Camouflage in Animals (Visual Illusion in Nature)

Hiding in plain sight

A leaf insect sits still on a branch, its wings veined and edged like real leaves. A flatfish settles on the seabed and shifts its pattern to match the sand. A fawn lies in dappled woodland light, its pale spots breaking its outline into pieces that read as leaf litter rather than an animal.

These aren't illusions in the way the Necker cube is: nothing about them is ambiguous or impossible. But they exploit the same features of vision that drawn illusions do. Visual systems are built to find edges, outlines and solid shapes, and camouflage, shaped by evolution rather than by an artist, works by defeating them. The game above lets you feel it: find the moths on the bark, and compare how long the two kinds of pattern take.

Three ways to disappear

Camouflage isn't one trick but several, each aimed at a different step in how vision picks objects out of a scene.

Background matching is the simplest: the animal's colours and textures resemble its usual surroundings, so there's little contrast at its edges. Finding the boundary where one texture or brightness gives way to another is one of the first things any visual system does, in predators and prey alike, and background matching starves it of signal.

Disruptive colouration works differently. Bold markings placed across the body, especially where they touch or cross the outline, break the shape into pieces that don't look as though they belong together. That defeats the visual system's habit of grouping contours into a single object, the same grouping studied in Gestalt psychology. Innes Cuthill and colleagues tested it by pinning triangular paper "moths", baited with mealworms, to oak trees for wild birds. Moths whose markings touched their edges survived better than moths with similar markings kept inside the outline (Cuthill et al., 2005). The game uses the same two designs. A later study with human searchers pointed out a complication: the inside markings were also more densely packed, which made those moths match the bark less well, so background matching may have contributed to the result (Fraser et al., 2007).

Countershading deals with light and shadow. Sunlight from above makes an animal's back brighter than its belly, and that gradient reveals its rounded shape. Many animals, from deer to sharks to penguins, are darker on top and paler underneath, which cancels out the lighting and makes them look flatter and less like a solid object. The American painter and naturalist Abbott Handerson Thayer described this in 1896 (Thayer, 1896), and it's now a standard part of evolutionary biology, even though some of his other claims, such as that flamingos are pink to match the sunset, were widely mocked.

Whose illusion is it?

It's the viewer who is fooled, not the camouflaged animal: a bird scanning for insects, or prey watching for a predator. Camouflage sits at the edge of what counts as an illusion, since much of it reduces the information available rather than creating a false impression. But the mechanisms it targets, edge detection, contour grouping and judging shape from shading, are the same ones that drawn illusions exploit, as reviews of the field emphasise (Stevens & Merilaita, 2009).

An evolutionary arms race

Predators and prey keep pushing each other. Predators get better at searching: a bird that has recently found several insects of one pattern becomes temporarily better at spotting that pattern, a "search image". Prey shift towards whatever patterns currently escape detection. Where hunting by sight is intense, camouflage becomes more refined over generations. Some animals take the opposite approach and make themselves conspicuous on purpose; see eyespots and mimicry.