Painted by Nature

General biases in animal sensory systems are the most likely explanation for Thayer’s Law.

© 2026 Andrew Hodgson

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

— Abbott Thayer (1896)

Among all the wild absurdities to which Mr. Thayer has committed himself, probably the wildest is his theory that flamingos are concealingly colored because their foes mistake them for sunsets. He has never studied flamingos in their haunts, he knows nothing personally of their habits or their enemies or their ways of avoiding their enemies … and certainly has never read anything to justify his suppositions; these suppositions represent nothing but pure guesswork, and even to call them guesswork is a little over-conservative, for they come nearer to the obscure mental processes which are responsible for dreams.

— Theodore Roosevelt (Gould 2010)

Universal biases favoring the perceptual mixtures up~dark and bright~down appear to be responsible, via mate choice, for a widespread animal coloration pattern in which the upper parts of bodies are darker and the lower parts are brighter. Hundreds of examples of this can be viewed on the Thayer Effect Pinterest page. The pattern, known as Thayer's Law, or countershading, has been recognized and studied for over a century. It was described in detail under the title “The Law Which Underlies Protective Coloration” by Abbott Handerson Thayer in The Auk (1896), and further by Thayer and his son in a book called 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 the color arrangement contributes to concealment due to how it interacts with light from above during the day. It's thought that an animal's upper body causes a shadow to be cast on its lower body, and this self-shadowing 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 presumably “obliterate” 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 its ubiquity and the amount of research that's been dedicated to demonstrating it. Some studies, including Rowland's, have had success in showing that countershading can sometimes be effective for hiding from predators, but she says 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, along with several potential adaptive mechanisms.

Within the general effect, ornaments used specifically for attracting mates often have the same structure. Mating colors with an upper blue and lower red portion, for example, occur in three-spined stickleback Gasterosteus aculeatus, the peacock spiders Maratus azureus, M. fletcheri, M. neptunus, M. amabilis and M. linnaei, Wilson's birds-of-paradise Cicinnurus respublica, wild turkeys Meleagris gallopavo, cassowaries (Casuarius), fan-throated lizards Sitana ponticeriana and mandrill's Mandrillus sphinx. Thus, widely unrelated species choose this configuration of color and direction through mate choice. The resulting patterns in these cases, by definition, are either detrimental or have nothing to do with survival or avoiding detection.

The concept of obliteration by countershading is a questionable explanation for various reasons aside from it being inapplicable to sexually selected traits. The direction sunlight comes from throughout the day is variable, and it scatters, so light isn’t shining straight down on animals very often. The shadow an animal casts depends on its shape. The top of a frog, snake, turtle or butterfly wing, for instance, doesn't exactly 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 directly from the side.

Thayer made the argument that flamingos are pink because it makes them hard to see in the sunset, as illustrated in the opening painting. Strangely, in this case, obliteration doesn't apply because, from a predator's viewpoint, the sun is behind a flamingo rather than above it. Obliteration assumes, of course, that predators aren't able to adapt to the effect by evolving to detect prey as effectively as they would in its absence. It's easy to imagine adaptations that would solve the problem of potential food being slightly harder to see at noon from a particular angle in front of a specific background, potentially including hunting at different times of day, at night, from different angles, or increased sensitivity to motion, shape, vibrations, scent, infrared radiation or the Thayer effect itself.

Countless predatory animals participate in the Thayer effect in addition to those expected to benefit most from crypsis. Most sharks, apparently, including 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. So do the tiger Panthera tigris, snow leopard Panthera uncia and jaguar Panthera onca. It could be, of course, that countershading helps predators sneak up on prey. In the case of many animals, however, like the whale shark, a filter feeder, it seems unlikely that plankton is keen enough to escape based on the lack of a vertical coloration difference in the look of a giant, fast-swimming shark, or that being two contrasting colors to hide from prey gives any predator a meaningful edge over one that's a either a single color or brighter on top and darker below. 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, or prey, the rest of the time, when the sun isn't shining straight down on it, the coloration pattern would make it more conspicuous.

Another problem with explaining dorsoventral color gradients in animal bodies is 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. 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, hardly the look of an animal with a strong evolutionary need to hide from predators, at any angle, and this goes for thousands of species.

Perhaps most importantly, the traditional adaptationist perspective 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 hunting with prey coloration patterns as a significant determinant of their success. Alternatively, a universal bias in the animal brain provides a single explanation for every case of the Thayer effect at once. Animals simply choose mates that exhibit the effect more frequently than those who don’t, or those who have the opposite look. In this case Thayer was right about his law, and the need for a comprehensive interpretation, to look for a single underlying cause, but he mistakenly dismissed sexual selection and aesthetic preferences, even though they represent a far more sensible solution to the problem of how effective it is to predict that an animal, of almost any kind, will be brighter on the bottom and darker on the top.