Ferris Jabr, How Beauty Is Making Scientists Rethink Evolution, The New York Times Magazine, 9 January 2019
The extravagant splendor of the animal kingdom can’t be explained by natural selection alone — so how did it come to be?
A male flame bowerbird is a creature of incandescent beauty. The hue of his plumage transitions seamlessly from molten red to sunshine yellow. But that radiance is not enough to attract a mate. When males of most bowerbird species are ready to begin courting, they set about building the structure for which they are named: an assemblage of twigs shaped into a spire, corridor or hut. They decorate their bowers with scores of colorful objects, like flowers, berries, snail shells or, if they are near an urban area, bottle caps and plastic cutlery. Some bowerbirds even arrange the items in their collection from smallest to largest, forming a walkway that makes themselves and their trinkets all the more striking to a female — an optical illusion known as forced perspective that humans did not perfect until the 15th century.
Yet even this remarkable exhibition is not sufficient to satisfy a female flame bowerbird. Should a female show initial interest, the male must react immediately. Staring at the female, his pupils swelling and shrinking like a heartbeat, he begins a dance best described as psychotically sultry. He bobs, flutters, puffs his chest. He crouches low and rises slowly, brandishing one wing in front of his head like a magician’s cape. Suddenly his whole body convulses like a windup alarm clock. If the female approves, she will copulate with him for two or three seconds. They will never meet again.
The bowerbird defies traditional assumptions about animal behavior. Here is a creature that spends hours meticulously curating a cabinet of wonder, grouping his treasures by color and likeness. Here is a creature that single-beakedly builds something far more sophisticated than many celebrated examples of animal toolmaking; the stripped twigs that chimpanzees use to fish termites from their mounds pale in comparison. The bowerbird’s bower, as at least one scientist has argued, is nothing less than art. When you consider every element of his courtship — the costumes, dance and sculpture — it evokes a concept beloved by the German composer Richard Wagner: Gesamtkunstwerk, a total work of art, one that blends many different forms and stimulates all the senses.
This extravagance is also an affront to the rules of natural selection. Adaptations are meant to be useful — that’s the whole point — and the most successful creatures should be the ones best adapted to their particular environments. So what is the evolutionary justification for the bowerbird’s ostentatious display? Not only do the bowerbird’s colorful feathers and elaborate constructions lack obvious value outside courtship, but they also hinder his survival and general well-being, draining precious calories and making him much more noticeable to predators.
Numerous species have conspicuous, metabolically costly and physically burdensome sexual ornaments, as biologists call them. Think of the bright elastic throats of anole lizards, the Fabergé abdomens of peacock spiders and the curling, iridescent, ludicrously long feathers of birds-of-paradise. To reconcile such splendor with a utilitarian view of evolution, biologists have favored the idea that beauty in the animal kingdom is not mere decoration — it’s a code. According to this theory, ornaments evolved as indicators of a potential mate’s advantageous qualities: its overall health, intelligence and survival skills, plus the fact that it will pass down the genes underlying these traits to its children. A bowerbird with especially bright plumage might have a robust immune system, for example, while one that finds rare and distinctive trinkets might be a superb forager. Beauty, therefore, would not confound natural selection — it would be very much a part of it.
Charles Darwin himself disagreed with this theory. Although he co-discovered natural selection and devoted much of his life to demonstrating its importance, he never claimed that it could explain everything. Ornaments, Darwin proposed, evolved through a separate process he called sexual selection: Females choose the most appealing males “according to their standard of beauty” and, as a result, males evolve toward that standard, despite the costs. Darwin did not think it was necessary to link aesthetics and survival. Animals, he believed, could appreciate beauty for its own sake. Many of Darwin’s peers and successors ridiculed his proposal. To them, the idea that animals had such cognitive sophistication — and that the preferences of “capricious” females could shape entire species — was nonsense. Although never completely forgotten, Darwin’s theory of beauty was largely abandoned.
Now, nearly 150 years later, a new generation of biologists is reviving Darwin’s neglected brainchild. Beauty, they say, does not have to be a proxy for health or advantageous genes. Sometimes beauty is the glorious but meaningless flowering of arbitrary preference. Animals simply find certain features — a blush of red, a feathered flourish — to be appealing. And that innate sense of beauty itself can become an engine of evolution, pushing animals toward aesthetic extremes. In other cases, certain environmental or physiological constraints steer an animal toward an aesthetic preference that has nothing to do with survival whatsoever.
These biologists are not only rewriting the standard explanation for how beauty evolves; they are also changing the way we think about evolution itself. For decades, natural selection — the fact that creatures with the most advantageous traits have the best chance of surviving and multiplying — has been considered the unequivocal centerpiece of evolutionary theory. But these biologists believe that there are other forces at work, modes of evolution that are much more mischievous and discursive than natural selection. It’s not enough to consider how an animal’s habitat and lifestyle determine the size and keenness of its eyes or the number and complexity of its neural circuits; we must also question how an animal’s eyes and brain shape its perceptions of reality and how its unique way of experiencing the world can, over time, profoundly alter both its physical form and its behavior. There are really two environments governing the evolution of sentient creatures: an external one, which they inhabit, and an internal one, which they construct. To solve the enigma of beauty, to fully understand evolution, we must uncover the hidden links between those two worlds.
