ANIMALS ARE LIVING color. Wasps buzz with painted warnings. Birds shimmer their iridescent desires. Fish hide from predators with body colors that dapple like light across a rippling pond. And all this color on all these creatures happened because other creatures could see it.
The natural world is so showy, it’s no wonder scientists have been fascinated with animal color for centuries. Even today, the questions how animals see, create, and use color are among the most compelling in biology.
Until the last few years, they were also at least partially unanswerable—because color researchers are only human, which means they can’t see the rich, vivid colors that other animals do. But now new technologies, like portable hyperspectral scanners and cameras small enough to fit on a bird’s head, are helping biologists see the unseen. And as described in a new Science paper, it’s a whole new world.
Visions of Life
The basics: Photons strike a surface—a rock, a plant, another animal—and that surface absorbs some photons, reflects others, refracts still others, all according to the molecular arrangement of pigments and structures. Some of those photons find their way into an animal’s eye, where specialized cells transmit the signals of those photons to the animal’s brain, which decodes them as colors and shapes.
It’s the brain that determines whether the colorful thing is a distinct and interesting form, different from the photons from the trees, sand, sky, lake, and so on it received at the same time. If it’s successful, it has to decide whether this colorful thing is food, a potential mate, or maybe a predator. “The biology of color is all about these complex cascades of events,” says Richard Prum, an ornithologist at Yale University and co-author of the paper.
In the beginning, there was light and there was dark. That is, basic greyscale vision most likely evolved first, because animals that could anticipate the dawn or skitter away from a shadow are animals that live to breed. And the first eye-like structures—flat patches of photosensitive cells—probably didn’t resolve much more than that. It wasn’t enough. “The problem with using just light and dark is that the information is quite noisy, and one problem that comes up is determining where one object stops and another one starts. ” says Innes Cuthill, a behavioral ecologist at the University of Bristol and coauthor of the new review.
Color adds context. And context on a scene is an evolutionary advantage. So, just like with smart phones, better resolution and brighter colors became competitive enterprises. For the resolution bit, the patch light-sensing cells evolved over millions of years into a proper eye—first by recessing into a cup, then a cavity, and eventually a fluid-filled spheroid capped with a lens. For color, look deeper at those light-sensing cells. Wedged into their surfaces are proteins called opsins. Every time they get hit with a photon—a quantum piece of light itself—they transduce that signal into an electrical zap to the rudimentary animal’s rudimentary brain. The original light/dark opsin mutated into spin-offs that could detect specific ranges of wavelengths. Color vision was so important that it evolved independently multiple times in the animal kingdom—in mollusks, arthropods, and vertebrates.
In fact, primitive fish had four different opsins, to sense four spectra—red, green, blue, and ultraviolet light. That four-fold ability is called tetrachromacy, and the dinosaurs probably had it. Since they’re the ancestors of today’s birds, many of them are tetrachromats, too.
But modern mammals don’t see things that way. That’s probably because early mammals were small, nocturnal things that spent their first 100 million years running around in the dark, trying to keep from being eaten by tetrachromatic dinosaurs. “During that period the complicated visual system they inherited from their ancestors degraded,” says Prum. “We have a clumsy, retrofitted version of color vision. Fishes, and birds, and many lizards see a much richer world than we do.”
In fact, most monkeys and apes are dichromats, and see the world as greyish and slightly red-hued. Scientists believe that early primates regained three-color vision because spotting fresh fruit and immature leaves led to a more nutritious diet. But no matter how much you enjoy springtime of fall colors, the wildly varicolored world we humans live in now isn’t putting on a show for us. It’s mostly for bugs and birds. “Flowering plants of course have evolved to signal pollinators,” says Prum. “The fact that we find them beautiful is incidental, and the fact that we can see them at all is because of an overlap in the spectrums insects and birds can see and the ones we can see.”
Covered in Color
And as animals gained the ability to sense color, evolution kickstarted an arms race in displays—hues and patterns that aided in survival became signifiers of ace baby-making skills. Almost every expression of color in the natural world came about to signal, or obscure, a creature to something else.
For instance, “aposematism” is color used as a warning—the butterfly’s bright colors say “don’t eat me, you’ll get sick.” “Crypsis” is color used as camouflage. Color serves social purposes, too. Like, in mating. Did you know that female lions prefer brunets? Or that paper wasps can recognize each others’ faces? “Some wasps even have little black spots that act like karate belts, telling other wasps not to try and fight them,” says Elizabeth Tibbetts, an entomologist at the University of Michigan.
But animals display colors using two very different methods. The first is with pigments, colored substances created by cells called chromatophores (in reptiles, fish, and cephalopods), and melanocytes (in mammals and birds). They absorb most wavelengths of light and reflect just a few, limiting both their range and brilliance. For instance, most animals cannot naturally produce red; they synthesize it from plant chemicals called carotenoids.
The other way animals make color is with nanoscale structures. Insects, and, to a lesser degree, birds, are the masters of color-based structure. And compared to pigment, structure is fabulous. Structural coloration scatters light into vibrant, shimmering colors, like the shimmering iridescent bib on a Broad-tailed hummingbird, or the metallic carapace of a Golden scarab beetle. And scientists aren’t quite sure why iridescence evolved. Probably to signal mates, but still: Why?
Decoding the rainbow of life
The question of iridescence is similar to most questions scientists have about animal coloration. They understand what the colors do in broad strokes, but there’s till a lot of nuance to tease out. This is mostly because, until recently, they were limited to seeing the natural world through human eyes. “If you ask the question, what’s this color for, you should approach it the way animals see those colors,” says Tim Caro, a wildlife biologist at UC Davis and the organizing force behind the new paper. (Speaking of mysteries, Caro recently figured out why zebras have stripes.)
Take the peacock. “The male’s tail is beautiful, and it evolved to impress the female. But the female may be impressed in a different way than you or I,” Caro says. Humans tend to gaze at the shimmering eyes at the tip of each tail feather; peahens typically look at the base of the feathers, where they attach to the peacock’s rump. Why does the peahen find the base of the feathers sexy? No one knows. But until scientists strapped to the birds’ heads tiny cameras spun off from the mobile phone industry, they couldn’t even track the peahens’ gaze.
Another new tech: Advanced nanomaterials give scientists the ability to recreate the structures animals use to bend light into iridescent displays. By recreating those structures, scientists can figure out how genetically expensive they are to make.
Likewise, new magnification techniques have allowed scientists to look into an animal’s eye structure. You might have read about how mantis shrimp have not three or four but a whopping 12 different color receptors, and how they see the world in psychedelic hyperspectral saturation. This isn’t quite true. Those color channels aren’t linked together—not like they are in other animals. The shrimp probably aren’t seeing 12 different, overlapping color spectra. “We are thinking maybe those color receptors are being turned on or off by some other, non-color, signal,” says Caro.
But perhaps the most important modern innovation in biological color research is getting all the different people from different disciplines together. “There are a lot of different sorts of people working on color,” says Caro. “Some behavioral biologists, some neurophysiologists, some anthropologists, some structural biologists, and so on.”
And these scientists are scattered all over the globe. He says the reason he brought everyone to Berlin is so they could finally synthesize all these sub-disciplines together, and move into a broader understanding of color in the world. The most important technology in understanding animal color vision isn’t a camera or a nanotech surface. It’s an airplane. Or the internet.