Countershading, or Thayer’s Law, is a form of camouflage. Countershading, in which an animal’s pigmentation is darker dorsally, is often thought to have an adaptive effect of reducing conspicuous shadows cast on the ventral region of an animal’s body. In essence the distribution of light on objects that are lit from above will cause unequal reflection of light on a solid body of uniform colour, such shadows could provide predators with visual cues to a prey's shape and projection. Countershading therefore, reduces the ease of detection of prey by potential predators by counterbalancing the effects of shadowing.
Countershading is observed in a variety of animals: pronghorn antelope, White-tailed deer, squirrels, birds, and various lepidopteran larvae. Marlins also are an example of countershading.
Alternatively, in many marine animals (including various species of fish, particularly sharks, penguins and cephalopods) this form of camouflage may work through background matching; when seen from the top, the darker dorsal area of the animal blends into the darkness of the water below, when seen from below, the lighter ventral area blends into the sunlight from the surface.
Furthermore, countershading could also result from differential selection pressures on dorsal and ventral surfaces, from the need to protect against the damaging properties of UV light, or abrasion.
Abbott Handerson Thayer was one of the first to conduct extensive research on and to write about certain aspects of protective colouration in nature. In 1892, he wrote about the function of countershading in nature, in which he accounted for the white undersides of animals. For this reason countershading is sometimes called Thayer’s Law.
Military camouflage sometimes uses the same principle; Thayer even obtained a patent in 1902 to paint warships using a countershaded scheme.
- Synodontis nigriventris, an "upside-down" catfish (with reverse countershading)
- Edmunds, M. & R.A. Dewhirst (1994). The survival value of countershading with wild birds as predators. Biological Journal of the Linnean Society 51(4): 447-452.
- Ruxton, G.D., M.P. Speed & D.J. Kelly (2004). What, if anything, is the adaptive function of countershading? Animal Behaviour 68(3): 445-451.
- Speed, M.P., D.J. Kelly, A.M. Davidson, G.D. Ruxton(2005). Countershading enhances crypsis with some bird species but not others. Behavioral Ecology 16(2): 327-334.