For ornithologist Richard Prum, manakins are among the most beautiful creatures in the world. He first started studying these small South American birds in 1982, and he’s been privy to many of their flamboyant performances. One species has a golden head and moonwalks. Another puffs up a white ‘beard’ and hops about like a “buff gymnast.” Yet another makes alarmingly loud noises with its club-shaped wing bones. Each of the 54 species has its own combination of costumes, calls, and choreography, which males use in their mating displays. To Prum, this is a great example of “aesthetic radiation,” where a group of animals has evolved “54 distinctive ideals of beauty.”
That’s not a common view among evolutionary biologists. Most of Prum’s colleagues see outrageous sexual traits as reliable advertisements. The logic goes that only the fittest manakins could coordinate their movements just so. Only the healthiest peacocks could afford to carry such a cumbersome tail. Their displays and dances hint at their good genes, allowing females to make adaptive decisions.
But Prum says that view is poorly supported by years of research, and plainly makes no sense when you actually look at what birds do. How could there be adaptive value in every single minute detail of a manakin’s plumage and performance? And why have some species replaced certain ancestral maneuvers (like pointing one’s tail to the sky) with new moves (like pointing one’s bill to the sky) that surely provide no better information? “It’s clearly arbitrary,” says Prum. “I wrote that in a 1997 paper, but the reviewers hated it. They said you can’t claim that unless you falsify every adaptive hypothesis we can imagine. And if you can’t find an adaptive explanation, you haven’t worked hard enough to discover it.”
That struck him as absurd. Worse, it’s stubbornly cold. It’s a theory of aesthetics that tries to shove aesthetics under the rug, implicitly denying that manakins and other animals could be having any kind of subjective experience. It has even crept into our understanding of ourselves: Evolutionary psychologists have put forward poorly conceived adaptive explanations for everything from female orgasms to same-sex preferences. “These ideas have saturated the popular culture. In the pages of Vogue, and in cosmetic surgery offices, you read that beauty is a revealing indicator of objective quality,” says Prum. “That’s why I had to write the book.”
The book in question, which publishes tomorrow, is The Evolution of Beauty: How Darwin's Forgotten Theory of Mate Choice Shapes the Animal World—and Us. It’s a “natural history of beauty and desire”—a smorgasbord of evolutionary biology, philosophy, and sociology, filtered through Prum’s experiences as a birdwatcher and his diverse research on everything from dinosaur colors to duck sex. Through compelling arguments and colorful examples, Prum launches a counterstrike against the adaptationist regime, in an attempt to “put the subjective experience of animals back in the center of biology” and to “bring beauty back to the sciences.”
The central idea that animates the book is a longstanding one that Prum has rebranded as the “Beauty Happens hypothesis.” It starts with animals developing random preferences—for colors, songs, displays, and more—which they use in choosing their mates. Their offspring inherit not only those sexy traits, but also the preference for them. By choosing what they like, choosers transform both the form and the objects of their desires.
Critically, all of this is arbitrary—not adaptive. Songs and ornaments and dances evolve not because they signal good genes but because animals just like them. They’re not objectively informative; they’re subjectively pleasing. Beauty, in other words, just happens. “It’s a self-organizing process, by which selection will arrive at some standard of beauty all by itself, in the absence of any adaptive benefit—or, indeed, despite maladaptive disadvantage,” says Prum.
The Beauty Happens idea isn’t an anthropomorphic one; Prum’s arguing that animals have evolved to be beautiful to themselves, not to him. It’s not a new idea either. A century ago, geneticist Ronald Fisher wrote about extreme traits and the desire for those traits co-evolving in a runaway process. “But [Fisher’s hypothesis] has been viewed as a curious idea that’s irrelevant to nature—that’s the status in most textbooks,” says Prum. He’s on a mission to re-emphasize it, and to show that aesthetics and beauty aren’t mushy subjects that science should shy away from.
It’s been an uphill struggle, partly because the arbitrary nature of the idea is so distasteful to some. Prum recalls discussing his ideas with a “well-respected, center-of-the-road, evolutionary biologist,” who took it all in and said: But that’s nihilism! “That’s when I realized that I had a marketing problem,” he says. “This is what fills me with joy to study, what literally gives me goosebumps in the office, and when I express it to my colleague, he doesn’t have a reason to get out of bed in the morning.”
The originator of these ideas—Charles Darwin himself—suffered from similar problems. In The Descent of Man, he put forward an explicitly aesthetic view of sexual selection, in which animal beauty evolves because it’s pleasurable to the animals themselves. And despite the book’s title, Darwin spent many of its pages focusing on the choices of females, casting them as agents of their own evolution and arguing that their preferences were a powerful force behind nature’s diversity.
