How Beauty Is Making Scientists Rethink Evolution
The extravagant splendor of the animal kingdom can’t be explained by natural selection alone — so how did it come to be?
By Ferris Jabr
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.
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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.
A male plum-throated cotinga.CreditKenji Aoki for The New York Times
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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.
A male lesser bird-of-paradise.CreditKenji Aoki for The New York Times
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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.”
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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.
Yellow plumes from a male lesser bird-of-paradise.CreditKenji Aoki for The New York Times
Image
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.)
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Unlike
natural selection, which preserved traits that were useful “in the
struggle for life,” Darwin saw sexual selection as exclusively concerned
with reproductive success, often resulting in features that jeopardized
an animal’s well-being. The peacock’s many-eyed aureole, mesmerizing
yet cumbersome, was a prime example and remains the mascot of sexual
selection today. “A great number of male animals,” Darwin wrote, “as all
our most gorgeous birds, some fishes, reptiles and mammals, and a host
of magnificently colored butterflies have been rendered beautiful for
beauty’s sake.”
Darwin’s peers
embraced the idea of well-armed males dueling for sexual dominance, but
many scorned the concept of animal aesthetics, in part because it was
grounded in animal consciousness and female desire. In one critique, the
English biologist St. George Mivart stressed “the fundamental
difference which exists between the mental powers of man and brutes” and
the inability of “vicious feminine caprice” to create enduring colors
and patterns. The English naturalist Alfred Russel Wallace, who
independently formed many of the same ideas about evolution as Darwin,
was also deeply critical. Wallace was particularly tormented by Darwin’s
suggestion of beauty without utility. “The only way in which we can
account for the observed facts is by the supposition that color and
ornament are strictly correlated with health, vigor and general fitness
to survive,” Wallace wrote. In other words, ornamentation could be
explained only as a heuristic that animals use to judge a potential
mate’s fitness — a view that came to dominate.
In the early 1980s,
while researching the history of sexual selection, Prum read a seminal
1915 paper and a 1930 book on the subject by the English biologist and
statistician Ronald Fisher, who buttressed Darwin’s original idea with a
more sophisticated understanding of heredity. At first, Fisher argued,
females might evolve preferences for certain valueless traits, like
bright plumage, that just happened to correspond with health and vigor.
Their children would tend to inherit the genes underlying both their
mother’s preference and their father’s trait. Over time, this genetic
correlation would reach a tipping point, creating a runaway cycle that
would greatly exaggerate both preference and trait, glorifying beauty at
the expense of the male’s survival. In the early 1980s, the American
evolutionary biologists Russell Lande and Mark Kirkpatrick gave Fisher’s
theory a formal mathematical girding, demonstrating quantitatively that
runaway sexual selection could happen in nature and that the ornaments
involved could be completely arbitrary, conveying no useful information
whatsoever.
Although Fisherian
selection was certainly not ignored, it was ultimately overshadowed by a
series of hypotheses that seemed to rescue beauty from purposelessness.
First, the Israeli biologist Amotz Zahavi proposed a counterintuitive
idea called the handicap principle, which put a new spin on Wallace’s
utilitarian explanation for sexual ornaments. 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 1982, the evolutionary biologists
W.D. 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. Scores of researchers joined the hunt for measurable benefits
of choosing an attractive mate: both direct benefits, like better
parenting or more desirable territory, and indirect benefits, namely
some evidence that more alluring males really did have “good genes”
underlying various desirable qualities, like disease resistance or
higher-than-average intelligence.
A male Guianan cock-of-the-rock.CreditKenji Aoki for The New York Times
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After
more than 30 years of searching, most biologists agree that although
these benefits exist, their prevalence and importance is uncertain. 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. A 2012
meta-analysis of 90 studies on 55 species found only “equivocal” support
for the good-genes hypothesis.
Prum
thinks the evidence for the heritable benefits of choosing a beautiful
mate is scant because such benefits are themselves rare, whereas
arbitrary beauty is “nearly ubiquitous.” Over the years, the more he
contemplated runaway selection, the more convinced he became that it was
a far more powerful and creative evolutionary force than natural
selection, which he regards as overhyped and boring. “Animals are agents
in their own evolution,” he told me during one conversation. “Birds are
beautiful because they are beautiful to themselves.”
In
the summer of 1985, 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. Instead of singing with
his throat, he repeatedly lifted his wings behind his back and vibrated
his feathers furiously against one another, producing two electronic
blips followed by a shrill buzzing ring — a sound Prum transcribes as
“Bip-Bip-WANNGG!”
