Sounds Wild and Broken, page 14
In 1915, statistician Ronald Fisher puzzled over the aesthetic tastes of animals in the breeding season. Darwin had proposed that sexual ornaments evolved to satisfy the preferences of mates. But why, Fisher wondered, do animals have such strong desires for “seemingly useless ornament”? His answer starts by noting that the evolutionary success of any animal depends not only on the survival of its offspring, but on how attractive these offspring are when they mature and try to mate.
Fisher reasoned that aesthetic tastes are grounded in the need to distinguish healthy from unhealthy mates. These preferences are shaped by the ecology of each species. Carrion flies, he wrote, love the aroma of rotting flesh, but the same odor on mammalian breath indicates tooth abscesses. Evolution thus favors the development of aesthetic tastes particular to each species, giving animals what he called a rough index of the “general vigour and fitness” of potential mates. Fisher then offered his key insight: Once established, preferences will favor further elaborations in the “splendour and perfection” of the mating display. Attractiveness becomes its own evolutionary force. Animals whose displays meet or exceed the aesthetic standards of their species will leave many offspring because they attract many mates or mates of high quality. Aesthetic preference and exaggeration of breeding displays become linked through evolution, egging one another on, in a process that feeds on itself.
The process of exaggeration continues even if the display “ceases to be any index of vitality whatever.” Then the breeding display is favored by evolution only because it is attractive, not because it signals health. Fisher predicted that breeding displays would ratchet up their extravagance until predation or physiological limits put an end to further increases.
In letters to Darwin’s grandson, Charles Galton Darwin, Fisher outlined a mathematical demonstration of his idea. He also speculated, without supporting evidence, about how the process might work in humans, viewing sexual choice in our own species through a racist, eugenic lens. He claimed that only the “higher races of mankind” developed standards of beauty that reflected “moral character.” Like many early twentieth-century scientists, Fisher took what was a sound insight into evolution, an insight that provides not a shred of support for racist ideologies, then twisted it to fit his white supremacist views. Modern theoreticians have shed and rejected the racism, and confirmed Fisher’s mathematical findings about the coevolution of sexual preferences and displays, especially work by Russell Lande and Mark Kirkpatrick in the 1980s, followed by Andrew Pomiankowski and Yoh Iwasa in the 1990s. These biologists concluded that the process of coevolution and elaboration that Fisher outlined has firm mathematical and logical foundations. The evolution of aesthetic preferences and breeding displays can indeed, they concluded, balloon initially modest mating signals into extreme displays. Biologist Richard Prum has even proposed that the theory underlying the process is so “extremely robust” that it should be regarded as the “intellectually appropriate null model” of sexual evolution, the default against which other ideas are tested.
Fisher and many contemporary biologists present this process as one in which female preferences drive male displays. But evolution transcends such restricted views of sexual roles. Any inherited display can coevolve with any inherited preference, regardless of sex. If inheritance happens culturally, when animals learn preferences from older generations, as has been documented in insects and vertebrate animals, Fisher’s process of exaggeration can also proceed. In all cases, it is the preference that kicks off and guides the process. Animal acoustic diversity has its roots in the sensory perceptions and preferences of listeners, which are then elaborated through coevolution of preference and display.
A survey by biologist Zofia Prokop and her colleagues of contemporary field studies of animal breeding displays found supporting evidence for Fisher’s process. Across ninety studies—with subjects as varied as crickets, moths, cod, voles, toads, swallows, and more—the researchers found that inheritance of attractiveness was more common than inheritance of bodily vitality. If this result holds across the animal kingdom, then the mating preferences of parents can indeed produce attractiveness in offspring, even if such attractiveness serves no other purpose than to increase mating success.
Fisher speculated that his process starts with preferences that indicate the health of breeding animals. But any mating preference can serve as a seed for the process. If the sensory system is tuned to a particular frequency or tempo of sound, perhaps to help find prey, then songs in this range will be particularly attractive. In small populations, accidental changes can also kick off the coevolutionary elaboration of taste and display. For example, when just a few members of a species are isolated from the rest of their kind—colonists on an island or inhabitants of an outpost on the edge of a species’ range—they may have mating preferences that are not representative of their species. These small clusters of atypical mating preferences arise through the randomness inherent in picking out a tiny subset of a population. This is exacerbated by genetic drift, the random ups and downs of gene frequencies from one generation to another, fluctuations that are especially pronounced in small populations. Drift also affects behaviors such as the songs of some birds whose forms pass from generation to generation not through genes but by social learning. Any quirk can set off Fisher’s process in a direction that depends on the initial particularities of mating preferences.
