page_title: Countershading
section_title: Evidence
hierarchical_section_title: Countershading / Evidence
caption_attribution_description: English: This Glaucus atlanticus was washed up on Surfers Paradise Beach in Queensland Australia. It is about 35 mm in length. The species swims upside-down and is accordingly countershaded for that position. فارسی: لیسه دریایی آبی‌رنگ گونه‌ای لیسه دریایی است که در تیره خاکستری‌لیسه‌سانان قرار دارد.
context_page_description: Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration 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, fish, and insects, both predators and prey, and has occurred since at least the Cretaceous period.
  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.
context_section_description: 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.
caption: The sea slug Glaucus atlanticus swims and is countershaded upside-down.
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