The Contrast Effect

Universal preferences select for contrasting color patterns in animals and culture

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 primitive perceptual dimensions of vision, such as bright~dark and warm color~cool color, provide a visual example of aesthetic indifference, or the fact that juxtapositions of more and less exciting perceptual qualities can be amusing whether they happen to be made up of direct, consciously recognized opposites or not. 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 generate the chromatic structure of aesthetically selected traits in animals and decorative human artifacts.

The aesthetic value of juxtapositions between darkness with warm colors and brightness with cool colors is of particular interest because the perceptual dimension bright~dark is at least partially distinct from that of color vision, and therefore we shouldn’t necessarily expect black to behave as an aesthetic opposite to warm colors or white to behave as an opposite to cool colors. The fact that they do suggests the psychological experience of darkness is somehow more similar to that of blue than it is to red, and that of brightness is closer to red than blue, or, emotionally, that darkness feels bluer and brightness feels redder. This probably happens due to correlations between the amount of excitement opposite qualities convey to the brain, red and brightness both conveying more and blue and darkness less.

The list below gives possible contrasting juxtapositions of commonly recognized colors, six of the rainbow plus black and white. They all tend to describe aesthetic phenomena in both biology and culture, implying that contrast indifference is a universal psychological trait in animals, or that we’re amused by contrast regardless of the colors that are used to make it up.

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 estimates about half of all bird species have contrasting plumage colors, including most woodpeckers, 80-95% of parrots, over 95% of hummingbirds, and all penguins. About 60% of fishes, all octopuses and cuttlefish (through controllable coloration), thousands of mammals, 70-80% of butterflies, and likely hundreds of thousands of beetles show contrasting coloration. Various photographic examples of the effect can be viewed on the Pinterest board Bright~Dark.

It’s common for such opposing colors to appear or increase in animals during the mating season, or to be expressed by one sex and not the other. 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 as an element of the allure of a part of the body that specifically 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. Such cases are evidence in favor of a universal color contrast bias, and against traditional explanations of color contrast in animals such as adaptive crypsis or thermoregulation.

The lists below, generated by Perplexity, give specific examples of species that use contrasting coloration reproductively, sorted according to the color pair used. None of the lists are comprehensive, and they don’t necessarily represent the relative numbers of taxa that express the color pairs, but they demonstrate the use of all the contrasting colors given above in the context of sexual selection, mostly in birds but also fish, insects, reptiles, crustaceans and mammals.

Black~White (23)

Birds: black-and-white warbler, bobolink, pied wagtail, rock ptarmigan male in courtship, common loon, common eider male, oystercatcher, pied avocet, magpie-lark, eurasian magpie, australian magpie, black-billed magpie, pied crow, pied kingfisher, gentoo penguin, chinstrap penguin, adelie penguin.

Reptiles: Myanmar river turtle (breeding male; black-striped silvery-white shell).

Mammals: blackbuck, sable antelope, northern white-cheeked gibbon, southern white-cheeked gibbon.

Insects: twelve-spotted skimmer.

Black~Red (12)

Birds: red-backed fairywren, red-winged blackbird, scarlet tanager, vermilion flycatcher.

Reptiles: south Indian rock agama, Roux’s forest lizard (breeding male; black body and red head/crest), Blanford’s rock agama (courtship male; dull-black body and red head/crest), marine iguana (Santa Cruz breeding male; red and black).

Insects: American rubyspot, calico pennant, large red damselfly.

Fish: southern redbelly dace.

Black~Orange (17)

Birds: Baltimore oriole, Bullock’s oriole, Blackburnian warbler, northern red bishop, black-winged red bishop, brambling, black-tailed godwit, common blackbird.

Reptiles: oriental garden lizard, Darwin’s large fan-throated lizard (male blue–black–orange courtship dewlap; tricolor), superb large fan-throated lizard (male blue–black–orange courtship dewlap; tricolor; experimentally studied signal).

Mammals: southern yellow-cheeked crested gibbon, crowned lemur.

Fish: kelp bass, guppy.

Insects: orange-tip, orange sulphur.

Black~Yellow (24)

Birds: American goldfinch, yellow-headed blackbird, Scott’s oriole, Audubon’s oriole, yellow-crowned bishop, yellow bishop, yellow-winged blackbird, bobolink, magnolia warbler, hooded warbler, Townsend’s warbler, Audubon’s warbler, western tanager, common blackbird (yellow bill).

Insects: eastern tiger swallowtail, calico pennant, widow skimmer, twelve-spotted skimmer, large red damselfly, common hawker.

Mammals: lion, southern yellow-cheeked crested gibbon, western black crested gibbon.

Fish: Burton’s mouthbrooder.

