Why All the Fuss about Sex?

The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick!

— Charles Darwin

NATURE USUALLY GETS DOWN TO BUSINESS. Let's think about sleep. When I go to bed, I pull back the sheets, put my head on the pillow, and I'm in dreamland. I do not have a sleeping ritual, I don't dance, sing, chant, or perfume myself. I just go to sleep. So do most animals. Eating is the same way. When a howler monkey finds an edible leaf, she plucks it and eats it; a heron just throws back his head and swallows the fish he speared out of the water; and a cheetah doesn't do a celebratory dance before she starts to devour the gazelle she just brought down, even though she sprinted at her personal best of 75 mph to do so. Granted, in our own species we can sometimes make a bigger deal out of eating, especially when a meal coincides with a special event. But for the most part, we are little different from the howler, the heron, and the cheetah. Take a bite, give it a good chew, and gulp it down. Much of life for most animals is like that — the job is to just get it done.

Sex is different: a just-get-it-done policy won't get it done. In humans and most other animals, extensive courtship rituals precede the sex act. Most of our sexual rituals are laden with accessories, including candles and music, poems and flowers, and even special wardrobes. The list goes on, but it is no less diverse for animals. Animals sing and dance, they perfume themselves, they show off their colors and even light themselves up, all in the hope of attracting a mate. Although we distinguish ourselves in the language and technology we deploy in courtship, all animals have evolved spectacular, even obscene, morphologies and behaviors as both sexual lures and strategies for consummation. The colors of butterflies and fishes, the songs of insects and birds, the sexual odors of moths and mammals all evolved in the service of sex. The same is true for many of the traits in our own species that make women sigh and men gasp when someone of striking beauty crosses their paths. These aspects of sexual beauty evolved not because they make their bearers live longer but because they enable them to mate more and thus pass on more offspring and genes to the next generation.

Sexual beauty is everywhere, woven through the fabric of all sexually reproducing animals. We humans strive for beauty; we pay for it; we judge whether others have it; and if they do, we treat them better. Animals and humans both go to extreme lengths to appear beautiful to those who judge them. Peacocks evolve magnificent tails that cause peahens to sway, fishes sport bright colors that catch the eye of the other sex, crickets chirp endearingly to their mates, and spiders dance and vibrate their webs to show off. We humans take a more active role in engineering our beauty than do most other animals. Perfumes, fashion, cars, and music have all been employed in the service of sexual beauty, as have the surgeon's knife and a pharmacopeia of drugs. But to enhance one's beauty, either through the painstakingly slow process of evolution or the more immediate gratification of beauty-engineering, one must have some notion of what is beautiful.

This book is about sexual beauty, where it comes from and what it is for. Of course, many have been inspired to write in appreciation of natural beauty and the enchanting mating behaviors that occur in wild animals. Their emphasis is usually on the details of beautiful male traits: How does having such a long tail benefit the peacock? How many carotenoids does the male guppy need to eat to be so brilliantly orange? How many syllables can a songbird pack into his complex vocal repertoire to make him even more sexy to females? These are interesting questions, but they represent only one-half of the equation of sexual beauty, because they ignore what is going on inside the head of those who actually judge beauty. Such studies often assume that the female brain must evolve tools to figure out what is beautiful. But instead, the converse is often true. The brain has a long evolutionary history that biases how it assesses the entire world around it, not just the world of sex; and it functions within the framework of numerous neurobiological and computational constraints. I argue that instead of the brain having to evolve to detect beauty, the brain determines what is beautiful, and all of its constraints and contingencies give rise to a breathtaking diversity of sexual aesthetics throughout the animal kingdom. In this book, I will show that to understand what beauty is, we need to understand the brain that perceives it.

I will expand our understanding of sexual beauty by asking how the details of an animal's brain give rise to its sexual aesthetics, which, in turn, drive the evolution of beauty in that species. Specifically, I argue that beauty only exists because it pleases the eyes, ears, or noses of the beholder; more generally, that beauty is in the brain of the beholder. Some of the brain's neural circuitry has evolved to sense and respond to sexual beauty so that animals can find a good mate. But the brain also has other things on its mind besides sex. Other adaptations of the brain, such as those that help an animal find food, avoid becoming food, or recognize the difference between its mother and its father, can have unintended but important consequences on how that brain defines beauty. Only when we understand the biological basis of sexual aesthetics can we understand how sexual aesthetics drive the evolution of sexual beauty.

I have a unique perspective to offer on these issues as I have spent the past forty years studying the sexual behavior of a tiny, bumpy frog in Central America. This work has opened my eyes and mind to both the diversity of sexual behavior in the animal kingdom and a core unifying theory that I have developed called sensory exploitation. The key idea is simple: features of the female's brain that find certain notes of the males' mating call attractive existed long before those attractive notes evolved. Thus, females are the biological puppeteers, making the males sing exactly what their brains desire. Beauty is indeed in the brain of the beholder, and in most cases, that means the female's brain, although I will review numerous cases where males judge female beauty and where there is mutual display and assessment of beauty by both sexes. This simple idea contributed to a paradigm shift in the study of sexual selection, one in which the importance of the sexual brain as a driver of evolution finally was acknowledged.

In this chapter, I will give some background on how scientists have come to understand the evolution of beauty and also explain which sex usually evolves this beauty and why. In the next one, I'll focus on the bumpy frog that has...