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At
the time, Prum had not fully developed his evolutionary theory of
beauty, but he immediately suspected that the club-winged manakin was
emblematic of nature’s capacity for pushing creatures to aesthetic
extremes. The bird’s singular vibrato haunted him for years. In the
early 2000s, when Prum had become a professor of biology at the
University of Kansas, he and his graduate student Kimberly Bostwick
revealed that the demands of courtship had drastically altered the
bird’s anatomy, turning it into a living violin. Male club-winged
manakins had feathers with contorted shafts that rubbed against each
other 100 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.
Although there are no
published studies of the club-winged manakin’s aeronautics, Prum says
it’s obvious from observation that the birds fly awkwardly — even the
females. 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. “Some of the evolutionary consequences of sexual
desire and choice in nature are not adaptive,” Prum writes in his recent
book. “Some outcomes are truly decadent.”
In
the following decade, as Prum’s hearing declined, he withdrew from
field research and birding, but he still managed to make a series of
groundbreaking scientific discoveries: He helped confirm that feathers
evolved in dinosaurs long before the emergence of birds, and he became
one of the first scientists to deduce the colors of a dinosaur’s plumage
by examining pigment molecules preserved in fossilized 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 1997 and 2015, 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. But after speaking with numerous researchers in the
field of sexual selection, I learned that all of Prum’s peers are well
aware of his work and that many already accept some of the core tenets
of his argument: namely that natural and sexual selection are distinct
processes and that, in at least some cases, beauty reveals nothing about
an individual’s health or vigor. 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. In conversation, Prum’s brilliance is obvious,
but he has a tendency to be dogmatic, sometimes interrupting to dismiss
an argument that does not agree with his own. 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. When I asked him which studies he thought
offered the strongest support of “good genes” and other benefits, he
paused for a while before finally responding that it was not his job to
review the literature.
A male painted bunting.CreditKenji Aoki for The New York Times
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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. Prum thinks that
long ago, an earlier version of the bird’s courtship dance incidentally
produced a feathery susurration. 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. “Not everything,” he said, “has this explicit
causal explanation.”
Prum’s indifference to
the ultimate source of aesthetic taste leaves a conspicuous gap in his
grand theory. 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. It’s entirely conceivable that an animal might
be inherently partial to, say, a warbling mating call or bright yellow
feathers, and that these predilections would have nothing to do with
advantageous genes. Yet such inclinations are inarguably the product of
an animal’s neurobiology, which is itself the result of a long
evolutionary history that has adapted the animal’s brain and sensory
organs to specific environmental conditions. In the past two decades, a
cohort of biologists have dedicated themselves to studying how an
animal’s “sensory bias” — its ecological niche and its particular way of
experiencing the world — sculpts its appearance, behavior and desires.
Like Prum, they don’t think beauty has to be adaptive. But where Prum
celebrates caprice, they seek causality.
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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. “Let me see if I
can find a big, beautiful boy,” she said.
A
paper-clip-size fish swam toward us. I leaned in for a closer look. His
silver body was decorated with a single black dot and a stripe of
iridescent blue; his lengthy tail, shaped like a knight’s blade, was
streaked with yellow. “Oh, yeah, there’s a guy courting,” Cummings said.
“He’s coming up to that female, trying to impress her.” The fish, a
male swordtail, seemed almost manic in his effort to be noticed. He
darted back and forth in front of the female, shimmying as he went, his
scales reflecting whatever light managed to breach the murk.
A
little while later, we drove the few miles back to her campus
laboratory, where shelves of fish tanks lined several rooms and Ernst
Haeckel’s resplendent illustrations of jellyfish undulated across the
walls. 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. After college,
she moved to James Cook University in Townsville, Australia, where she
studied marine ecology and discovered the work of the biologists John
Lythgoe and John Endler, both of whom were interested in how the type of
light in an animal’s environment shaped its visual system.
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?
The tips of the outer tail feathers of a male king bird-of-paradise.CreditKenji Aoki for The New York Times
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After
earning a postgraduate degree in Australia in 1993, Cummings began a
Ph.D. at the University of California, Santa Barbara. 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. A female’s preference for a blaze of silver
or blue was not arbitrary; it was a consequence of the particular
wavelengths of light that traveled farthest through her underwater
niche. 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. “Some people think female preferences
just somehow emerge,” Cummings says, “but what has been overlooked is
that in many cases, it’s a result of environmental constraints. It’s not
always random.”
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. Many plants have evolved flower parts that absorb or
reflect ultraviolet light, forming patterns like rings, bull’s-eyes and
starbursts. 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.
