Sensory Bias
What an odd thing it is to see an entire species — billions of people — playing with, listening to, meaningless tonal patterns, occupied and preoccupied for much of their time by what they call ‘music.’
— Oliver Sacks, Musicophilia (2008)
For beneath the first depth, namely that of the earliest civilizations — which are but the foreground of the long backward reach of the prehistory of our race — there rest the centuries, millenniums, indeed the centuries of millenniums of primitive man, the mighty hunter, the more primitive root-and-bug collector, back for more than half a million years. And there is a third depth, even deeper, and darker, below that, below the ultimate horizon of humanity. For we shall find the ritual dance among the birds, the fish, the apes and the bees.
— Joseph Campbell, The Masks of God: Primitive Mythology (2020)
Currently popular scientific approaches to beauty don’t address its structure, and they ignore the problem of its persistence and consistency across widely unrelated species. The most prevalent perspective regarding music, dance and elaborate shapes and coloration maintains that they evolve to demonstrate a good body or good genes, somehow more reliably than other attributes, indirectly benefiting a mate with a coevolving preference for the trait by allowing them to capture and pass on the genetic goodness to their own offspring. Such approaches don’t explain the complexity of sexually selected traits, or why, of all the things animals could do to prove themselves, and the vast number of more direct vitality-determining biases that could evolve, countless numbers of varied species have relentlessly converged on the particular scenario of preferring a complicated song, dance, shape or pattern.
We shouldn’t think of this convergence as somehow inevitable, that songs for instance represent such a large subset of the possible vocalizations an animal can make that the event of evolving to sing is hard to avoid statistically, any more than we think it’s inevitable a large proportion of human cultures would resort to singing instead of talking as a means of relaying relevant information simply because we’ve run out of more direct alternatives. The same argument applies to other aesthetic phenomena. They come about entirely because we select for them over less desirable stimuli, not because they’re inevitable or likely as members of a limited set of possibilities.
The second most popular scientific explanation for beauty and preferences, also a coevolutionary approach that doesn’t address the consistency, complexity or persistence of aesthetic traits is Fisherian runaway sexual selection (Fisher 1930, O’Donald 1967, Lande 1981, Kirkpatrick 1982), in which preference/trait pairs result from mathematically conceivable genetic feedback loops for no particular reason, or “beauty happens,” as Prum puts it in The Evolution of Beauty (2018), which otherwise argues against good genes as a realistic mechanism. Fisher’s model assumes an initial period in which a trait favored slightly by natural selection also randomly happens to be favored by certain mates, causing genes for the trait and those for the preference to increasingly occur together in succeeding generations. The preference and trait reinforce each other in a positive feedback loop, and the process continues until sexual selection for the trait and natural selection against it are in balance. The sequence of steps involved and the number of times they would need to play out independently but somehow lead to similar results across species makes this an extremely unlikely scheme compared to good genes, and especially compared to the idea that preferences simply exist by default.
The sensory bias model of sexual selection is a recently developed (West-Eberhard 1979, 1984; Ryan 1990, Ryan and Keddy-Hector 1992, Basolo 1996), relatively simple and intuitive alternative to good genes and Fisherian runaway. It predicts that side effects arise in the process of sensory system and brain evolution, causing preferences that generate selection for matching traits despite potentially reducing survival, arguably circling back, after more than 100 years of science, to Darwin’s original thinking (1896):
When male animals utter sounds in order to please the females, they would naturally employ those which are sweet to the ears of the species; and it appears that the same sounds are often pleasing to widely different animals, owing to the similarity of their nervous systems, as we ourselves perceive in the singing of birds and even in the chirping of certain tree-frogs giving us pleasure.
Sensory bias can be distinguished from other models in that the preference for a trait evolves prior to and in a different context than the trait preferred, non-coevolutionarily. Such biases could be originally adaptive, evolving through direct selection, by improving prey detection ability for instance, or they could be “hidden,” in that they’re an incidental by-product with no original, subsequent or current fitness-related function (see Ryan 1990, Enquist and Arak 1993, Endler and Basolo 1998, Arnqvist 2006).
