Painted by Nature

General sensory biases selecting for mixtures of upwardness with darkness and downwardness with brightness in bodies are the most likely explanation for widespread dorsal darkness and ventral brightness in animals, rather than differential survival and countershading or obliteration.

“Animals are painted by nature, darkest on those parts which tend to be most lighted by the sky’s light, and vice versa.”

— Thayer (1896)

Universal biases favoring the perceptual mixtures dynamic — dark and bright — static appear to be responsible, via mate choice, for a widespread pattern in bird and other animal coloration in which dynamic body parts are darker than more static ones. Examples of numerous species with darker extremities, meaning such parts as wings, arms, legs, tails or long ornamental feathers or hair, or those which undergo the most dynamic motion from the perspective of another animal, can be seen on the Pinterest page Darker Extremities. Reversing the pattern should make an animal look more exciting, less pleasant and less interesting.

The perceptual mixtures dynamic — dark and bright — static are related to up — dark and bright — down, where the duality dynamic versus static has been replaced by up versus down. Dynamism is evidently in the same mental category with upwardness and stasis is in a category with downwardness, as described in More and Less Exciting Things and Categories of the Mind. As a consequence animals think of both upwardness and motion as interesting opposites to darkness and that both downwardness and stasis are more amusing mixed with brightness. Mate choice, coupled with the set of biases in question, has led to the nearly universal effect that animals are brighter on the bottom and centrally and darker further from the center, in more dynamic parts, and on top.

The idea that these preferences are universal comes from the fact that extremely similar effects are seen in human language and art, so similar that they can’t realistically be considered to have evolved in response to independent causes. Various otherwise nonsensical, idiomatic English phrases, in which the literal meaning is disconnected from our interpretation, can be understood as exploiting the same preferences.

Within the general pattern of dorsal to ventral, darker to brighter coloring to be observed among animals, ornaments used specifically for attracting mates with an up — dark/bright — down structure are extremely common. Mating colors with an upper blue and lower red portion across the head or face occur in Three-spined Stickleback (Gasterosteus aculeatus), Wild Turkeys (Meleagris gallopavo), Mandrill’s (Mandrillus sphinx) and Cassowaries (Casuarius). Widely unrelated species choose this configuration of color and direction for features smaller than the body as a whole, features which should be unrelated to being detected, especially given that by definition they did not evolve through differential survival. In humans the face is darker above and brighter below due to brown, blue or green and black color in our eyes and typically dark eyebrows and eyelashes coupled with redness in the lips, mouth and tongue below, a configuration presumably preferable to blue lips and red eyes. It seems relevant, as well, that women tend to accentuate the gradient by wearing makeup which darkens the area around the eyes and brightens the lips, more often than the other way around.

Up — dark and bright — down preferences appear to be responsible for the fact that animals tend to be darker on the dorsal surface and brighter on the ventral surface. Examples of animals exhibiting the effect can be viewed on the Thayer Effect Pinterest page. This pattern in the coloration of animals across species, known as countershading, Thayer’s Law or the Thayer Effect, was recognized and first published under the title “The Law Which Underlies Protective Coloration” in The Auk (1896) by Abbott Handerson Thayer and further described by Thayer and his son Gerald in a book named Concealing-Coloration in the Animal Kingdom (1909), in which they state:

“If an object be colored so that its tones constitute a gradation of shading and of coloring counter to the gradation of shading and of coloring which light thrown upon it would produce, and having the same rate of gradation, such object will appear perfectly flat; — retaining its length and breadth, but losing all appearance of thickness; and when seen against a background of color and pattern like its own will be essentially indistinguishable at a short distance. All persons who have seen the models which illustrate this, know that they prove it. Now, if this stands proved, the fact that a vast majority of creatures of the whole animal kingdom wear this gradation, developed to an exquisitely minute degree, and are famous for being hard to see in their homes, speaks for itself”.

The effect is attributed to differential survival, based on the idea that when animals are darker on the top and brighter on the underside they’re harder to see because of the way this arrangement interacts with light from above during the day. An animal’s upper body causes a shadow to be cast on its lower body and this self-shadow in turn gives it more contrast and makes it stand out more as a distinct solid object, which is related to the way darkness and shadows are used to make an object look solid and distinct in a painting. Brightness on the underside and darkness on top “obliterates” the shadow and hides the animal more effectively against its background.

Rowland (2009) calls countershading one of the most common visual characteristics of animals. She points out that little empirical evidence exists at present to support an obliterative, self-shadowing-based mechanism, despite the time that’s passed since the inception of the idea, but that more recent studies, including her own, have had success in showing that countershading can be effective in hiding from predators and that other possible reasons should be explored, such as thermoregulation. She gives examples of dorsal to ventral darker to lighter coloration in frogs, sharks, lizards, turtles, snakes, water bugs, penguins, tropical rainforest birds, shrimp, mice, rats, mole rats, squirrels, bats, lemurs, monkeys and other animals, and several potential mechanisms for the effect.