Michael Ryan, a
professor of zoology whose lab and office are just a few floors below
Cummings’, has spent more than 30 years investigating how the quirks of
an animal’s anatomy determine its aesthetic preferences — a career he
details in his recent book, “A Taste for the Beautiful.” Since 1978,
Ryan has been traveling to Panama to study a mud-colored frog called the
túngara. Like the club-winged manakin, the túngara has a unique form of
beauty that is not visual but aural. At dusk, male túngara frogs gather
at the edges of puddles and sing to seduce females. 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?
According
to the adaptive model of beauty, the chucks must convey something about
the males’ fitness. 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.) Moreover, the
túngara frog has an inner organ tuned to 2,200 hertz, which is close to
the dominant frequency of a chuck. Together, these facts seem to
indicate that the túngara’s puddle-side serenade is an example of
adaptive mate choice: Females evolved ears tuned to chucks because they
indicate the biggest and most sexually skilled males.
Ryan’s
research revealed a stranger story. When he examined the túngara frog’s
family tree, he discovered that eight frog species closely related to
the túngara also have inner ear organs sensitive to frequencies of about
2,200 hertz, yet none of them produce chucks in their mating call. Ryan
thinks that eons ago, the ancestor of all these species probably
evolved an inner ear tuned to roughly 2,200 hertz for some
long-abandoned purpose. The túngara later revived this neglected
auditory channel, probably by happenstance. 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.
Like the
glistening scales on the surfperch and swordtails that Cummings studied,
the túngara’s costly mating call did not evolve to convey any pragmatic
information about health or fitness. But that doesn’t mean that these
traits were arbitrary. They were the result of specific, discernible
aspects of the animals’ environments, anatomy and evolutionary legacy.
“I took a real beating when I suggested this idea in 1990,” Ryan says.
“It was very widely criticized. 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. Prum sees sensory bias as just another way
to maintain the predominant “adaptive paradigm” that refuses to
acknowledge his theory of aesthetic evolution. Tellingly, Prum and Ryan
do not discuss each other’s work in their recent books.
A male king bird-of-paradise.CreditKenji Aoki for The New York Times
Image
While
mulling over the similarities and discrepancies between Prum’s ideas
and those of his peers, I kept returning to a passage in his book. In
2010, 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. At first,
layers of fuzzy filaments, similar to a chick’s down, most likely helped
dinosaurs repel water and regulate body temperature. But what explains
the development of broad, flat feathers like those found on Anchiornis?
Flight is the intuitive answer, but the first planar feathers were
probably too primitive for flight or gliding, lacking the distinct
asymmetry that makes birds’ feathers aerodynamic. 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. “The innovative
planar feather vane, however, creates a well-defined, two-dimensional
surface on which it is possible to create a whole new world of complex
color patterns within every feather,” Prum writes. 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.
Most
of the scientists I spoke with said that the old dichotomy between
adaptive adornment and arbitrary beauty, between “good genes” and
Fisherian selection, is being replaced with a modern conceptual
synthesis that emphasizes multiplicity. “Beauty is something that arises
from a host of different mechanisms,” says Gil Rosenthal, an
evolutionary biologist at Texas A&M University and the author of the
new scholarly tome “Mate Choice.” “It’s an incredibly multilayered
process.”
The environment constrains a
creature’s anatomy, which determines how it experiences the world,
which generates adaptive and arbitrary preferences, which loop back to
alter its biology, sometimes in maladaptive ways. 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. 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. From a distance, its flowers
resembled medieval illustrations of comets, big and bold with long,
trailing streamers. Up close, I was struck by the complexity of a single
blossom: a large yellow star wreathed a cluster of five tubular petals,
shaped like angel’s trumpets and pooled with nectar. A tuft of
pollen-tipped filaments fizzed through the very center. Viewed from
above, the flowers looked like huddles of tiny birds with their beaks
pressed together and wings flared. The name “columbine” comes from the
Latin for “dovelike.”
Why are flowers beautiful? Or, more precisely: Why are flowers beautiful to us?
The more I thought about this question, the more it seemed to speak to
the nature of beauty itself. Philosophers, scientists and writers have
tried to define the essence of beauty for thousands of years. The
plurality of their efforts illustrates the immense difficulty of this
task. Beauty, they have said, is: harmony; goodness; a manifestation of
divine perfection; a type of pleasure; that which causes love and
longing; and M = O/C (where M is aesthetic value, O is order and C is
complexity).