Sensory bias appears to have played a role in the evolution of “courtship trembling” behavior in Neumania papillator water mites (Proctor 1991, 1992), complex mating calls in the Tungara frog Physalaemus pustulosus (Ryan and Rand 1990), complex swords in swordtail fishes (Basolo 1990, 1995, 1996), long, flowing tails in widowbirds (Pryke and Andersson 2002), orange spots in the guppy Poecilia reticulata (Rodd et al. 2002), song structure in Costa’s hummingbird Calypte costae (Clark and Feo 2009), song repertoires in birds (Collins 1999), nuptial food gifts in insects (Sakaluk 2000), egg-spots in Cichlids (Egger et al. 2011), mud pillar (Christy 1995) and sand hood (Christy 2003) building in the fiddler crabs Uca beebei and Uca musica, red pelage and skin colors in primates (Fernandez and Morris 2007), pollinator attraction in orchids (Schiestl and Cozzolino 2008), Anoline lizard head bobbing patterns (Fleishman 1992) and mate color preferences in birds (Møller and Erritzøe 2010). Preferences for entirely novel signals, like red leg bands in zebra finches Taeniopygia guttata (Burley et al. 1982), white crests in finches (Burley and Symanski 1998) and gene transfer-induced red color in zebrafish (Owen et al. 2012) indicate that preferences can exist independently of favored traits.
That organisms so widely separated taxonomically, ecologically and neurologically as humans, in every culture, whales (Payne and McVay 1971), bats (Behr and Helversen 2004), mice (Holy and Guo 2005), frogs (Searcy and Andersson 1986), about 5,000 species of birds (Hartshorne 1973), numerous different types of insects and other animals have an apparent preference for complex, song-like auditory stimuli strongly suggests the involvement of common elements in sensory systems, and it’s difficult to imagine so many different populations and species being subject to a common ecological factor that could possibly drive direct, adaptive selection leading all of them to a desire for song.
There’s no reason to believe other animals have strange, unidentifiable but somehow functional reasons for performing and enjoying songs and dances, particularly when we know that for us the reason is only the pleasure we take in experiencing the patterns that make them up. The problem with assuming independent origins of preferences for song in many different singing animals might be illustrated by the absurdity of the idea that unique preferences for the behavior had to evolve separately in every human culture rather than being in place to begin with, especially given the lack of a function for human song (Darwin 1871):
As neither the enjoyment nor the capacity of producing musical notes are faculties of the least use to man in reference to his daily habits of life, they must be ranked amongst the most mysterious with which he is endowed.
The 300 distinct notes and 66 types of songs of the winter wren Troglodytes troglodytes (Kroodsma 1980), or the 58 elements, 15 separate behaviors and 10 plumate ornaments distinguished by Scholes (2006) in the dance of Carola’s parotia (Parotia carolae) represent behaviors that are undeniably more complicated than what would suffice for the purpose of conveying a signal of condition. Unless we assume the songs of wrens came into existence fully formed, without increasing in complexity over time, these birds have been amused by every shorter, simpler evolutionary version of the song in addition to the current one. Parotias must have been impressed by every historical parotia dance, and by every new element as it was added, at the same time consistently rejecting potential mates who didn’t perform movements approaching the extremely complicated sequence currently in use. The existence of such traits is much easier to understand with a general preference for complexity in place prior to and throughout their evolution.
Humans have no adaptive business being so affectionate about flowers, given that we don’t get food from or pollinate any of them naturally. One common theoretical approach in such cases is to trace the origin of a currently inexplicable trait, an attraction to flowers in this case, back to a common ancestor with a species that does have a reason to express it. For humans, an attraction to flowers would seem to go back at least beyond our most recent common ancestor with the many species of nectar-feeding, flower-pollinating birds, or prior to the split between mammals and reptiles, around 300 million years, probably before the first flowers existed. Notably, human interaction with flowers isn’t restricted to those produced by plants. We paint abstract versions of them on objects and canvases, carve or mold them into architecture, stitch them into clothing and tattoo them on our bodies, an indication that animals enjoy the shape in general, not only in the practical, ecological context of foraging.