The concept of obliteration by countershading is a questionable explanation of the effect for various reasons. The direction sunlight comes from throughout the day is variable, and it scatters, so light is not shining straight down on animals very often. The shadow an animal casts depends on its shape. The top of a frog, a snake or turtle, for instance, usually doesn’t cast a shadow on its lower surface. The background against which an animal is seen is variable, and they don’t spend all their time oriented to be viewed from the side. Also many predatory animals participate in the Thayer effect in addition to those expected to benefit most from hiding. Most sharks, including for example the Great White Carcharodon carcharias, the Tiger shark Galeocerdo cuvier and the Whale Shark Rhincodon typus exhibit a dark to light coloration gradient from the dorsal to ventral surface. The Tiger Panthera tigris, the Snow Leopard Panthera uncia and the Jaguar Panthera onca exhibit the pattern. It could be that countershading helps predators sneak up on prey, but in the case of many animals, like the Whale Shark, Rhincodon typus, a filter feeder, it seems unlikely that plankton is keen enough to escape based on such a subtle visual cue on the side of a swimming shark, or that being two contrasting colors to hide from planktonic shrimp gives a shark a meaningful edge over one that’s a single color. One could argue that if countershading does conceal an animal when the sun is casting light across its body in such a way that it would be visually obliterated from the perspective of a predator, the rest of the time, when the sun is not shining straight down on it, the coloration pattern would make it more conspicuous.

That each instance of the effect, in every species where we see it arise and persist, requires an individual explanation from a traditional evolutionary, adaptationist perspective, and one which accounts for the many different circumstances under which an animal’s coloration interacts with light may be the most important difficulty for a theory of countershading as protection. Thayer made the argument that Flamingos are pink because it makes them hard to see in the sunset, for instance.

Another problem with explaining dorsoventral color gradation in animal bodies is the lack of its uniformity. It’s common for the darker dorsal area to be interrupted by other, light colors and the brighter ventral surface to be interrupted by patches of darker color. As Rowland (2009) points out, the upper surface of a whale shark is darker than the lower surface but it’s also covered with regularly spaced bright white spots and lines. The Mandarin Duck Aix galericulata has bright to dark ventral to dorsal coloration but the upper surface is also red, white, orange, green, blue and tan, which is hardly the look of an animal with a need to hide from predators, from any angle.

The phenomenon is so common that it’s reasonable to expect something universal about its cause. Thermoaesthetics provides a single, simple explanation for every case at once while countershading requires many independent, case by case explanations, one for each species in which the pattern occurs, along with a significant and ongoing relationship to one or more predators with good eyes.

Otherwise, one could assume animals like each other more with this color configuration than the opposite. We treat upwardness and darkness as opposites even though they don’t appear to be opposites in the outside world. Experiments showing universal psychological connections between upwardness and high-pitch, downwardness and low-pitch, and also high- and low-pitch with brightness and darkness, respectively (Spence 2011), suggest the existence of the connections up — bright and down — dark. People react faster to stimuli combining high-pitch with brightness, and thus they spend more time interpreting the mixed signal high-pitch — dark. To the extent that this corresponds to an animal being more intrigued by the mixture than the combination, or liking it more, and to the extent we relate upwardness to brightness in our minds, Thayer’s pattern can be explained as a result of a universal sensory bias for complexity similar to that of a liquid crystalline brain. The tops of animals are darker because we see the upper portion of each other as more exciting than the bottom by default. An up — dark/bright — down coloration scheme is one way to compensate for this aesthetically. This is related to the metaphors “up is exciting” and “down is less exciting,” as well as the hue heat hypothesis. People hold a hot and dark object longer than a hot bright one (Ziat et al. 2016), reacting to the heat more slowly when temperature and light are contradictory. Imagining upwardness replacing heat in this experiment, presumably people would spend more time interpreting, or favor, an object which is darker on top and brighter on the bottom.

Works Cited

Rowland, Hannah M. “From Abbott Thayer to the Present Day: What Have We Learned About the Function of Countershading?.” Philosophical Transactions of the Royal Society B: Biological Sciences 364.1516 (2009): 519–527.

Spence, Charles. “Crossmodal Correspondences: A tutorial review.Attention, Perception, & Psychophysics 73.4 (2011): 971–995.

Thayer, Abbott H. “The Law which Underlies Protective Coloration.” The Auk 13.2 (1896): 124–129.

Ziat, Mounia, et al. “A Century Later, the Hue-Heat Hypothesis: Does Color Truly Affect Temperature Perception?” Haptics: Perception, Devices, Control, and Applications Lecture Notes in Computer Science, 2016, pp. 273–280., doi:10.1007/978–3–319–42321–0_25.

Life in terms of the nonliving. Universal animal preferences for mixtures of simple, more and less exciting psychological opposites.

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