Soon he was birding in the ample fields and woods around his home. He wore the grooves off two records of bird calls. By the time Prum was in seventh grade, he was leading bird walks at the local state park. 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 , bird specimens. 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 pounds.
Some old time beauties after portraits by the English masters, with embellishment and comment
After graduating from Harvard in , 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 , 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.
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.
In one critique, the English biologist St. The English naturalist Alfred Russel Wallace, who independently formed many of the same ideas about evolution as Darwin, was also deeply critical. At first, Fisher argued, females might evolve preferences for certain valueless traits, like bright plumage, that just happened to correspond with health and vigor. Although Fisherian selection was certainly not ignored, it was ultimately overshadowed by a series of hypotheses that seemed to rescue beauty from purposelessness.
Extravagant ornaments, Zahavi argued, were not merely indicators of advantageous traits as Wallace had said — they were a kind of test. If an animal thrived despite the burden of an unwieldy or metabolically expensive ornament, then that animal had effectively demonstrated its vigor and proved itself worthy of a mate. Similarly, in , the evolutionary biologists W. Hamilton and Marlene Zuk proposed that some ornaments, in particular bright plumage, signaled that a male was resilient against parasites and would grant his children the same protection. Many scientists began to think of sexual selection as a type of natural selection.
After more than 30 years of searching, most biologists agree that although these benefits exist, their prevalence and importance is uncertain.
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A few compelling studies of frogs, fish and birds have shown that females who choose more attractive males typically have children with more robust immune systems and a greater chance of survival. On the whole, however, the evidence has not equaled the enthusiasm. In the summer of , around the same time that biologists were rekindling their interest in sexual selection, Prum and the nature documentarian Ann Johnson who would later choose him as her husband traveled to Ecuador to continue studying manakins.
The first morning, while hiking through a cloud forest, Prum heard odd bell-like notes, which he took to be the murmurings of parrots. Later that day, on the same trail, he heard the strange sounds again and followed them into the forest. He was astonished to find that the source was a male club-winged manakin, a small cinnamon-bodied species with a red cap and black-and-white mottled wings.
The manakin was jumping around in a showy manner that suggested he was courting females. Male club-winged manakins had feathers with contorted shafts that rubbed against each other times a second — faster than a hummingbird beats its wings. Whereas a vast majority of birds have light, hollow bones in service of flight, Bostwick has recently shown via CT scans that male club-winged manakins have solid ulnas — wing bones — which they need to withstand the intense quivering.
Female manakins have inherited related anomalies as well. The self-perpetuating pressure to be beautiful, Prum argues, has impeded the survival of the entire species. Because the females do not court males, there can be no possible advantage to their warped bones and feathers. All the while, he never stopped thinking about sexual selection. Prum formally presented his theory of aesthetic evolution in a series of scientific papers published between and , proposing that all sexual ornaments and preferences should be regarded as arbitrary until proven useful.
Despite his recent Pulitzer nomination, Prum still stings from the perceived scorn of his academic peers. At the same time, nearly every researcher I spoke to said that Prum inflates the importance of arbitrary preferences and Fisherian selection to the point of eclipsing all other possibilities. Although he admits that certain forms of beauty may be linked to survival advantages, he does not seem particularly interested in engaging with the considerable research on this topic.
Like Darwin, Prum is so enchanted by the outcomes of aesthetic preferences that he mostly ignores their origins. Toward the end of our bird walk at Hammonasset Beach State Park, we got to talking about club-winged manakins. I asked him about their evolutionary history. Over time, this sound became highly attractive to females, which pressured males to evolve adaptations that made their rustling feathers louder and more noticeable, culminating in a quick-winged strumming.
But why, I asked Prum, would females be attracted to those particular sounds in the first place? To Prum, it was a question without an answer — and thus a question not worth contemplating. Even if we were to accept that most beauty blooms from arbitrary preferences, we would still need to explain why such preferences exist at all. But where Prum celebrates caprice, they seek causality. Molly Cummings, a professor of integrative biology at the University of Texas at Austin, is a leading researcher in the field of sensory ecology.
When I visited her last spring, she drove us to one of her field laboratories: a grassy clearing populated with several large concrete basins. The surface of one basin was so packed with woolly algae and pink-flowered water lilies that we could hardly see the water. Cummings began pushing some of the vegetation out of the way, forming shady recesses that permitted our gaze at the right angle. A paper-clip-size fish swam toward us. I leaned in for a closer look. He darted back and forth in front of the female, shimmying as he went, his scales reflecting whatever light managed to breach the murk.
As we toured the facilities, Cummings told me about the arc of her career. While an undergraduate at Stanford University, she spent a summer scuba diving in the giant kelp forests at Hopkins Marine Station, adjacent to the world-renowned Monterey Bay Aquarium.
Cummings thought about the fish she had observed in California and Australia. She was astounded by the dynamic beauty of surfperch in the kelp forest: the way they communicate through the color and brightness of their skin, flashing blue, silver and orange to attract mates. Equally impressive was the diversity of their aquatic habitats. Some patches of water were sparkling and clear; others were cloudy with algal muck. In Australia, sunlight bathed the many vibrant species of reef fish almost constantly, but they lived against a kaleidoscopic backdrop of coral.