To explain why insects and birds share a preference for flowers one would predict it was present in the brains of the ancestors of both groups, going back over 500 million years, to a time long before flowers, which, according to Sauquet et al. (2017), appeared somewhere between 140 and 250 million years ago. Preferences for song and dance, judging by their effects, in the courtship of both insects and birds, for instance, probably go back to the same early times, persisting with no regard for the taxonomic distance between species, across massive discrepancies in body size, brain size and ecology, suggesting that every animal has some capacity to understand and admire them by default, as a consequence of universal, inevitable aspects of sensory system structure.
The proposition that every singing species of bird evolved an inclination for musical sounds independently is hardly different from proposing that every species of bird separately evolved the ability to fly, except that in the first case there’s no practical reason for them to do so, and therefore even less reason to believe it happened more than once. There are obvious benefits to flying. It seems very likely that doing so has allowed birds to be, according to Darwin, the second most aesthetic type of animal, and it’s very unlikely to be a coincidence that birds and humans also have in common such an unusual degree of freedom from ecological limitations related to resource availability, predation and unsuitable environmental conditions. This isn’t to say animals don’t evolve the tendency to be attracted to a specific flowering plant, or to enjoy the particular type of singing or dancing of their own species more than others; it’s that they do so in the context of preexisting, ever-present biases for these types of stimuli.
Even if we did historically pollinate a particular flower, for some hardly imaginable, naturally selected purpose, this would only explain our attraction to that kind of flower, not most or all of them. This problem applies in general because so many particular flowers are pollinated by multiple animal species, which themselves pollinate multiple kinds of flowers. Assuming animals like flowers to begin with helps explain the success angiosperms have had in using floral shapes to exploit us. If humans had a history, evolutionarily, of performing a song or dance in a courtship display to compete for mates, traditional thinking would only explain the enticement of whatever particular song we were singing or dance we were doing, not the massive variety of those we’ve created, and no obvious advantage would accrue to a population expanding the use of song or dance outside the context of sex into that of religion, patriotism, ceremony, celebration and general entertainment.
Musical patterns apparently flow through the animal brain from the auditory cortex to the motor areas, translating outwardly into alternating body shapes and actions with the same, semi-predictable, simultaneously regular and random (disorder~order) structure as the music, as though the substance of the brain is offering little resistance. That humans relate dance to music and integrate the two phenomena in practice shows we have an automatic, unconscious way of recognizing some degree of aesthetic overlap or equivalence between them, probably because they have similar effects on the brain.
There’s a growing list of animals that dance to human music, meaning the equivalence is likely recognized universally. Cotton-top tamarin monkeys respond emotionally to human music rendered in a tamarin style (Snowdon and Teie 2010), and research shows that harbor seals (Verga et al. 2022), cockatoos (Patel et al. 2009), rats (Ito et al. 2022) and chimpanzees (Hattori and Tomonaga 2020) enjoy dancing to human music. In these cases, animals that shouldn’t understand and care about music, or dance, or know that the two are related, unconsciously understand all three things, undermining coevolutionary arguments and supporting a universal sensory bias perspective.
