Countershading

Camouflage to counteract self shading

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. This pattern is found in many species of mammals, reptiles, birds, fish, and insects, both in predators and in prey. 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. This pattern of light and shade makes the object appear solid, and therefore easier to detect. 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. A related mechanism, counter illumination, adds the creation of light by bioluminescence or lamps to match the actual brightness of a background. Counter illumination camouflage is common in marine organisms such as squid. It has been studied up to the prototype stage for military use in ships and aircraft, but it too has rarely or never been used in warfare. The reverse of countershading, with the belly pigmented darker than the back, enhances contrast and so makes animals more conspicuous. It is found in animals that can defend themselves, such as skunks. The pattern is used both in startle or deimatic displays and as a signal to warn off experienced predators. However, animals that habitually live upside down but lack strong defences, such as the Nile catfish and the Luna moth caterpillar, have upside down countershading for camouflage.

The English zoologist Edward Bagnall Poulton, author of The Colours of Animals (1890) discovered the countershading of various insects, including the pupa or chrysalis of the purple emperor butterfly, Apatura iris, the caterpillar larvae of the brimstone moth, Opisthograptis luteolata and of the peppered moth, Biston betularia. However he did not use the term countershading, nor did he suggest that the effect occurred widely.]] The New Hampshire artist Abbott Handerson Thayer was one of the first to study and write about countershading. In his 1909 book Concealing Coloration in the Animal Kingdom, he correctly described and illustrated countershading with photographs and paintings, but wrongly claimed that almost all animals are countershaded. For this reason countershading is sometimes called Thayer's law. Thayer wrote:

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. Thayer obtained a patent in 1902 to paint warships, both submarines and surface ships, using countershading, but failed to convince the US Navy to adopt his ideas. Hugh Bamford Cott in his 1940 book Adaptive Coloration in Animals described many instances of countershading, following Thayer in general approach but criticising Thayer's excessive claim ("He says 'All patterns and colors whatsoever of all animals that ever prey or are preyed upon are under certain normal circumstances obliterative.'") that effectively all animals are camouflaged with countershading. Cott called this "Thayer straining the theory to a fantastic extreme". Both Thayer and Cott included in their books photographs of a non countershaded white cockerel against a white background, to make the point that in Thayer's words "a monochrome object can not be 'obliterated', no matter what its background" or in Cott's words "Colour resemblance alone is not sufficient to afford concealment". Cott explained that

Countershading is observed in a wide range of animal groups, both terrestrial, such as deer, and marine, such as sharks. It is the basis of camouflage in both predators and prey. It is used alongside other forms of camouflage including colour matching and disruptive coloration. Other countershaded marine animals include blue shark, herring, and dolphin; while fish such as the mackerel and sergeant fish are both countershaded and patterned with stripes or spots. quote| It tones the canvas on which are painted the Leopard's spots, the Tiger's stripes It is the dress almost universally worn by rodents It is the essential uniform adopted by Conies, Asses, Antelopes, Deer It is repeated extensively among the marsupials It provides a basic livery for the great majority of snakes, lizards, and amphibians. Among insects it reaches a fine state of perfection in different caterpillars and grasshoppers. It is, however, in rivers, and in the surface waters of the sea, that countershading reaches its maximum development and significance.|Hugh Cott The ornithischian dinosaur Psittacosaurus similarly appears to have been countershaded, implying that its predators detected their prey by deducing shape from shading. Modelling suggests further that the dinosaur was optimally countershaded for a closed habitat such as a forest.

Another form of animal camouflage uses bioluminescence to increase the average brightness of an animal to match the brightness of the background. This is called counter illumination. It is common in mid water pelagic fish and invertebrates especially squid. It makes the counter illuminated animal practically invisible to predators viewing it from below. As such, counter illumination camouflage can be seen as an extension beyond what countershading can achieve. Where countershading only paints out shadows, counter illumination can add in actual lights, permitting effective camouflage in changing conditions, including where the background is bright enough to make an animal that is not counter illuminated appear as a shadow.