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 several hundred animals 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…
