Updated: Mar 2
"Animals are painted by nature, darkest on those parts which tend to be most lighted by the sky's light, and vice versa."
Biases favoring the mixtures dynamic—dark and bright—static, which appear to be responsible for a widespread pattern in bird and other animal coloration in which dynamic body parts are darker than statics ones, are related to the biases up—dark and bright—down, where the duality dynamic versus static has been replaced by the duality up versus down, assuming dynamism is in the same thermological category with upwardness and stasis is thermological with downwardness as predicted. Up—dark and bright—down biases appear to be responsible for the fact that animals tend to be darker on the dorsal surface and brighter on the ventral surface. This pattern in the coloration of animals across species, known as countershading, 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 that "...the 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...". The pattern is currently 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 the animal more contrast and makes it stand out more as a distinct solid object, an effect 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 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.
A general sensory bias, in this case a pair of biases favoring the hypothetically aesthetic mixtures up—dark and its reverse bright—down over the thermological combinations up—bright and dark—down, might provide a more direct and parsimonious potential cause for the extremely widespread convergent evolution of coloration in animals toward upward bodily darkness and downward brightness. 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, 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, appear to 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 some animals, like the Whale Shark, 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 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 should make it more conspicuous. That each instance of the effect, in every species where we see it, requires an individual explanation from a traditional evolutionary 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 gradation in animal bodies is the lack of 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. Spots in numerous animals are an example. 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 the need to hide from predators at 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 thousands of 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 (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 can be interpreted as 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.
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.