How science came to see ultraviolet light in animals

How science came to see ultraviolet light in animals

The following is an excerpt from A huge world: how the minds of animals reveal the hidden worlds around us by Ed Yong.

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A huge world: how the minds of animals reveal the hidden worlds around us

In the 1880s, John Lubbock – banker, archaeologist, polymath – shared a ray of light with a prism and shone the resulting rainbow on ants. The ants ran away from the light. But Lubbock noticed that they also fled from a region just beyond the violet end of the rainbow, which looked dark to his eyes. However, this area was not dark for the ants. 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 “obvious to the ants as a distinct and separate color (of which we have no idea),” Lubbock wrote predictably. “It seems that the colors of objects and the general aspect of nature must give them a completely different look than what it does to us.”

At the time, some researchers believed that animals are either color-blind or see the same spectrum as we do. Lubbock showed that ants are exceptional. Half a century later, bees and elritsa also appeared to see ultraviolet light. The story changed: Some animals can see colors that we can not, 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 story changed again: UV vision is found 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 set to UV. Okay, well, 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 still feel it. So can some people.

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Our lenses usually block UV, but people who have lost their lenses due to operations or accidents may perceive UV as white-blue. This happened to the painter Claude Monet, who lost his left lens at the age of 82. He began to see the UV light reflected from water lilies and began to paint them as white-blue instead of white. Apart from Monet, most people cannot see UV, which probably explains why scientists were so eager to believe that the ability was rare. In fact, it is the opposite. Most animals that can see color can see UV. That’s the norm, and we’re weird.

Ultraviolet sight is so prevalent that very of nature must looks different from most other animals. * Water diffuses UV light, creating an ambient ultraviolet fog, against which fish can more easily see small UV-absorbing plankton. Rodents can easily see the birds’ dark silhouettes against the UV-rich sky. Reindeer can quickly distinguish mosses and lichens, which reflect some UV, on a slope covered by UV-reflecting snow. I could go on.

I will continue. Flowers use dramatic UV patterns to market their wares to pollinators. Sunflowers, marigolds and black-eyed Susans all look equally colored to human eyes, but bees can see the UV spots at the base of their petals, which form living bullseyes. Usually these forms are guides that indicate the position of the nectar. Sometimes they are traps. Crab spiders lurk on flowers to lay an ambush for 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 conspicuous to a bee, which makes the flowers they sit on so much more attractive. Instead of blending in, some of them attract their UV-sensitive prey by sticking 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 breasts.” To humans, all these birds look more or less the same. But thanks to their UV patterns, males and females look very different. The same goes for more than 90% of the songbirds whose sex is impossible for us, including barn swallows and mockingbirds.

It’s not just people who can’t see UV patterns. Because UV light is highly diffused by water, predatory fish that must detect prey at a distance are often insensitive to it. Their prey, in turn, has taken advantage of this weakness. The swordfish fishermen in Central American rivers look dull to us, but as Molly Cummings and Gil Rosenthal showed, males of some species have strong UV stripes along the flanks and tails. These markings are attractive to females, but they are invisible to the main predators of swordtail. And in places where these predators are more common, sword tails have more vivid UV markings. “They could get away with being super flamboyant” without attracting danger, says Cummings. Similar secret codes are found in Australia’s great barrier reef, home to ambon damselfish. To human eyes, it resembles a lemon with fins and looks identical to other closely related species. But Ulrike Siebeck found that her head is actually streaked with UV stripes, as if invisible mascara had run all over her face. Predators can not see these markings, but the ambons themselves use them to distinguish their own variety from other damselfish.

For us, UV feels enigmatic and intoxicating. It is an invisible nuance that lies right on the edge of our vision – a perceptual void that our imagination wants to fill. Researchers have often ascribed special or secret significance to it and treated it as a channel for secret communication. But apart from ambon damselfish and swordtails, most such claims have been founded. * The reality is that UV vision and UV signals are extremely common. “My personal opinion is that it’s just another color,” Innes Cuthill, who studies color vision, tells me.

Imagine what a bee can say. They are trichromates, with opsins that are most sensitive to green, blue and ultraviolet. If bees were scientists, they might be amazed at the color we know as red, which they cannot see and which they can call “ultrayellow”. They may first claim that other beings cannot see ultra-yellow, and then they wonder why so many do. They may ask if it is special. They may photograph roses through ultra-yellow cameras and rhapsodize about how different they look. They may wonder if the large two-legged animals that see this color exchange secret messages through their blushed cheeks. They may eventually realize that it is just another color, especially in their absence from their sight. And they may be wondering what it would be like to add it to their Umwelt and reinforce their three dimensions of color with a fourth.

Extracts from A huge world © 2022 by Ed Yong. Used with 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 extract may be reproduced or reprinted without the written permission of the publisher.

Meet the author

Ed Yong

About Ed Yong

Ed Yong is a scientific writer and author of A huge world: how the minds of animals reveal the hidden worlds around us. (Random House, 2022)

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