Blue~White (20)

Birds: eastern bluebird, mountain bluebird, indigo bunting, tree swallow, barn swallow, blue-and-white swallow, blue jay, white-throated magpie-jay, ultramarine flycatcher, white-winged fairywren, blue-footed booby, belted kingfisher.

Reptiles: Sagaing blue crested agama (breeding male; blue head/body and white lip-to-flank band; multicolor), eastern fence lizard (displaying male with white ventral ground and blue patches; black borders; other individuals may be cream), red-crowned roofed turtle (breeding male; blue–red–yellow–white head and neck; multicolor).

Mammals: nilgai, vervet monkey.

Fish: redfin shiner.

Insects: Adonis blue, common blue.

Blue~Red (22)

Birds: painted bunting, red-and-blue lory, eclectus parrot female, eastern rosella, crimson rosella, pesquet’s parrot, red-bellied macaw, American purple gallinule.

Reptiles: red-crowned roofed turtle (breeding male; blue–red–yellow–white head and neck; multicolor).

Mammals: mandrill, vervet monkey, drill.

Fish: three-spined stickleback, mandarinfish, red-and-blue betta (display morphs), Burton’s mouthbrooder, redfin shiner, red shiner, redside dace.

Crustaceans: blue crab female, red-clawed crab.

Insects: red-eyed damselfly.

Blue~Orange (28)

Birds: eastern bluebird, western bluebird, lazuli bunting, varied thrush, blue-winged pitta, fairy pitta, blue grosbeak, gouldian finch (black-headed morph), flame bowerbird male, superb starling.

Reptiles: red-headed rock agama male display, mwanza flat-headed rock agama, collared lizard male, Darwin’s large fan-throated lizard (male blue–black–orange courtship dewlap; tricolor), superb large fan-throated lizard (male blue–black–orange courtship dewlap; tricolor; experimentally studied signal).

Insects: agrias butterfly, blue-banded bee.

Mammals: golden snub-nosed monkey, blue-eyed black lemur.

Fish: peacock cichlid, mandarinfish, regal angelfish, blueface angelfish, flame angelfish, orange-shoulder tang, guppy, rainbow darter, orangethroat darter.

Blue~Yellow (26)

Birds: blue-and-yellow macaw, blue-throated macaw, blue tit, eurasian blue tit (female and male display), blue-and-yellow tanager, turquoise tanager, paradise tanager, blue-crowned motmot, european bee-eater, white-throated bee-eater, yellow-billed blue magpie, taiwan blue magpie, yellow-crowned amazon, blue-and-gold tanager.

Reptiles: red-crowned roofed turtle (breeding male; blue–red–yellow–white head and neck; multicolor).

Insects: blue-and-yellow damselflies, banded demoiselle, broad-bodied chaser, common hawker.

Fish: yellowtail damselfish, blue devil damsel male, palette surgeonfish, queen angelfish, regal demoiselle, redside dace, desert pupfish.

Green~White (19)

Birds: resplendent quetzal male, white-tailed trogon, white-throated magpie-jay, green magpie, green jay (northern subspecies), white-crowned parrot, white-bellied parrot, green-and-white hummingbird, andean emerald, white-necked jacobin male, snowy-bellied hummingbird, collared trogon male, white-tailed hillstar.

Insects: green tiger beetle, green orchid bee male, banded demoiselle.

Fish: green swordtail (green morph), green chromis, green humphead parrotfish (terminal male).

Green~Red (41)

Birds: red-and-green macaw, military macaw, great green macaw, red-fronted macaw, red-winged parrot male, red-fronted parakeet, red-crowned parakeet, red-lored amazon, red-crowned amazon, rose-ringed parakeet male, green woodpecker, red-headed barbet male, red-throated barbet male, fire-tufted barbet, coppersmith barbet, green-winged pytilia male, cuban tody, jamaican tody, narina trogon male, collared trogon male, elegant trogon male, mountain trogon male, Cuban trogon, fiery-throated hummingbird male, Anna’s hummingbird male, red-billed streamertail male, red-collared lorikeet, red-necked tanager, Papuan lorikeet, red-headed lovebird male, red-faced lovebird, fischer’s lovebird, red-bellied macaw, red-shouldered macaw, red-crested turaco, Guinea turaco, Livingstone’s turaco, Hartlaub’s turaco.

Reptiles: marine iguana (Española breeding male; bright green and red), common green forest lizard (breeding male; green body and red head/throat).

Fish: sockeye salmon.

Green~Orange (22)

Birds: orange-bellied leafbird male, golden-fronted leafbird male, orange-bellied parrot male, orange-cheeked parrot, orange-fronted parakeet, orange-winged amazon, orange-breasted bunting male, rufous-tailed jacamar male, green oropendola, orange-bellied trogon male, collared trogon (orange-bellied variant), emerald toucanet, orange-throated sunangel, olive-bellied sunbird male, variable sunbird male, orange-breasted trogon male, orange-breasted green pigeon male, whistling green pigeon, thick-billed green pigeon.

