The Contrast Effect

For what seemed an immensely long time, I gazed without knowing, even without wishing to know, what it was that confronted me. At any other time I would have seen a chair barred with alternate light and shade. Today the percept had swallowed up the concept. I was so completely absorbed in looking, so thunderstruck by what I actually saw, that I could not be aware of anything else. Where the shadows fell on the canvas upholstery, stripes of a deep but glowing indigo alternated with stripes of an incandescence so intensely bright that it was hard to believe that they could be made of anything but blue fire.

— Aldous Huxley, on mescalin, see Campbell (2018)

Aesthetic interactions between qualities from opposite sides of semi-independent primitive perceptual dimensions of vision such as bright~dark and warm color~cool color provide a particularly clear example of aesthetic indifference. Red can be coupled with blue, but also with darkness, green or purple, and blue can be coupled with brightness or any warm color to create a composition matching the structure of aesthetically selected traits in animals and decorative human artifacts.

The list below gives the possible contrasting combinations of the most widely recognized colors (rainbow colors plus black and white), and all of them tend to describe aesthetic phenomena in both biology and culture, meaning contrast indifference is a universal trait in animals.

Contrast effect: black~white, black~red, black~orange, black~yellow, blue~white, blue~red, blue~orange, blue~yellow, green~white, green~red, green~orange, green~yellow, purple~white, purple~red, purple~orange, purple~yellow.

Perplexity Computer estimates that about half of all bird species (including over 95% of hummingbirds and all penguins), 50-70% of fishes, 70-80% of butterflies, probably all octopuses and cuttlefish, thousands of mammals and most likely hundreds of thousands of beetles show contrasting coloration. Various photographic examples of the effect, mostly in animals, can be viewed on the Pinterest board Bright~Dark.

Considering this pattern as a whole, especially given how it holds across distantly related species, the most reasonable conclusion is that all animals with the ability to see them enjoy a mixture of brightness and/or warmer colors with darkness and/or cooler colors. The preferences apparently evolve along with and as a mostly harmless side effect of the practical ability to make distinctions between colors for reasons related to survival.

It's common for such opposing colors to appear or increase during mating seasons, and they often describe the color scheme of small, particular parts of an animal's surface, such as decorative ocelli in butterflies, birds and fishes, or they appear as an element of the allure of a part of the body that certainly serves the purpose of impressing a mate, such as the fan of a dancing pheasant, the tail of guppy or the elaborate abdomen of a peacock jumping spider, all of which are evidence against traditional explanations having to do with adaptive crypsis or thermoregulation.

Aesthetic indifference in the contrast effect is conveniently demonstrated by color patterns in manakins. Females are usually a plain olive green, so the colors of males must have been selected for aesthetically through mate choice. According to Perplexity, out of 55 manakin species, 28 are decorated with two contrasting colors, 21 with contrasting multicoloration, and only 6, about 11%, are a uniform black or green. Patterns that have evolved in males include black and white, black and red, black and yellow, black and orange, “super black” and bright blue, and green and yellow. Thus, white, red, orange and yellow can replace each other as amusingly contrasting opposites to black, suggesting manakins appreciate contrast in general, not only the colors of their own species. Rather than some preferring yellow and black over red and black or black and orange over black and white, it’s likely that all manikins simply prefer mixtures of more and less exciting colors, and that the preference existed before manakins differentiated into the current set of species.

This isn’t to say manakins of a particular species don’t prefer their own colors to those of other, related species. Part of the reason for the diversity of contrasting colors in closely related species that inhabit or previously inhabited the same territory is probably that they support conspecific recognition and prevent pointless matings, but this could be accomplished with many kinds of differentiating characteristics. The recurrence across species of contrasting patterns in itself needs to be explained regardless of a recognition function, along with why it evolved initially. The contrast preference must have evolved long before manakins, because the same kind of color pattern trend exists in countless other groups of related taxa.

Anole lizard dewlap colors can be blue and yellow, black and yellow, black and orange, black and red, blue and pink or other contrasting colors, much like manakins. Again, it’s more reasonable to assume the preexistence of a general preference for contrasting colors in the ancestors of anole lizards than a scenario in which each species evolved a preference for the particular pattern it displays in conjunction with the pattern.

In birds-of-paradise coloration is diversified across species into configurations exemplifying the mixtures black/green~red/orange/yellow (Raggiana), black/blue~red/yellow (Wilson’s), black/blue/green~red/white (splendid astrapia), black/blue~yellow, (Victoria’s rifflebird), black/blue~yellow/white (lophorina), black/green~orange/white (standardwing), black~yellow/red (twelve-wired), black/blue/green~yellow/white (Parotias), black~yellow/white (king of Saxony), black/purple~red (Manucodes) and so on.

The same bright blue color seen in blue (Chiroxiphia caudata), blue-backed (Chiroxiphia pareola) blue-capped (Lepidothrix coronata) and velvety (Lepidothrix coronata) manakins also occurs in the blue (Paradisornis rudolphi), Wilson’s (Cicinnurus respublica) and magnificent birds-of-paradise, supporting the idea that less exciting, cooler colors are modified by brightness, a quality from a largely distinct perceptual dimension (bright~dark) to be more exciting than they would be otherwise, and moderately exciting overall.

Peacock jumping spider color schemes are similar to manakins and birds-of-paradise, with opposite hues interacting more elaborately. Females of various species select males displaying increasingly intricate mixtures incorporating all the mixtures listed above as representatives of the contrast effect, with some species expressing almost all of them at once: Maratus amabilis, Maratus australis, Maratus azureus, Maratus gemmifer, Maratus fletcheri and others. The bright blues seen in manakins and birds-of-paradise show up again in peacock spiders, often as a backdrop for dark reds.

In response to the prompt Is bright blue more common than dark blue in sexually selected animal color patterns, Perplexity Computer answers:

Yes — across the animal kingdom, sexual selection strongly favors bright, saturated, often iridescent blue over dark or muted blue, and this pattern shows up consistently in birds, butterflies, and other taxa that have been studied in detail.

Oppositely, reds tend to be dark:

Red works differently from blue — the sexiest reds tend to be deep, saturated, more “darkened” reds rather than pale or light ones, though the underlying mechanism and social meaning are distinct from blue’s brightness story.

The contrast effect shows indifference in culture as well, as with the colors of sports uniforms, clothing, flags, heraldry, holliday colors, superheroes, stained glass windows, mandalas, Ajanta Cave paintings, Persian tile work, logos, animation, video games and so on, meaning it evolves to fit preferences outside the context of mate choice, and that the underlying preferences can predate the effect instead of coevolving with it.

Repeated evolution of these same patterns in distant taxonomic groups implies the action of biases that came into existence with color vision, and it undermines arguments based on survival-related benefits like thermoregulation, parasite resistance or crypsis, which ignore the scope of the pattern as well as the fact that there's no connection to survival in cases of mate choice or human cultural objects. Assuming brightness and redder colors to be more exciting perceptually than darkness and bluer colors, the contrast effect is complex with regard to excitement, and fits the present thesis.