Perhaps no living scientist is as enthusiastic — or doctrinaire — a champion of Darwinian sexual selection as Richard Prum, an evolutionary ornithologist at Yale University. In May 2017, he published a book, “The Evolution of Beauty,” that lucidly and passionately explains his personal theory of aesthetic evolution. It was nominated for the Pulitzer Prize for general nonfiction, but within the scientific community, Prum’s ideas have not been as warmly received. Again and again, he told me, he has asked other researchers for feedback and received either excuses of busyness or no reply at all. Some have been openly critical. In an academic review of Prum’s book, Gerald Borgia, one of the world’s foremost experts on bowerbirds, and the ethologist Gregory Ball described the historical sections as “revisionist” and said Prum failed to advance a credible case for his thesis. Once, over a lunch of burritos, Prum explained his theory to a visiting colleague, who pronounced it “nihilism.”
Last April, Prum and I drove 20 miles east of New Haven to Hammonasset Beach State Park, a 900-acre patchwork of shoreline, marsh, woodland and meadow on Long Island Sound, with the hope of finding a hooded warbler. Birders had recently seen the small but striking migratory species in the area. Before he even parked, Prum was calling out the names of birds he glimpsed or heard through the car window: osprey, purple martin, red-winged blackbird. I asked him how he was able to recognize birds so quickly and, sometimes, at such a great distance. He said it was just as effortless as recognizing a portrait of Abraham Lincoln. In Prum’s mind, every bird is famous.
Binoculars in hand, we walked along the park’s winding trails, slowly making our way toward a large stand of trees. Prum wore jeans, a quilted jacket and a beige hat. His thick eyebrows, round spectacles and sprays of white and gray hair give his face a vaguely owlish appearance. In the course of the day, we would see grazing mallards with emerald heads, tree swallows with iridescent turquoise capes and several sparrow species, each distinguished by a unique ornament: swoops of yellow around the eye, a delicate pink beak, a copper crown. On a wooded path, we encountered a lively bird flinging leaf litter into the air. Prum was immediately transfixed. This was a brown thrasher, he told me, describing its attributes with a mix of precision and fondness — “rufous brown, speckled on the breast, yellow eye, curved beak, long tail.” Then he reprimanded me for trying to take a picture instead of observing with my “binos.”
About two hours into our walk, Prum, who is a fast and fluid talker, interrupted himself midsentence: “Right there! Right there!” he said. “There’s the hooded! Right up against the tree!” Something gold flashed across the path. I raised my binoculars to my eyes and scanned the branches to our right. When I found him, I gasped. He was almost mythological in his beauty: moss-green wings, a luminescent yellow body and face and a perfectly tailored black hood that made his countenance even brighter by contrast. For several minutes we stood and watched the bird as it hopped about, occasionally fanning white tail feathers in our direction. Eventually he flew off. I told Prum how thrilling it was to see such a creature up close. “That’s it,” Prum said. “That moment is what bird-watching is about.”
As a child growing up in a small rural town in southern Vermont, Prum was, in his words, “amorphously nerdy” — keen on reading and memorizing stats from “The Guinness Book of World Records” but not obsessed with anything in particular. Then, in fourth grade, he got glasses. The world came into focus. He chanced upon a field guide to birds in a bookstore, which encouraged him to get outdoors. Soon he was birding in the ample fields and woods around his home. He wore the grooves off two records of bird calls. He befriended local naturalists, routinely going on outings with a group of mostly middle-aged women (conveniently, they had driver’s licenses). By the time Prum was in seventh grade, he was leading bird walks at the local state park.
In college, Prum wasted no time in availing himself of Harvard University’s substantial ornithological resources. The first week of his freshman year, he got a set of keys to the Museum of Comparative Zoology, home to the largest university-based ornithological collection in the world, which today has nearly 400,000 bird specimens. “I’ve been associated with a world-class collection of birds every moment of my adult life,” he says. “I joke with my students — and it’s really true — I have to have at least 100,000 dead birds across the hallway to function intellectually.” (He is now the head curator of vertebrate zoology at Yale’s Peabody Museum of Natural History.) He wrote a senior thesis on the phylogeny and biogeography of toucans and barbets, working on a desk beneath the skeleton of a moa, an extinct emu-like bird that stood 12 feet tall and weighed 500 pounds.
After graduating from Harvard in 1982, Prum traveled to Suriname to study manakins, a family of intensely colored birds that compete for mates with high-pitched songs and gymnastic dance routines. In 1984, he began graduate studies in biology at the University of Michigan, Ann Arbor, where he planned to reconstruct the evolutionary history of manakins through careful comparisons of anatomy and behavior. In the process, a colleague introduced him to some research papers on sexual selection, piquing his interest in the history of this fascinating yet seemingly neglected idea.
Darwin was contemplating how animals perceived one another’s beauty as early as his 30s: “How does Hen determine which most beautiful cock, which best singer?” he scribbled in a note to himself sometime between 1838 and 1840. In “The Descent of Man,” published in 1871, he devoted hundreds of pages to sexual selection, which he thought could explain two of the animal kingdom’s most conspicuous and puzzling features: weaponry and adornment. Sometimes, males competing fiercely for females would enter a sort of evolutionary arms race, developing ever greater weapons — tusks, horns, antlers — as the best-endowed males of each successive generation reproduced at the expense of their weaker peers. In parallel, among species whose females choose the most attractive males based on their subjective tastes, males would evolve outlandish sexual ornaments. (It’s now well known that all sexes exert numerous different evolutionary pressures on one another and that in some species males choose ornamented females, but to this day, many of the best-studied examples are of female preference and male display.)
