Countershading

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 == Application ==

In animals es are effectively flattened by countershading, making them nearly invisible against a desert background. There are three in the image. 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. Mesozoic marine reptiles had countershading. Fossilised skin pigmented with dark-coloured eumelanin reveals that ichthyosaurs, leatherback turtles and mosasaurs had dark backs and light bellies. 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. Counter-illumination 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. Military Countershading, like counter-illumination, has rarely been applied in practice for military camouflage, though not because military authorities were unaware of it. Both Abbott Thayer in the First World War and Hugh Cott in the Second World War proposed countershading to their countries' armed forces. They each demonstrated the effectiveness of countershading, without succeeding in persuading their armed forces to adopt the technique, though they influenced military adoption of camouflage in general. Cott's gun is "invisible except to the most minute scrutiny by someone who knows exactly where to look and what to look for. The other gun is always highly visible." 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. Cott was posted to the Middle East, and Kerr unsuccessfully intervened, pleading for guns to be painted Cott's way and Cott to be brought home. The Australian zoologist William Dakin in his 1941 book The Art of Camouflage followed Thayer in describing countershading in some detail, and the book was reprinted as a military handbook in 1942. Dakin photographed model birds, much as Thayer and Cott had done, and argued that the shoulders and arms of battledress should be countershaded. Countershading was described in the US War Department's 1943 Principles of Camouflage, where after four paragraphs of theory and one on its use in nature, the advice given is that: Inventors have continued to advocate military usage of countershading, with for example a 2005 US patent for personal camouflage including countershading in the form of "statistical countercoloring" with varying sizes of rounded dark patches on a lighter ground. Research by Ariel Tankus and Yehezkel Yeshurun investigating "camouflage breaking", the automated detection of objects such as tanks, showed that analysing images for convexity by looking for graded shadows can "break very strong camouflage, which might delude even human viewers." More precisely, images are searched for places where the gradient of brightness crosses zero, such as the line where a shadow stops becoming darker and starts to become lighter again. The technique defeated camouflage using disruption of edges, but the authors observed that animals with Thayer countershading are using "counter-measures to convexity based detectors", which implied "predators who use convexity based detectors." 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 == Function ==

by 'painting out' the self-shadowing of the body or object. The result is a 'flat' appearance, instead of the 'solid' appearance (with visual convexity) of the body before countershading. Hannah Rowland, reviewing the subject 100 years after Abbott Thayer, observed that countershading, which she defines as "darker pigmentation on those surfaces exposed to the most lighting" is a common but poorly understood aspect of animal coloration. She noted there had been "much debate" about how countershading works. She considered the evidence for Thayer's theory that this acts as camouflage "by reducing ventral shadowing", and reviewed alternative explanations for countershading. 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." Thayer's original argument, restated by Cott, 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. 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. == Evidence ==

Despite demonstrations and examples adduced by Cott and others, little experimental evidence for the effectiveness of countershading was gathered in the century since Thayer's discovery. Experiments in 2009 using artificial prey showed that countershaded objects do have survival benefits and in 2012, a study by William Allen and colleagues showed that countershading in 114 species of ruminants closely matched predictions for "self-shadow concealment", the function predicted by Poulton, Thayer and Cott. == Mechanism ==

Evolutionary developmental biology has assembled evidence from embryology and genetics to show how evolution has acted at all scales from the whole organism down to individual genes, proteins and genetic switches. In the case of countershaded mammals with dark (often brownish) upper parts and lighter (often buff or whitish) under parts, such as in the house mouse, it is the Agouti gene which creates the difference in shading. Agouti encodes for a protein, the Agouti signalling peptide (ASP), which specifically inhibits the action of the Melanocortin 1 receptor (MC1R). In the absence of the Agouti protein, alpha-melanocyte-stimulating hormone stimulates the cells bearing MC1R, melanocytes, to produce dark eumelanin, colouring the skin and fur dark brown or black. In the presence of the Agouti protein, the same system produces the lighter-coloured, yellow or red phaeomelanin. A genetic switch active in the cells of the embryo that will become the belly skin causes the Agouti gene to become active there, creating the countershading seen in adult mammals. == Reverse countershading ==

is reverse countershaded, a form of aposematism (warning coloration). '' swims and is countershaded upside-down. 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. This pattern of animal coloration is found in animals such as the skunk and honey badger with strong defences—the offensive stink of the skunk, and the sharp claws, aggressive nature and stink of the honey badger. 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. Similarly in the sea slug Glaucus atlanticus, the reverse countershading is associated with inverted habits. These animals are thus employing countershading in the usual way for camouflage. ==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. ==See also==