Darwin’s contemporaries were having none of it. They believed that animals didn’t have rich subjective worlds, lacking the mental abilities that had been divinely endowed to humans. And the idea of female animals making fine-grained choices seemed doubly preposterous to the Victorian patriarchy. One scientist wrote that female whims were so fickle that they could never act as a consistent source of selection. Alfred Russel Wallace, the co-discoverer of evolutionary theory, also rejected Darwin’s ideas, insisting that beauty must be the result of adaptation, and that sexual selection is just another form of natural selection. In a feat of sheer chutzpah, he even claimed that his view was more Darwinian than Darwin’s in a book called Darwinism. “I can still remember wanting to throw Wallace around the room when I read that,” says Prum, who accuses the man of turning sexual selection into an ‘intellectually impoverished theory.’”
That legacy still infects evolutionary biology today. Consider orgasms, which Prum does at length in a later chapter. “There’s an entire field on the evolution of orgasm that’s devoid of any discussion of pleasure,” he says. “It’s stunningly bad science, and once more, it places male quality at the causal center.” For example, some researchers suggested that contractions produced during female orgasm are adaptations that allow women to better “upsuck”—no, really—the sperm of the best males. Others theorists suggested that female orgasm is the equivalent of male nipples—an inconsequential byproduct of natural selection acting on the opposite sex. Both ideas trivialize the sexual agency of women, Prum says, and completely fail to engage with the thing they’re actually trying to explain--women’s subjective experiences of sexual pleasure.
“It should come as no surprise that science does such a poor job of explaining pleasure because it’s left the actual experience of pleasure out of the equation,” he writes. That is, when biologists think about mate choice, whether in manakins or people, they focus only on the outcomes of the choice, and neglect the actual act of choosing. The result is a sexual science that’s bizarrely sanitized—an account of pleasure that’s totally anhedonic.
His counter-explanation is simple: women preferred to have sex with men who stimulated their own sexual pleasure, leading to co-evolution between female desire and male behaviors that met those desires. That’s why, compared to our closest ape relatives, human sex is much longer, involves a variety of positions, and isn’t tied to fertility cycles. It’s also why female orgasm isn’t necessary for actual procreation. “It may be the greatest testament to the power of aesthetic evolution,” Prum writes. “It’s sexual pleasure for its own sake, which has evolved purely as a consequence of women’s pursuit of pleasure.”
By his admission, this is speculative. He hopes that his book—which also includes hypotheses about human bodies, cultural standards of attractiveness, sexual identity, and more—will spur more research that’s grounded in an appreciation of aesthetics. But he also notes that there are other species in which experiments have confirmed the power of female choice.
In 2005, a woman named Patricia Brennan joined Prum’s lab with an interest in animal genitals—and in ducks. Most birds don’t have penises, but male ducks have huge, corkscrew-shaped ones that they extrude into females at high speed. But Brennan showed that female ducks have equally convoluted vaginas, which spiral in the opposite direction and include several dead-end pockets. Why?
Duck sex is intense and violent. Several males will often try to force themselves onto a female, and they use their ballistic penises to deposit sperm as far inside their mates as possible. But Brennan, by getting drakes to launch their penises into variously shaped glass tubes, showed that a female’s counter-spiraling vagina can stop the progress of her partner’s phallus. If she actually wants to mate, she can change her posture and relax the walls of her genital tract to offer a male easy passage. As a result, even in species where 40 percent of sexual encounters are forced, more than 95 percent of chicks are actually sired by a female’s chosen partner.
I wrote about Brennan’s work back in 2009, and I’ve since heard it repeatedly called “that duck penis study.” But really, it’s a duck vagina story. It’s a story of females asserting their agency, even in the face of persistent violence. “And when females get sexual autonomy, what do they do with it?” says Prum. “They make aesthetic choices, and the result is this aesthetic explosion over time.” By retaining their capacity to choose, female ducks force male plumage, displays and songs to continually evolve to court those choices. Sexual autonomy is an evolutionary engine of beauty.
“That research was transformative for me,” says Prum. It’s one of several reasons why The Evolution of Beauty is an explicitly feminist book. It’s disdainful about the male biases that characterize much of evolutionary psychology. Instead, it consistently centers female choice and repeatedly draws on feminist scholarship.
“If you say anything about a feminist science, you get a lot of negative blowback immediately,” says Prum. “But this isn’t a science that accommodates itself to feminist principles. It’s about the discovery of feminist concepts in biology itself.” By his reckoning, freedom of choice isn’t a matter of ideology. It arises from evolution, and it shapes subsequent evolution—and it’s about time that biologists recognized that.
“It’s a sad thing that, given the promise of evolutionary biology, we’ve really failed to lead culture in any meaningful way, whether in thinking about racism, sexism, or economic disparity,” says Prum. “We’re just hanging at the rear end. And there’s a real prospect for that to change because of all the power of evolutionary theory to be relevant to people and people’s lives.”
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