Thoughts On Animal Acoustic Diversity

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. 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. 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.

One More Chance

Within ten years, colonist songs had almost completely diverged from those of the ancestral population on another island. 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. 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. 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. 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. Each species brings a different set of predispositions and foibles, which are then elaborated through the reciprocal evolution of preference and display. Yes, there is order in the sounds of a forest or seashore, revealing the physical and ecological laws of the world. 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.

Back In Business

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. 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. 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.

It Takes A Lot To Laugh

Listen to the animals around us. 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. 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. Larger species, on average, sing at lower pitches, with slower trills and melodies, than their smaller kin. 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. 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.