Countershading Countershading , or Thayer ' s Law , is a form of camouflage . Countershading is the pattern of animal coloration in which an animal ’ s pigmentation is darker on the upper side and lighter on the underside of the body . This pattern is found in many species of mammals , reptiles , birds , and fish . When light falls on a uniformly coloured object such as a sphere from above , 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 acts as a visual cue which makes the object easier to detect . Countershading reduces the ease of detection of predators and prey 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 zoologists such as Hugh Cott . 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 , like the Nile catfish and the luna moth caterpillar , have upside - down countershading for camouflage . Early research . 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 submarine s and surface ship s , 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 calls this " Thayer straining the theory to a fantastic extreme " . Both Thayer and Cott include 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 explains that Application . In animals . 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 . Among predatory fish , the gray snapper , " Lutianus griseus " , is effectively flattened by its countershading , while it hunts an " almost invisible " prey , the hardhead fish , " Atherina laticeps " which swims over greyish sands . Other countershaded marine animals include blue shark , herring , and dolphin ; while fish like the mackerel and sergeant fish are both countershaded and patterned with stripes or spots . A related mechanism : 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 was a protege of John Graham Kerr who had quarrelled with Norman Wilkinson in the First World War about dazzle camouflage for ships . Wilkinson remained influential in 1939 as an inspector of camouflage , so a political argument developed . Cott was invited to camouflage a 12 - inch rail - mounted gun , alongside a similar gun camouflaged conventionally . 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 guns were then photographed from the air from various angles , and in Peter Forbes ' s view " the results were remarkable . " 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 . 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 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 tank s , 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 . " Function . Hannah Rowland , reviewing countershading 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 . Camouflage theories of countershading , Rowland wrote , include " self - shadow concealment which results in improved background matching when viewed from the side " ; " self - shadow concealment that flattens the form when viewed from the side " ; " background matching when viewed from above or below " ; and " body outline obliteration when viewed from above " . These are examined in turn below . 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 grey 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 . Early researchers including Alfred Russel Wallace , Frank Evers Beddard , Cott and Craik argued that 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 countershaded , and the prey of these birds would see only the underside of the bird . Rowland concluded that each possible role for 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 like 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 . Finally in 2012 , however , 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 . Reverse countershading . 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 like the skunk with strong defences . 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 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 also 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 .