Countershading

, are countershaded. 's 1909 book on camouflage of a Luna caterpillar Actias luna a) in position b) inverted. 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.…

, are countershaded.]] 's 1909 book on camouflage of a Luna caterpillar Actias luna a) in position b) inverted.]] 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. File:Countershaded Rail-mounted Gun Camouflaged by Hugh Cott 1940.jpg|Rail-mounted guns countershaded by Hugh Cott (top) and conventionally camouflaged (middle), August 1940. The British authorities agreed Cott's countershading worked, but refused to adopt it. File:7.2_inch_howitzer_of_51st_Heavy_Regiment.jpg|BL 7.2-inch howitzer with countershaded barrel, September 1944 File:Sherman Firefly 9-08-2008 15-05-43.JPG|A preserved Sherman Firefly; its gun barrel is countershaded and disruptively patterned to disguise its length. File:Focke-Wulf Fw 190D-9 outside USAF.jpg|True (graduated from dark to light) countershaded Focke-Wulf Fw 190

Flattening and background matching when viewed from the side

Cott, like Thayer, argued that countershading would make animals hard to see from the side, as they would "fade into a ghostly elusiveness". 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." Kiltie measured the effect of the countershading of the eastern gray squirrel, Sciurus carolinensis, showing that when the squirrel is horizontal the self-shadowing of the belly is partly concealed, but that when the squirrel is vertical (as when climbing a tree trunk) this effect did not occur. Thayer's original argument, restated by Cott, 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. Shading is a powerful cue used by animals in different phyla to identify the shapes of objects. 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.

Background matching from above or below

, like many pelagic fish, is dark above, pale below, camouflaging it against the ocean depths and the bright surface. ]] A completely different function of animal (and military vehicle) coloration is to camouflage the top and bottom surfaces differently, to match their backgrounds below and above respectively. This was noted, for example, by Frank Evers Beddard in 1892: ]] Early researchers including Alfred Russel Wallace, Beddard, Cott and Craik argued that in marine animals including pelagic fish such as marlin and mackerel, as well as dolphins, sharks, and penguins the upper and lower surfaces are sharply distinct in tone, with a dark upper surface and often a nearly white lower surface. They suggested that when seen from the top, the darker dorsal surface of the animal would offer camouflage against the darkness of the deep water below. When seen from below, the lighter ventral area would similarly provide the least possible contrast with the sunlit ocean surface above. There is some evidence for this in birds, where birds that catch fish at a medium depth, rather than at the surface or on the seabed, are more often coloured in this way, and the prey of these birds would see only the underside of the bird. 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.

Outline obliteration from above

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. Using a graphics tool, she demonstrated that this effect can be flattened out by countershading. Since predators are known to use edges to identify prey, countershading may therefore, she argues, make prey harder to detect when seen from above.

Non-camouflage theories

Non-camouflage theories include protection from ultraviolet light; thermoregulation; and protection from abrasion. All three of these "plausible" theories remained largely untested in 2009, according to Rowland.

Examples in animals

File:Tragelaphus scriptus (male).jpg|Bushbuck, Tragelaphus scriptus, appears almost perfectly even in tone, showing that its countershading has cancelled out its self-shading. The white spots and markings help to disrupt the 'solidity' of the animal further. File:Sylvia borin (Örebro County).jpg|Many birds, such as this garden warbler, Sylvia borin, are countershaded. The lighter belly makes the bird appear almost evenly coloured when seen from the side. File:Carolina_anole.jpg|The Carolina anole lizard, Anolis carolinensis, is smoothly countershaded. File:Adelie Penguin2.jpg|Adelie penguins, Pygoscelis adeliae, are white below and dark above, presumably to enable them to blend with the sea surface when seen from below, and with deep water when seen from above. File:Smerinthus ocellatus caterpillar on apple tree.jpg|The caterpillar larva of the eyed hawkmoth, Smerinthus ocellatus, is reverse countershaded, making it appear flat when upside-down in feeding position. File:Eyed_hawkmoth_larvae_reverse_countershaded.jpg|When the eyed hawkmoth caterpillar is turned upright, as here, its countershading adds to the shading caused by sunlight, rather than "painting it out", so its body appears strongly rounded in this position. File:Striped Skunk (Mephitis mephitis) DSC 0030.jpg|Striped skunk, Mephitis mephitis, has conspicuous warning coloration with reversed countershading, alerting predators to its powerfully defensive stink. File:Axis axis (Nagarhole, 2010).jpg|Chital deer, Axis axis. The animals in the background are effectively countershaded with their bodies horizontal, but the upright stag in the foreground is made conspicuous by its light belly. The spotting is disruptive.

Notes

References

Bibliography

Pioneering books

General reading

Journals

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