see minuscule 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 hill covered with UV-reflecting snow. I could go on.

I am going 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 spot the UV spots at the base of their petals, which form vibrant florets. Usually these shapes are guides that indicate the position of nectar. Sometimes they are traps. Crab spiders lurk on flowers to ambush pollinators. To us, these spiders seem to match the colors of their chosen flowers, and they have long been treated as masters of camouflage. But they reflect so much UV that they are very noticeable to a bee, which makes the flowers they sit on much more attractive. Rather than stand out, 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 pretty much the same. But thanks to their UV patterns, males and females look very different. The same is true for over 90% of songbirds whose sex is indistinguishable to us, including barn swallows and mockingbirds.

It’s not just people who can’t see UV patterns. Because UV light is strongly scattered by water, predatory fish that need to see prey from a distance are often insensitive to it. Their prey, in turn, exploited this weakness. The swordtails of Central American rivers look drab to us, but as Molly Cummings and Gil Rosenthal showed, males of some species have strong UV streaks along their flanks and tails. These markings are attractive to females, but they are invisible to the swordtail’s main predators. And in places where those predators are more common, swordtails have more vibrant UV markings. “They can 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 appears identical to other closely related species. But Ulrike Siebeck discovered that his head was actually covered with UV streaks, as if invisible mascara had been all over his face. Predators cannot see these markings, but the ambons themselves use them to distinguish their own species from other damsels.

To us, UV feels puzzling and intoxicating. It’s an invisible hue that lies just on the edge of our vision – a perceptual void that our imaginations are eager to fill. Scientists have often assigned a special or secret meaning to it, treating it as a channel for secret communication. But aside from the ambon damselfish and swordtails, most such claims have foundered.* The reality is that UV vision and UV signals are very common. “My personal opinion is it’s just a different color,” says Innes Cuthill, who studies color vision.

Imagine what a bee could say. They are trichromats, with opsins most sensitive to green, blue and ultraviolet. If bees were scientists, they would be amazed at the color we know as red, which they can’t see and which they might call “ultra yellow.” They first claim that other beings cannot see ultra-yellow, and later wonder why so many do. They may ask if it is special. They can photograph roses with ultra-yellow cameras and tell them how different they look. They may wonder if the large bipedal animals that see this color exchange secret messages through their red cheeks. They might eventually realize it’s just a different color, especially since it’s not in their view. And they might wonder what it would be like to add it to their Umwelt and amplify their three dimensions of color with a fourth.

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taken from An immense 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 extract may be reproduced or reprinted without the written permission of the publisher.