Evolutionary
psychologists, eagerly applying adaptive logic to every facet of
behavior and cognition, have speculated that the human perception of
beauty emerges from a set of ancient adaptations: Perhaps men like women
with large breasts and narrow waists because those features signal high
fertility; symmetrical faces may correlate with overall health; maybe
babies are irresistibly cute because their juvenile features activate
the caregiving circuits in our brains. Such claims sometimes verge on
the ludicrous: The philosopher Denis Dutton has argued that people
around the world have an intrinsic appreciation for a certain type of
landscape — a grassy field with copses of trees, water and wildlife —
because it resembles the Pleistocene savannas where humans evolved. In a
TED Talk, Dutton explains that postcards, calendars and paintings
depicting this universally beloved landscape usually include trees that
fork near the ground because our ancestors relied on their conveniently
low branches to scramble away from predators.
Of
course, it is undeniable that we, like all animals, are products of
evolution. Our brains and sensory organs are just as biased as any other
creature’s. Our inherited anatomy, physiology and instincts have
undoubtedly shaped our perception of beauty. In their recent books,
Richard Prum and Michael Ryan synthesize research on animals and people,
exploring possible evolutionary explanations for our own aesthetic
tastes. Ryan is particularly interested in the innate sensitivities and
biases of our neural architecture: He describes how our visual system,
for example, may be wired to notice symmetry. Prum stresses his
conviction that in humans, as in birds, many types of physical beauty
and sexual desire have arbitrarily co-evolved without reference to
health or fertility. What complicates their respective arguments is the
overwhelming power of human culture. As a species, we are so thoroughly
saturated with symbolism, ritual and art — so swayed by rapidly changing
fashions — that it is more or less impossible to determine just how
much an aesthetic preference owes to evolutionary history as opposed to
cultural influence.
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A male Mandarin duck.CreditKenji Aoki for The New York Times
Image
Perhaps
more than any other object of aesthetic obsession, flowers expose the
futility of trying to contain beauty in a single theoretical framework.
Consider how flowers came to be and how we grew to love them: 150
million years ago many pollen-producing plants depended on the wind to
spread their pollen and reproduce. But certain insects, perhaps beetles
and flies, began to eat those protein-rich pollen grains, inadvertently
transporting them from one plant to another. This proved to be a much
more efficient means of fertilization than capricious air currents.
Plants with the richest and most obvious sources of pollen were
especially successful. Likewise, insects that were particularly adept at
finding pollen had an advantage over their peers.
Through
a long process of co-evolution, plants and pollinators transformed one
another. Some plants began to modify their leaves into proto-flowers:
little flags that marked the location of their pollen. Bold colors and
distinctive shapes helped them stand out in a tangle of green. Strong
aromas and ultraviolet beacons played upon pollinators’ senses. Nectar
sweetened the deal. Insects, birds and mammals began competing for
access, evolving wings, tongues and brains better suited to the quest
for floral sustenance. As the pressure from both parties intensified,
plants and their pollinators formed increasingly specific relationships,
hurtling each other toward aesthetic and adaptive extremes — a bird
that hums and hovers like an insect, an orchid that mimics the
appearance and scent of a female bee.
Many
millions of years later, flowers enchanted yet another species. Perhaps
the initial attraction was purely utilitarian: the promise of fruit or
grain. Maybe we were captivated by their consonance of color, form and
aroma. Whatever the case, we adopted numerous flowering plants into an
expanding circle of domesticated species. We brought them into
greenhouses and laboratories, magnifying their inherent beauty, creating
new hybrids and tailoring their features to our individual tastes. We
contracted orchid delirium and tulip mania, and we have never fully
recovered. The flower began as a plea and became a phenomenon.
If
there is a universal truth about beauty — some concise and elegant
concept that encompasses every variety of charm and grace in existence —
we do not yet understand enough about nature to articulate it. What we
call beauty is not simply one thing or another, neither wholly
purposeful nor entirely random, neither merely a property nor a feeling.
Beauty is a dialogue between perceiver and perceived. Beauty is the
world’s answer to the audacity of a flower. It is the way a bee spills
across the lip of a yawning buttercup; it is the care with which a satin
bowerbird selects a hibiscus bloom; it is the impulse to recreate water
lilies with oil and canvas; it is the need to place roses on a grave.
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-https://www.nytimes.com/2019/01/09/magazine/beauty-evolution-animal.html?action=click&fbclid=IwAR2Z10Als0Ec5L5bNI1qWsOi_nKPxL78r7oAxqdEfF0uql3JvPJCCHDcCSE&module=Well&pgtype=Homepage§ion=The+New+York+Times+Magazine
Ferris Jabr is a contributing writer for the magazine. He last wrote about a neuroscientist who has made groundbreaking discoveries by puréeing brains.
A version of this article appears in print on , on Page 22 of the Sunday Magazine with the headline: Beauty and the Beasts. Order Reprints | Today’s Paper | Subscribe
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