Reptiles: green iguana (breeding male).

Insects: orange-and-green jewel beetle.

Fish: green swordtail (orange-and-green morph).

Green~Yellow (33)

Birds: green honeycreeper male, yellow-throated euphonia male, thick-billed euphonia, green-headed tanager, bay-headed tanager, yellow-tufted honeyeater, yellow-throated honeyeater, yellow-fronted canary, eurasian siskin male, european greenfinch male, great tit, eurasian golden oriole female, golden weaver male, yellow-crowned amazon, yellow-headed amazon, yellow-naped amazon, yellow-shouldered amazon, budgerigar (wild type), yellow-collared macaw, superb parrot male, regent parrot male, red-rumped parrot male, green rosella, yellow-fronted parakeet, yellow-billed lorikeet, green woodpecker, levaillant’s woodpecker, grey-headed woodpecker male, yellow-bellied greenbul, green-backed camaroptera, green-and-yellow tanager, yellow-crested pygmy parrot.

Reptiles: green iguana (breeding male).

Purple~White (12)

Birds: violet-backed starling male, purple heron, purple sunbird male in breeding plumage, splendid sunbird male, violet-crowned hummingbird, violet-capped hummingbird male, costa’s hummingbird male, amethyst-throated sunangel male, violet-backed hyliota male.

Insects: purple emperor male upperside, lesser purple emperor male upperside, white admiral male upperside.

Purple~Red (12)

Birds: gouldian finch (red-headed morph), pompadour cotinga male, purple-throated fruitcrow male, purple-throated mountaingem male, Anna’s hummingbird male, purple grenadier male, violet-eared waxbill male, purple-naped lory, stella’s lorikeet (dark morph), red-and-blue lory, many-colored fruit dove male.

Insects: giant purple emperor male upperside.

Purple~Orange (9)

Birds: orange-breasted sunbird male, vinaceous-breasted amazon, amethyst-throated hummingbird male, purple grenadier male, purple honeycreeper female.

Insects: lesser purple emperor male upperside, purple hairstreak male upperside, small purple-and-gold moth, purple-and-gold pyrausta moth.

Purple~Yellow (16)

Birds: gouldian finch (purple-breasted morph), violaceous euphonia male, thick-billed euphonia, yellow-throated euphonia male, yellow-crowned euphonia male, purple sunbird male in breeding display, amethyst sunbird male, splendid sunbird male in display, yellow-winged cacique male, magnificent riflebird male, vinaceous-breasted amazon.

Insects: lesser purple emperor male butterfly (yellow-banded form), mint moth, moth butterfly.

Fish: royal gramma, yellow-headed jawfish.

It’s extremely difficult to explain so many instances of contrasting breeding colors without the existence of a general contrast bias in animals. Otherwise one has to imagine a bias for every particular contrasting color pair evolving separately, dozens of times even to explain the biases implied by the small sample of instances given above, and perhaps thousands of times to account for the actual number of independent cases of contrasting breeding colors. The most parsimonious solution would be to assign biases for all pairs of contrasting colors to the common ancestors of the species that express them, ultimately to the earliest animals.

Especially in species with extreme sexual dimorphism, we know that expressing contrasting colors is risky. One sex can’t afford to do so, usually females, and choosy partners take on the risk that their offspring, having inherited genes for colorfulness, will be captured by a predator and fail to reproduce. Genes for contrasting colors in bodies evolve and persist as though biases for contrast are inevitably built into sensory systems. This wouldn’t be an unprecedented scenario. Sensory biases for songs, dances and flowers seem to be just as inevitable, occurring in thousands of species across the most distantly related taxa, with no obvious adaptive value.

Contrasting breeding colors are distributed across taxa in ways that suggest universal biases are responsible. Close relatives often evolve to express different contrasting color pairs, and it’s common for a single species to express several at once, unlikely outcomes in the absence of a general bias.

Parrots in the genus Amazona show at least six distinct contrasting color pairs:

Amazon parrots: yellow-crowned amazon (blue~yellow, green~yellow), red-lored amazon (green~red), red-crowned amazon (green~red), orange-winged amazon (green~orange), yellow-headed amazon (green~yellow), yellow-naped amazon (green~yellow), yellow-shouldered amazon (green~yellow), vinaceous-breasted amazon (purple~orange, purple~yellow).