Drift can, in just a few generations, elevate a rare mating preference to dominance in a small population. For example, after a small group of finches colonized one of the Galápagos Islands, their songs changed rapidly from a simple slight downslur of frequencies to a more pronounced, two-part sweep. Within ten years, colonist songs had almost completely diverged from those of the ancestral population on another island. Likewise, the songs of common birds such as the red-capped robin, western gerygone, and singing honeyeater on Rottnest Island off the west coast of Australia differ markedly from those of the mainland. Despite the fact that many mainland bird populations sing uniform songs across ranges of thousands of kilometers, these island birds sing with their own cadences and rhythms. Island-dwelling robins and honeyeaters have simpler songs than those of the mainland, but the gerygone sings more song types on the island, using rhythms unknown on the mainland. The isolation of these small peripheral populations frees them from the genetic and cultural exchange that enforces uniformity on the mainland. There is a parallel here with cultural change in human societies. Margins are, in the words of essayist and journalist Rebecca Solnit, “where authority wanes and orthodoxies weaken.” Islands and other marginal habitats, then, are incubators of novelty and change.
The coevolution of taste and display can be an accelerant for both the diversity of sound and the process of speciation. Small differences are magnified, accounting for the profuse diversity of animal mating displays. But as varied as they are, the differences among breeding displays are not arbitrary; they reflect the particular history and ecology of each species, inflated over time.
There is an improvisational quality to Fisher’s process. When musicians improvise, they take ideas, elements of the music, then pass them back and forth, elaborating and exploring as they listen and respond. Evolution works in analogous ways, although it makes its music by shaping the script of DNA and the learned experiences of animals. Each species brings a different set of predispositions and foibles, which are then elaborated through the reciprocal evolution of preference and display.
This view of sonic evolution has a refreshing openness to novelty and unpredictability, contrasting with more rule-based, utilitarian explanations of why sounds are so diverse. Yes, there is order in the sounds of a forest or seashore, revealing the physical and ecological laws of the world. But there is also unpredictable creativity in evolution’s work. When I listen to the diversity of birdsong or the varied calls of frogs and insects, I hear exuberant anarchy, evolution drunk on its own aesthetic energies. Other human listeners, though, are more impressed by the order and unity of wild sounds, comparing them with symphonies and orchestras, forms of music whose beauty and creativity emerge through coordinated and hierarchical relationships. Predictable order and capricious whimsy work together to produce the sonic marvels of our world. Human aesthetics, born in our evolutionary path as we developed speech and music, seem to love these tensions between order and tumult, unity and diversity.
The effects of physical laws on animal sounds are easier to measure and document than the unique improvisational history of each species. Fisher’s process is wraith-like. Its creative actions left no fossils of sound for us to discover. The ghost left marks of its passage, though, in subtle arrangements of genes and patterns of sound among closely related species.
In Fisher’s process, aesthetic tastes and the form of song displays coevolve. Changes in tastes encourage elaboration of displays, which then stimulate further exaggeration of tastes. This results in a genetic correlation between aesthetic preferences and the form of breeding displays. Animals with genes for extreme displays also have genes for extreme preferences. The limited genetic evidence to date, drawn from studies of fewer than fifty species, shows that, for most species, genes for display and mating preference are indeed correlated. Most of these studies are of insects and fish, animals whose breeding sounds are relatively simple to measure—trills, croaks, and chirps. The genetics of aesthetic preference in more complex sounds—the timbre of a hermit thrush’s slow, rich introductory note compared with later rapidly modulated notes, the melodic form of a humpback whale’s song, the fine details of the cadence and pacing of a mouse’s ultrasonic warbling—are unknown. Uncaged animals live in aesthetic territories whose behavioral genetics are uncharted. For now, we can conclude that in some species, the limited genetic evidence to date is consistent with Fisher’s idea.
Fisher’s process also leaves evidence more accessible to our everyday senses than statistical correlations among genes. Listen to the animals around us. Spring peepers, chorus frogs, wood frogs, and toads all call from the same American vernal ponds, yet they make a range of sounds that far exceeds the need to tell one from another or transmit sound through vegetation: bell-like peeps, rhythmic rasps, strangled quacks, and sweet trills. The katydids of the Amazon forest tap, chirp, thrum, whir, and whistle, using many tempi, displays whose diversity bears the marks of aesthetic extravagance. The astonishing diversity of birdsong transcends mere utilitarian need to signal vigor.
These everyday experiences can be analyzed more formally using evolutionary trees derived from DNA. Each tree represents the history of origins and splitting of animal species, a family pedigree for the species in question. By mapping the form of songs or other breeding displays onto the trees, we can trace how sounds changed over time. In these trees we read both the predictable marks of physical constraint and the caprices of history. The body size of animals—from the length of bird beaks to the size of chirping insect wings—strongly affects the frequency and speed of song. Larger species, on average, sing at lower pitches, with slower trills and melodies, than their smaller kin. Likewise, the environmental and biological context—density of surrounding vegetation, presence of predators and competitors—shapes the form of songs, molding each species to its surroundings. But alongside these factors there is a sprite-like unpredictability about evolutionary changes in rhythms, melodies, modulations, timbre, loudness, crescendos and decrescendos, and pacing, elements that in a human context we’d call musical form or style.
When songs are mapped onto evolutionary trees, we see that they expand and contract unaccountably through time. Their cadences and timbres shift with seemingly no governing law or direction. A biologist presented with the news that a new species has been discovered might, with the help of an evolutionary tree and information about the animal’s body size and habitat, hazard a good guess about the most general qualities of the song of this species, such as frequency and perhaps tempo. But they would be unable to predict other qualities of the song. These evolutionary patterns do not prove that Fisher’s process caused the elaboration of sound. But they are consistent with his ideas and, for now, inexplicable by any other known evolutionary process.