How did fish evolve effective and reliable sexual ornaments if the lighting and scenery in their homes were so variable?
After earning a postgraduate degree in Australia in , Cummings began a Ph. For several years, she studied various species of surfperch, repeatedly diving in the kelp forests with a Plexiglas-protected spectrometer to quantify and characterize the light in different habitats. At night, she would use powerful diving lights to stun surfperch and take them back to the lab, evading the hungry seals that routinely trailed her in hopes of making a meal of the startled fish.
After hundreds of dives and careful measurements, Cummings discovered that water itself had guided the evolution of piscine beauty. Whichever males happened to have scales that best reflected these wavelengths were more likely to catch the eye of females. In her studies, Cummings showed that surfperch living in dim or murky waters generally preferred shiny ornaments, while surfperch inhabiting zones of mercurial brightness favored bold colors.
Later, Cummings found that Mexican swordtails occupying the upper layers of rivers, where the clear water strongly polarized incoming sunlight, had ornaments that were specialized to reflect polarized light — like a stripe of iridescent blue. These findings parallel similar studies suggesting that female guppies in Trinidad prefer males with orange patches because they first evolved a taste for nutritious orange tree fruits that occasionally fell into the water. What a creature finds attractive depends on more than the unique qualities of its environment, however; attraction is also defined by which of those qualities cross the threshold of awareness.
Consider the difference between what we see when we look at a flower and what a bumblebee sees. Like us, insects have color vision. Unlike us, insects can also perceive ultraviolet light. Most creatures are oblivious to these ornaments, but to the eyes of many pollinators, they are unmistakable beacons. There is an entire dimension of floral beauty invisible to us, not because we are not exposed to ultraviolet light, but because we do not have the proper biological hardware to perceive it.
Their mating call has two elements: The main part, dubbed the whine, sounds precisely like a miniaturized laser gun; sometimes this is followed by one or more brief barks, known as chucks. A long and complex mating call is risky: It attracts frog-eating bats. Yet there is a high payoff. Ryan has shown that whines followed by chucks are up to five times as appealing to females as whines alone.
But why? As it happens, larger males, which produce the deepest and sexiest chucks, are also the most adept at mating, because they are closer in size to females. Frog sex is a slippery affair, and a diminutive male is more likely to miss his target. Ryan thinks that eons ago, the ancestor of all these species probably evolved an inner ear tuned to roughly 2, hertz for some long-abandoned purpose. Male frogs that happened to burp out a few extra notes after whining were automatically favored by females — not because they were more suitable mates, but simply because they were more noticeable.
But now sensory bias is considered an important part of the evolution of these preferences. During our walk at Hammonasset, while admiring seabirds from shore-side cliffs, I asked Prum about sensory bias. He said it could not possibly explain the staggering diversity and idiosyncrasy of sexual ornaments — the fact that every closely related sparrow species has a unique embellishment, for example. In , Prum and his colleagues revealed that a crow-size dinosaur called Anchiornis huxleyi was beautifully adorned: gray body plumage, an auburn mohawk and long white limb feathers with black spangles.
Why dinosaurs originally evolved feathers has long perplexed scientists. But what explains the development of broad, flat feathers like those found on Anchiornis? In his book, Prum advocates for an alternative hypothesis that has been gaining support: Large feathers evolved to be beautiful. The aesthetic possibilities of fuzzy down are limited. Only later did birds co-opt their big, glamorous plumes for flight, which is probably a key reason that some of them survived mass extinction 66 million years ago.
Birds transformed what was once mere frippery into some of the most enviable adaptations on the planet, from the ocean-spanning breadth of an albatross to the torpedoed silhouette of a plunging falcon. Yet they never abandoned their sense of style, using feathers as a medium for peerless pageantry. A feather, then, cannot be labeled the sole product of either natural or sexual selection. A feather, with its reciprocal structure, embodies the confluence of two powerful and equally important evolutionary forces: utility and beauty.
Beauty reveals that evolution is neither an iterative chiseling of living organisms by a domineering landscape nor a frenzied collision of chance events. Rather, evolution is an intricate clockwork of physics, biology and perception in which every moving part influences another in both subtle and profound ways. Its gears are so innumerable and dynamic — so susceptible to serendipity and mishap — that even a single outcome of its ceaseless ticking can confound science for centuries.
On my last day in Austin, while walking through a park, I encountered a common grackle hunting for insects in the grass. His plumage appeared black as charcoal at first, but as he moved, it shimmered with all the colors of an oil slick. Every now and then, he stopped in place, inflated his chest and made a sound like a rusty swing set.
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Perhaps dissatisfied with the local fare, or uncomfortable with my presence, he flew off. In his absence, my attention immediately shifted to something his presence had obscured — a golden columbine bush. Please enter recipient e-mail address es. The E-mail Address es you entered is are not in a valid format. Please re-enter recipient e-mail address es. You may send this item to up to five recipients. The name field is required. Please enter your name. The E-mail message field is required.
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