How Does Camouflage Work in Animals?

The simplest form is background matching, where colour and texture resemble the surroundings. A stick insect on a twig or a flatfish on sand is doing this.

But an animal that matches perfectly can still be given away by its outline and its shadow. That is why several other strategies exist, and most well-camouflaged animals combine more than one at the same time.

Bold patterns can be camouflage rather than display. High contrast stripes and blotches break up the recognisable edge of a body, so a predator's eye struggles to assemble it into a shape.

This is why a leopard's spots work in dappled shade and why zebra stripes are so striking. Military camouflage borrowed the idea directly, and ships in both world wars were painted in bold geometric patterns intended to confuse estimates of course and speed rather than to hide them.

Look at almost any fish, deer or bird and you will notice the back is darker than the belly. This is countershading, and it cancels the shading produced by light coming from above.

Without it, an animal lit from above looks obviously three dimensional and solid. With it, the darker top and lighter underside even out and the body reads as flat. The effect is subtle and extremely widespread, which is a good sign of how well it works.

Some animals do not hide so much as impersonate. This is masquerade, and the disguise is usually something a predator would not bother eating:

• •Leaf insects resembling leaves down to the veins and damage
• •Caterpillars shaped and coloured like bird droppings
• •Decorator crabs attaching seaweed and sponges to their shells
• •Fish shaped like drifting dead leaves

Cephalopods are the extreme case. Octopus, squid and cuttlefish have skin containing chromatophores, pigment sacs controlled by muscles, layered over reflective cells. They can change colour and pattern in under a second and also alter skin texture to mimic rough surfaces.

The strange part is that cephalopods appear to be colourblind by conventional measures, so how they match colour so well is still debated, with one hypothesis being that their oddly shaped pupils allow colour to be judged through focus. Chameleons change colour by a different mechanism involving nanocrystals in the skin, and mostly for signalling rather than hiding.

The opposite of camouflage is aposematism, where an animal advertises itself with bright warning colours because it is genuinely dangerous or foul tasting. Poison dart frogs and wasps both do this.

This creates an opening for mimicry. Harmless hoverflies have evolved wasp-like stripes, gaining protection without the cost of producing venom. Where several genuinely dangerous species converge on the same warning pattern, as many stinging insects have, predators only need to learn one lesson, which benefits all of them.

It is easy to look at a patterned animal and assume the pattern hides it, but scientists try to test such claims. One common method uses artificial prey. Researchers pin paper or card shapes with different patterns onto tree trunks, sometimes with a small piece of food attached, and then record which ones birds find and eat over several days. If disruptive patterns survive longer than plain ones of the same average colour, that supports the idea that breaking up outlines helps prey escape detection.

Computer experiments add another angle. People search for hidden targets on screens, and the time they take to spot each pattern can be measured precisely. The peppered moth provides a famous natural example. In parts of industrial Britain during the nineteenth century, soot darkened tree bark, and a dark form of the moth became much more common, as pale moths were easier for birds to see. After clean air laws reduced pollution in the twentieth century, the pale form became common again, matching the change in background.

The open ocean has nothing to hide against: no rocks, no weeds, just water in every direction. Many animals there use transparency instead. Jellyfish, comb jellies, some shrimp and many larval fish are almost see-through, because their tissues let light pass with little absorption or scattering. Achieving this is hard, since organs such as eyes and guts need pigment or contain food, so transparent animals often keep these parts small or shaped to cast as little shadow as possible. Glass frogs in rainforests use partial transparency on land too.

In the dim twilight zone below the surface, the main danger comes from predators looking upwards, which see prey as dark silhouettes against faint light from above. Hatchetfish, many squid and some shrimp answer this with counter-illumination. They carry light-producing organs along their undersides and adjust the glow to match the brightness and colour of the light coming down, erasing their shadow. The Hawaiian bobtail squid does this with the help of luminous bacteria living in a special organ, supplying them with nutrients in return for light.

Whether an animal is well hidden depends on who is looking. A tiger's orange coat seems an odd choice for a hunter that relies on stealth, but its main prey, such as deer, have only two types of colour-sensitive cone in their eyes, like most mammals. They cannot easily tell orange from green, so to a deer the tiger blends into vegetation far better than it does to us. Birds raise the opposite problem. Many can see ultraviolet light, so an insect that looks perfectly matched to a human may stand out to a bird that picks up patterns we cannot see.

This is why researchers increasingly photograph animals with cameras that record ultraviolet as well as visible light, then use computer models of a particular predator's vision to estimate what that predator would actually see. Movement adds another test. Even a perfect match can fail once an animal moves, because detecting motion is one of the strongest abilities of almost any visual system. Many camouflaged animals therefore freeze when danger is near. Some stick insects and mantises go further, swaying gently back and forth as they walk so that they resemble a leaf or twig moving in the breeze rather than a creature crossing a branch.

Zebra stripes are often explained as camouflage, either by breaking up the outline of each animal or by dazzling a lion trying to single one out from a running herd. The evidence for this is weak. Lions and hyenas hunt mostly at dusk and at night, and studies modelling their eyesight suggest that beyond fairly short distances they cannot resolve the stripes at all, so a zebra probably looks like a plain grey horse. Zebras also stand out clearly in open grassland and are noisy and conspicuous in herds, which hardly fits a strategy of hiding.

The explanation with most support today involves biting flies. Researchers comparing zebras and their relatives found that stripes tend to be strongest in regions where horseflies and similar insects are most active. In field experiments, horseflies approached horses wearing striped coats about as often as plain ones but landed on them far less, apparently failing to slow down properly as they came in. Zebras have thin coats, and these flies can carry serious diseases, so avoiding them could matter a great deal. Other ideas, such as cooling or social recognition, have not been ruled out entirely, so the question is not completely closed.

Camouflage works through background matching, disruptive patterns, countershading and outright masquerade, usually in combination, and the best camouflaged animals are solving the problems of outline and shadow as much as colour.