In the voices around us, we hear a great meeting of evolutionary forces, like the confluence of lively rivers: Fisher’s mercurial processes, the genetic imperative to avoid interbreeding with the wrong species, the benefits of honest signaling of bodily health, the many shapes and sizes of animal bodies, the guiding walls of physical environments, and the diverse ways that animals find their sonic place in complex communities of competitors, cooperators, and predators. The result is a glorious, creative, turbulent flow from headwaters at least three hundred million years old.
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Honest signals. Sensory bias. Coevolution of preference and display. What do these workings of evolution mean for living animals?
Every species lives within its own aesthetic. The spring peeper hears the peep of neighbors through inner ears tuned to the range of frequencies used in its breeding displays. The sense of hearing is the first gate on the path to aesthetic judgment for the spring peeper, just as it is for all animals that find and select mates by sound. The anatomy and sensitivity of each species’ ear frame this portal to aesthetic experience.
The next door is the narrower one, the unique preferences of each animal for the pacing, timbre, amplitude, and melodic structure of the call. A peeper’s ear is stimulated by many sounds, including sometimes the sounds of closely related frogs. But only one sound causes her to reach out and tap the bulge-throated singer and initiate mating. Her sonic discernment might serve many ends: picking out a vigorous mate, staying away from transmissible diseases, avoiding interbreeding with other populations, or ensuring that offspring will, when the time comes, have songs that other frogs find attractive. For the frog, though, this long backstory of how preferences came to be resolves into an experience in the moment. Vibrations in air, when they are patterned just right, wake knowledge embedded in the frog’s genes, body, and nervous system. She hears and understands.
Aesthetic experience is thus a meeting of the outer world with the knowledge that all animals carry within. The result is subjective, depending on the sensory abilities and preferences of each species and individual within the species. Only a spring peeper truly comprehends the peep.
How this experience manifests in froggy subjective experience is unknowable. Even among humans, we cannot project our own experiences onto others. I hear sounds both as aural sensations and sometimes as bodily experiences of light and motion. For others among my family and friends, the same sounds evoke color, and every pitch has its own hue. The senses live in a net of relations, a web whose shape differs subtly among us. Imagining the experience of sound in other humans is therefore difficult. Imagining experience in other species is harder still, best approached in a mode of gentle conjecture. The spring peepers’ large mouths and noses are very sensitive to aromas, and so perhaps they experience sounds as odorous vapors or bursts. Or the peep may evoke a sense of movement in the chest, echoing its production, in the same way that our body sometimes feels itself in motion when we hear human music. Studies of frog physiology show that sound is transmitted to the inner ear not only through the eardrum but via forelimbs and lungs, making frog hearing perhaps more like the total bodily immersion that fish experience. We live in world of tantalizing otherness. So many experiences coexist, food for imagination and humility.
We humans can reach out to other species with science, empathy, and imagination, but such practices are also subjective, coming as they do from an animal with its own sensory biases and tastes, including our aesthetic preferences for some ideas over others. And so the history of the scientific study of sexual displays cleaves to the values of each age. We hear other animals sing through the filters of our preferences for what is a beautiful or ugly idea.
But subjectivity does not mean that we do not perceive truth. Aesthetic experience can, when it is rooted in deep engagement with the world, allow us to transcend the limits of the self and to understand more fully the “other.” Outer and inner worlds meet. Subjectivity gains a measure of objective insight. In an experience of beauty or ugliness is an opportunity to learn and expand.
Biologists seldom discuss aesthetics or beauty. When they do, it is in the context of the evolution of a restricted set of sexual displays, those that we humans find attractive or intriguing: strident songs and bright colors. Quieter sexual beauties are absent from biological theories of aesthetics. We pass over the quiet chip notes and camouflaged olive-green plumage of a female bird, even though evolution has likely caused male birds to be highly attentive to these forms of sexual beauty. Further, all animals make sophisticated choices about social relationships, food, habitats, and the rhythms of their activities across space and time. Each of these is mediated by a nervous system that integrates inner knowledge and outer information, resulting in motivation and thus action. Every species has its own neural architecture, but all species share nerve cells and neurotransmitters of the same kinds. Unless evolution has wrought an entirely different product—aesthetic experience—from human nerves than those of all our cousins, aesthetics are at the center of how nonhuman animals understand their worlds and make decisions. To presume otherwise is to suppose that humans and other animals are separated by an experiential wall. There is no neurological or evolutionary evidence for such a divide.
Consider the many manifestations of aesthetic experience in our lives. Almost every important decision and relationship in human lives is mediated by aesthetic judgment.
Where to live? We have profoundly moving responses to habitats, both to houses and their surroundings. In some we find great beauty or ugliness. In others our aesthetic sense yields only a bland whatever. These judgments then motivate us to spend a large portion of our resources to locate ourselves in the most beautiful of the choices available to us.
