Any particular trait that evolved through sexual selection was probably influenced by some combination of runaway processes, pressures to advertise fitness, and psychological preferences. Most sexually selected traits probably work as both ornaments and indicators. Some elements of their design evolved to provide hard-to-fake information about fitness; others evolved just because they happened to be exciting and entertaining. To understand the human mind as a set of sexually selected traits, we have to envision how ornamental and indicator functions can exist side by side in the same trait.
An indicator must accurately indicate a particular quantity. But this requirement does not determine every aspect of an indicator's design: there are always many design elements that are free to vary in ornamental ways. Almost all car speedometers can successfully indicate the car's speed, but there are hundreds of different speedometer designs used in different makes and models of car. All wristwatches indicate the time, but different watch designs may vary in every possible detail according to the aesthetic tastes of manufacturers and consumers. As long as speed, time, or some other indicated quantity is more or less intelligible, the indicator's design is free to vary according to aesthetic whimsy, exploring the fringes of ornamental style.
Actually, the handicap principle makes sexually selected traits a bit more constrained than watch designs. The Rolex Corporation has no incentive to mislead its customers about the time. Animals do have incentives to mislead potential mates about their fitness. Coins make a better analogy for sexually selected traits than do watch-faces. Numismatists are familiar with the two criteria of successful coins: they are hard to counterfeit (a requirement that increases with their monetary value), and they are attractive to the eye and the hand. Coins indicate value just as watches indicate time. But with coins there is a much greater incentive for fakery.
Counterfeiting has been a concern ever since 560 B.C., when King Croesus of Lydia invented true official coinage
(government-issued cast disks of standard weight, composition, and guaranteed value). To guard against counterfeiting, authorities produce coins according to the handicap principle. They endow coins with features that would be prohibitively expensive for a counterfeiter with low capital to imitate. In the ancient era, it was usually sufficient to produce coins with hard-to-make iron coining dies. By the 17th century, authorities had to invest in expensive rolling mills, sizing dies, and blanking presses to deter counterfeiting. The modern principles of coinage-accuracy of dimension, perfect reproduction of design, standard weight of an easily tested alloy—all evolved to make coins accurate indicators of monetary value.
And yet there has been enormous scope for coins to vary in ornamental ways. This ornamental elbow room is what gives numismatics its interest, just as sexual selection gives biodiversity its fascination. Ancient Greek coins, though commonly made of precious-metal alloys to a common basic design, were ornamented in different ways depending on the city-state of origin: owls for Athens, bees for Ephesus, the griffin for Abdera, the eagle of Zeus for Olympia, the lion of Leontini, the minotaur of Knossos, the quince of Melos, the silver-miner's pick at Damastium, the grapes of Naxos. The requirement that the famous Sicilian decadrachm of 480 B.C. must properly indicate its value did not determine its beautiful ornamentation, with triumphal chariots above a fleeing lion (symbolizing the recently conquered Carthage) on one side, and, on the other side, Arethusa. (Arethusa was a water nymph who escaped unwanted sexual attention from the river-god Alpheios by asking Artemis to transform her into a freshwater spring—an evolutionary counterproductive way to exercise female mate choice.) Within a few years of the invention of coinage, Greek city-states were not just worrying about overcoming counterfeiting; they were competing to make coins beautiful. While there were just a few principles to guarantee a coin's value, there were an infinite number of ways to ornament it with a pleasing design.
The principles of coinage, like those of sexual selection, are not just economic but aesthetic. While the economic principles of value-indication tend to produce similarities between coins, the aesthetic principles are more creatively protean, producing endless diversity. To understand the features of any given coin, it is not enough to appreciate the general requirements of money (durability, divisibility, portability), or the particular anti-counter-feiting principles of coinage (standard size, weight, composition, and design). One must also appreciate the aesthetic imperatives, from the universal sensory demands of the human hand and eye, to the historically contingent symbolism of a particular culture. Likewise for a sexually selected trait—one must understand how certain features indicate an animal's fitness, and how other features evolved as aesthetically pleasing ornaments, just because they happened to excite the senses and brains of the opposite sex. As anti-counterfeiting principles rarely suffice to explain every detail of a coin, in almost no case of a sexually selected trait does the handicap principle alone suffice to explain every detail. There is always some aesthetic slack.
In sexual selection, traits that began as indicators tend to grow more complexly ornamental because the sensory preferences of the opposite sex partially impose their own aesthetic agenda on the indicator. Conversely, traits that originate as pure runaway ornaments tend to acquire value as fitness indicators because aesthetically impressive ornaments tend to be costly and difficult to produce. Almost all sexually selected traits that last more than a few hundred generations probably function both as indicators and as ornaments. They may have originated mainly as one or the other, but soon imposed sufficient costs that they indicated fitness accurately, and soon acquired enough aesthetic complexity that they stimulated the senses of the opposite sex in ways that could not be reduced to indicating fitness.
The messy overlap between indicators and ornaments does not mean that we can afford to get messy about sexual selection theory. Zahavi's handicap principle is quite distinct from Fisher's runaway process. But they frequently work together, so we should not worry too much about trying to categorize every sexual trait as either an indicator or an ornament. Instead, we should use different models of sexual selection as lenses to view a given trait from different angles and different distances, to answer different evolutionary questions. The fitness-indicator principles are good at explaining why animals of a given species have such a strong consensus about what they like in a sexual trait: why all peahens like the peacocks to have large, symmetric, bright, many-eyed tails. The fitness-indicator perspective explains the perfectionism and conservatism of sexual tastes within each species. It also explains why large, long-lived animals have not degenerated to extinction under the pressure of harmful mutations. On the other hand, the ornamental principles are good at explaining why animals of different species develop such different tastes: the tails that attract peahens, for example, are not turn-ons for female turkeys or female albatrosses. The ornamental perspective explains the protean divergence of sexual tastes across species over macro-evolutionary time. It also explains why sexually reproducing life on our planet has split apart into millions of different species.
The ornamental view is especially important for appreciating the role of evolutionary contingency in shaping sexual traits, just as it is in appreciating the role of historical contingency in shaping coins. Once King Croesus invented official coinage, we could have predicted that most city-states of the ancient Mediterranean world would adopt coins, would make them hard to counterfeit, and would ornament them with some pleasing designs. However, we could not have predicted that the coin-engraver's art would reach its peak in 5th-century B.C. Syracuse, on the island of Sicily. It could have happened at some other time in Carthage, Crete, or Athens, but it didn't.
Likewise for the products of sexual selection. We can see that, once sexually reproducing animals evolved the capacity for mate choice, every animal species would then evolve some sort of fitness indicator; and that some indicators might be costly, exaggerated body parts, and others would be costly, ritualized courtship behaviors. But we could not have predicted that courtship behavior would reach an especially high degree of sophistication exactly 535 million years after the Cambrian explosion (when multicellular animals proliferated) in our particular species of bipedal ape. Nor could we have predicted that the courtship behavior would take the precise form of interactive conversations using arbitrary acoustic signals (words) arranged in three-second bursts (sentences) according to recursive syntactic rules. Perhaps it could have happened in an octopus, a dinosaur, or a dolphin. Perhaps it was likely that it would happen sometime, in some species of large-brained social animal. Rewind the tape of evolution, and the human mind would probably not have evolved, because sexual selection would have taken a different contingent route in our lineage of primates. But I suspect that in any replay of evolution on Earth, sexual selection would sooner or later have discovered that intelligent minds similar to ours make good courtship ornaments and good fitness indicators.
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