Animals will also dance spontaneously, without a musical cue, in response to the sight of another dancing animal. Campbell (2020) gives an account of chimpanzees playing and joining a group dance featuring spinning in place, forming flowing chains of spinning individuals and trotting with synchronized, asymmetric steps in circles around a pole:
Tschengo and another chimpanzee named Grande invented a game of spinning round and round like dervishes, which was then taken up by all the rest. “Any game of two together,” Dr. Köhler writes, ‘was apt to turn into his “spinning-top” play, which appeared to express a climax of friendly and amicable joie de vivre. The resemblance to a human dance became truly striking when the rotations were rapid, or when Tschengo, for instance, stretched her arms out horizontally as she spun round. Tschengo and Chica — whose favorite fashion during 1916 was this “spinning” — sometimes combined a forward movement with the rotations, and so they revolved slowly round their own axes and along the playground. The whole group of chimpanzees sometimes combined in more elaborate motion patterns. For instance, two would wrestle and tumble near post; soon their movements would become more regular and tend to describe a circle round the post as a center. One after another, the rest of the group approach, join the two, and finally march in an orderly fashion round and round the post. The character of their movements changes; they no longer walk, they trot, and as a rule with special emphasis on one foot, while the other steps lightly, thus a rough approximate rhythm develops, and they tend to “keep time” with one another….’ “It seems to me extraordinary,” Köhler concludes, “that there should arise quite spontaneously, among chimpanzees, anything that so strongly suggests the dancing of some primitive tribes.
Like us, although they have no reason to do so, or to find it amusing, chimps are equipped psychologically with both the desire to dance and the knowledge of how it’s done. Due to the existence of music and dance throughout the animal world, it’s predictable that sensory systems and brains embody an understanding of the qualities and dualities involved automatically, perceive them as opposites and find the idea of mixing them together in intricate patterns intriguing.
Mythology has the same problem with aesthetic themes recurring in widely unrelated cultures and stories that evolutionary biology has with certain themes (song, dance, contrasting colors, ornamental ocelli, long flowing tails, Thayer’s law) turning up in widely unrelated species, and, probably, the same kind of solution in the form of universal preexisting biases in sensory systems. As a potential way to understand common features of human psychology and resulting patterns in mythology, Campbell brought up the subject of sensory bias long before it was popular, in Primitive Mythology (1959), the first volume of The Masks of God series, citing an experiment by Adolf Portmann (1953) with the grayling butterfly, Eumenis semele, which showed that males pursue artificially darkened females “in preference even to the darkest female of the species,” early empirical evidence that animals have preferences for versions of each other that don’t exist in nature:
A new and very promising approach is opened, however, when it [mythology] is viewed in the light of biological psychology as a function of the human nervous system, precisely homologous to the innate and learned sign stimuli that release and direct the energies of nature — of which our brain itself is but the most amazing flower.
Wallin (1991) proposes a kind of sensory bias as the mechanism responsible for the evolution of singing behavior, pointing out the “dynamic dichotomy in music is similar to that which characterizes organismal systems,” and a “morphodynamic isomorphism” between the form of music and that of the brain, music’s physiological substrate.
Sensory bias is usually thought of as a proximal explanation, for instance with the bias ultimately having arisen in an ecological context such as predation and later being exploited by traits that it favors happening to evolve in members of the opposite sex. However, the ecological origin step is an unnecessary complication. Especially when the same bias must have originated repeatedly, and it can’t convincingly be connected to anything of value survival-wise, it’s more reasonable to assume the bias is ultimately an incidental result of the basic structure of the brain to begin with, that it transcends and predates any particular ecological adaptation. Thermoaesthetics hypothetically extends Wallin’s isomorphism beyond music and song to other amusing things such as dance, color patterns and language, by way of a more general isomorphism between the dichotomies of liquid crystalline matter in the brain and aesthetic material.
Perplexity AI
Prompt: “How many species sing songs.”
The number of species that sing songs is far greater than commonly perceived, with conservative estimates suggesting over 200,000 species globally. This includes approximately 4,000 songbird species, at least 200,000 insect species producing vibrational songs, numerous marine mammals, various amphibians, select terrestrial mammals, and an increasing number of documented fish species. The true number continues to grow as researchers discover previously unknown vocal behaviors in species once thought to be silent, revealing that song is a fundamental form of communication across the animal kingdom.
Prompt: “How many species perform courtship dances?”
Conservative estimates suggest that several thousand species across the animal kingdom perform some form of courtship dance. Birds alone contribute hundreds of dancing species across multiple families. When combined with the diverse arthropod dancers, marine species, and representatives from other animal groups, the total likely reaches into the thousands.