The same patterns appears in macaws, with several contrasting pairs represented and most species exhibiting more than one pair of colors:

Macaws: blue-and-yellow macaw (blue, yellow, green forehead, black throat, white face), blue-throated macaw (turquoise-blue, yellow, blue throat), military macaw (green, red forehead, blue flight feathers), great green macaw (pale green, red forehead, blue flight feathers), scarlet macaw (scarlet red, yellow, blue), red-and-green macaw (dark red, green, blue), red-fronted macaw (olive-green, red-orange forehead and shoulders, blue flight feathers), chestnut-fronted macaw (green, chestnut-brown forehead, red and blue in the wings), Cuban macaw (red, orange-yellow, blue), hyacinth macaw (cobalt blue, yellow eye-ring and chin patch), Lear’s macaw (dark blue, pale yellow patch at the base of the bill), glaucous macaw (grayish turquoise-blue, pale yellow facial skin), Spix’s macaw (blue, grayish-blue head, bare gray face), blue-winged macaw (green, red forehead and belly, blue in the wings), blue-headed macaw (green, blue head, blue flight feathers), golden-collared macaw (green, yellow neck collar, blackish-brown forehead), red-bellied macaw (green, maroon-red belly, yellow facial skin), red-shouldered macaw (green, blue forehead, red shoulders).

Atlantic puffins of both sexes transform dramatically in the spring from a plain, dark complexion to display an enlarged, yellow, orange and black bill that glows fluorescently with cyan-green light (see Dunning et al. 2018), and a slightly oblong, fleshy yellow ornament at the base of the beak. The mouth lining and legs turn bright orange, and a red ring forms around the eye with connected, triangular and spiky black lines. In the horned puffin the triangular patch above the eye is extended into a black horn, and the dark line below the eye runs all the way to the back of the head. Clearly, puffins are amused by the contrast of black with warm colors in general: black~red, black~orange and black~yellow are all effective aesthetically.

Female manakins 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, green and yellow and “super black” and bright blue. 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 over those of other, related species. Part of the reason for the diversity of contrasting colors in closely related taxa that inhabit or previously inhabited the same territory might be that they support conspecific recognition and prevent pointless matings, but this could be accomplished with any number of differentiating characteristics. The recurrence across species of contrasting patterns in itself needs to be explained regardless of a recognition function, and so does 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.

Sexually selected colors in the dewlap of anole lizards 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 these lizards than a scenario in which each species happened to evolve a specific preference for the pattern it displays without an initial interest in contrast.

In birds-of-paradise coloration is diversified across species into configurations exemplifying the same mixtures as parrots, puffins, manakins and anole lizards, with more colors per species: 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).

Peacock jumping spider color schemes are similar to manakins and birds-of-paradise, with opposite hues interacting even more elaborately. Females of the various species evidently select for males that display all the effects 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.

The bright blues seen in manakins and birds-of-paradise show up again in peacock spiders, often as a backdrop for dark reds. Bright blue is also a theme in blue (Chiroxiphia caudata), blue-backed (Chiroxiphia pareola) blue-capped (Lepidothrix coronata) and velvety (Lepidothrix coronata) manakins, as well as blue (Paradisornis rudolphi), Wilson’s (Cicinnurus respublica) and the magnificent (Diphyllodes magnificus) birds-of-paradise, supporting the idea cooler colors are mixed in sexually selected traits with brightness, a quality from a different perceptual dimension.

Perplexity research results appear to confirm that blues tend to be bright in sexually selected traits generally:

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.

Considering the contrast effect as a whole, every aesthetic instance throughout animals and culture, 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, which presumably provides advantages through the ability to tell light from dark and differentiate colors.

Many instances of contrasting coloration that biologists normally consider purely adaptive are probably better interpreted as either entirely aesthetic or multifunctional, and when the effect is adaptive it’s worth remembering that it might have originated sexually, that there seems to be relentless selective pressure pushing evolution in the direction of contrasting colors whether it’s ecologically beneficial or not.

Contrast bias appears to be present in the brain of every type of animal, no matter how distantly related. For all the differences between a peacock spider and a human, both species clearly appreciate contrast, so the mechanism that supports the bias isn’t dependent on any aspects of brain structure that vary between species. A spider has a brain with perhaps a couple of hundred thousand cells, and a volume of around 0.001-0.01 mm³, and it has the same color preferences as a human, with a brain of 86 billion cells and a volume of 1.2 million mm³. Contrast bias apparently persists despite any degree of evolutionary change, and despite being detrimental in that the ornamental traits they select for require energy to produce and tend to attract attention from predators, just as they attract attention from potential mates.

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, that aesthetic phenomena are made up of complex, contradictory mixtures of differentially exciting qualities. A mixture of red and blue is like a mixture of high and low pitch in a song or motion and stasis in a dance, with red being like high pitch and motion in contributing greater excitement to a perceptual pattern than blue, low pitch and stasis, which contribute an equally essential component of less excitement, the overall effect being moderation. As with other universal biases, the preference is likely embodied in the physical consistency of the brain, which is one of the few properties to have remained constant throughout evolutionary history.