“Why All the Fuss about Sex?” wonders Michael Ryan. He wonders enough to make it the title of the first chapter of his new book, A Taste for the Beautiful. One would be forgiven for thinking it would be a short chapter—consisting of the single word “duh”—but Ryan is a zoologist who studies evolution and animal behavior. So he has a slightly different take.

“I have a unique perspective to offer on these issues,” he writes, “as I have spent the past 40 years studying the sexual behavior of a tiny, bumpy frog in Central America.”

It’s not that he has a fetish; it’s that he uses these túngara frogs—along with bowerbirds, howler monkeys, fireflies, peacock spiders, collared lizards, corn borer moths, hairy caterpillars, surf perches, and bee orchids—to demonstrate how beauty, and the appreciation of it, may have evolved in animals. Like us.

Where’s beauty come from?

Ryan’s thesis is that we generally have beauty backwards. We take it as a given that certain traits are inherently attractive—a peacock’s tail; a zebra finch’s red beak; Daniel Craig’s crooked smile, tight abs, and posh accent—and the brain then has to figure out why. But Ryan argues that “instead of the brain having evolved to detect beauty, the brain determines what is beautiful.” Beauty is in the mind, not the eye, of the beholder. And that mind not only perceives beauty but actually shapes it.

The proverbial beholder is usually female. It is not that female individuals are limiting in the reproductive game, but their gametes are. Males are defined as those members of a species who continuously churn out billions of tiny sperm over the course their lives. Females harbor the eggs, which are so much larger than sperm; they contain everything the fetus needs to grow and thrive (minus half of a genome). Because eggs are larger, they take more energy to make, so there are fewer of them. Each female gets only a limited number over her lifetime, and once one is fertilized, she’s often tied up for a long time dealing with it while the male fertilizer keeps on churning out those sperm. This yields a sexual marketplace in which a surfeit of males has to compete to impress a limited number of females, who get to choose.

In so choosing, the females—their brains, anyway—dictate how the males display their wares. Ryan has spent his career examining how female brains evolved to want what they want, but also how those desires have acted as a selective pressure to drive the evolution of beautiful traits in males. Observing different species and their various courting strategies has led him to identify a couple of key mechanisms for how they developed. One is sensory exploitation, and its corollary is the idea of hidden preferences.

Evolving beauty

A mantra of Ryan’s seems to be that, while brains are most often thinking about sex, they aren’t always thinking exclusively about sex. One of the other main things they think about is food. Guppies are exceptionally well tuned to the color orange to home in on orange fruits that fall into the water; so male guppies evolved orange ornamentation. They exploited a sensory bias that already existed in females, subverting the extant foraging strategy into a mating strategy. The sensitivity to orange was being fulfilled by food, and male guppies just piggybacked on it.

Other times, females have latent desires that are completely unrealized until males evolve a trait that reveals them. This is what happened with platyfish. They are similar to swordtails, except they don’t have swords on their tails. But when researchers attached swords to the tails of the male platyfish, the female platyfish went wild for them. The females had a hidden preference for the swords, probably inherited from the ancestor they share with the swordtails. Ryan came up with this idea in 1990 and claims that it is now considered to be “one of the major factors driving the evolution of sexual beauty.”

Perhaps it is best for some of these hidden preferences to remain so. Ryan ends with the disturbing notion that men have a hidden—in the sense of generally unrealized by normal women—preference for exaggerated female forms, like those displayed by airbrushed supermodels and Barbie dolls. When exposure to these hypersexualized forms was minimal or difficult to come by, this preference remained where it belonged (in fantasyland), and men could be content with the real live women around them. But now that we are saturated with such images, Ryan echoes Naomi Wolf’s concern that “real naked women are just bad porn.”

And no, this won’t cause women to evolve to look like Barbie; a real woman with those dimensions would not be able to accommodate a liver or intestines. It will only fatten the wallets of porn producers and possibly get in the way of men forming some real relationships.