Countershading
Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration is darker on the top or upper side and lighter on the underside of the body.
When light falls from above on a uniformly coloured three-dimensional object such as a sphere, it makes the upper side appear lighter and the underside darker, grading from one to the other.
The classical form of countershading, discovered in 1909 by the artist Abbott Handerson Thayer, works by counterbalancing the effects of self-shadowing, again typically with grading from dark to light.
In theory this could be useful for military camouflage, but in practice it has rarely been applied, despite the best efforts of Thayer and, later, in the Second World War, of the zoologist Hugh Cott.
The precise function of various patterns of animal coloration that have been called countershading has been debated by zoologists such as Hannah Rowland (2009), with the suggestion that there may be multiple functions including flattening and background matching when viewed from the side; background matching when viewed from above or below, implying separate colour schemes for the top and bottom surfaces; outline obliteration from above; and a variety of other largely untested non-camouflage theories.
... 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.Thayer observed and painted a number of examples, including the Luna moth caterpillar Actias luna, both in its habitual upside-down feeding position, where its countershading makes it appear flat, and artificially inverted from that position, where sunlight and its inverted countershading combine to make it appear heavily shaded and therefore solid.
[9] Thayer obtained a patent in 1902 to paint warships, both submarines and surface ships, using countershading,[10] but failed to convince the US Navy to adopt his ideas.
[18] Among predatory fish, the gray snapper, Lutianus griseus, is effectively flattened by its countershading, while it hunts an "almost invisible" prey, the hardhead silverside, Atherina laticeps which swims over greyish sands.
Fossilised skin pigmented with dark-coloured eumelanin reveals that ichthyosaurs, leatherback turtles and mosasaurs had dark backs and light bellies.
[21][22] The ornithischian dinosaur Psittacosaurus similarly appears to have been countershaded, implying that its predators detected their prey by deducing shape from shading.
Cott carefully combined disruptive contrast to break up the gun barrel's outlines with countershading to flatten out its appearance as a solid cylinder.
The authorities hesitated, appearing to be embarrassed by the evidence that Cott was right, and argued that countershading would be too difficult to use as an expert zoologist would be needed to supervise every installation.
[33] She considered the evidence for Thayer's theory that this acts as camouflage "by reducing ventral shadowing", and reviewed alternative explanations for countershading.
[34] Rowland notes that Cott is here reviewing Thayer's theory and "reinforcing the view that a gradation in shading would act to eliminate the effects of ventral shadowing.
[35] Thayer's original argument, restated by Cott,[34] was that nature did the exact opposite with countershading that an artist did with paint when creating the illusion of solid three-dimensionality, namely counteracting the effect of shade to flatten out form.
Research with chicks showed that they preferred to peck at grains with shadows falling below them (as if illuminated from above), so both humans and birds may make use of shading as a depth cue.
[42] Rowland concluded that each possible role for coloration patterns lumped together as "countershading" needs to be evaluated separately, rather than just assuming it functions effectively.
[5] Rowland (2009) identified an additional mechanism of countershading not previously analysed, namely that a round body such as a cylinder illuminated and seen from above appears to have dark sides.
If countershading paints out shadows, the reverse, darkening the belly and lightening the back, would maximise contrast by adding to the natural fall of light.
[46] These animals do not run when under attack, but move slowly, often turning to face the danger, and giving deimatic or threat displays either to startle inexperienced predators, or as an aposematic signal, to warn off experienced ones.
The caterpillar of the Luna moth, as discovered by Thayer, is in Cott's phrase "countershaded in relation to [its] attitude", i.e. shaded with a light back grading to a dark belly, as is the Nile catfish, Synodontis batensoda for the same reason: these animals (and other caterpillars including Automeris io and the eyed hawkmoth, Smerinthus ocellatus) habitually live 'upside down' with the belly uppermost.
a) in position b) inverted.
