Offense and defense

The complex of biochemical, physiological, energetic and mechanical systems that make up an organism are extremely vulnerable to threats from the environment, and the greatest danger comes from other organisms. Since the beginning of evolution, each lifeform has been a vital resource for other ones, and it has has te need to defend itself, and to fight back.

Camouflage
Also see: List of camouflage methods

The most secure way to survive predation is avoiding to be recognised as a prey. Camouflage involves an organism modifying its own appearance for this purpose (it's called mimicry when it occurs in such a way to make it similar to another organism).

Crypsis
In crypsis, an organism blends itself in the environment, effectively becoming invisible (of course, it's also useful to predators to approach the prey). A disruptive pattern is among the simplest way to do this: the spot on a leopard's fur, or the mottled feathers of a nightjar break up the outline, as well as a complex system of appendages; zebras' stripes meld their shape with the other individuals in the herd. Generally, animals that live in the tall grass have vertical stripes, while those that live among bushes or in the underbrush have broad spots. Often, these animals have dark stripes or marks that hide the eyes, like giant pandas, raccoons, killer whales or common frogs.

The shadow is a primary mark of a three-dimensional shape: for this reason, many animals have a darker back, or generally an upper side darker than the lower side; several squids and fish resort to counter-illumination, that is, a ventral source of light. For low and squat organisms, lateral flanges that make the body wide and flat on the ground help to hide the shadow, especially if they have lighter fringed sides.

Some animals use self-decoration to hide themselves: the decorator crab covers itself with pebbles, sponges and seaweed, while caddisfly larvae use sand, twigs, or broken shells. Octopi, squids and some flatfish and lizard can actively change colour; foxes, hares and ermines change their coat with the season.

A very pecular form of crypsis is found in transparent species, such as comb jellies, young glass frogs, glass knifefish and others. Jellyfish's main tissue, mesoglea, is devoid of cells and rich in water, and thus often transparent, but muscle and other tissues can be made so if the proteins are organized in fibrils smaller than the wavelength of visible light (the effect works best under water; outside, a transparent organism would look like a bag of water). The ice fish lacks haemoglobin in its blood, making it completely clear, though this requires a greater heart strain and forces it to live in oxygen-rich water. Anyway, some functions (protection from harmful radiations, photosynthesis, sight) require light absorption and their organs can never be invisible. Some fish, such as herrings and marine hatchetfish, can imitate transparency with a flat and thin body covered in guanine-rich silvery, highly reflective scales.

Aposematism
Aposematism is the opposite of crypsis: rather than hiding, the organisms makes itself stand out in the environment signalling itself as an unappetizing prey. It's most common in toxic species (frogs and salamanders, bees and wasps, beetles, caterpillars, snakes and Heloderma lizards); it's usually a combination of black and a vivid colour such as red or yellow (or, rarely, blue). Green and brown are avoided, as they're common background colours, but of course this would depend from the dominant flora and geology: in a planet where most plants are red, green could be a common warning colour. In the (venomous) Hapalochlaena octopi, blue rings appear immediately before the bite (see "deimatic behaviour", below). In actually lethal species, though, aposematism is rare, since its success depend from the learning (and therefore, survival) of the predator.

Mimesis
Mimesis, or true mimicry, involves an organism trying to be as similar as possible to anther one. Examples of cryptic mimicry include stick insects and leaf insects, the leafy seadragon and the mata-mata turtle, or the orchid mantis. Species that imitate visible but unattractive objects include several butterflies that look like dry leaves, as well as the spider Celaenia excavata, that mimics bird droppings.

Batesian mimicry is a form of mimicry in which a non-dangerous organism mimics a dangerous organism. Several snakes, beetles, butterflies, etc. have a false aposematic coloration, for example the moth Sesia apiformis, which has the yellow-black stripes and the transparent wings of a wasp; the hawk-cuckoo imitates true hawks in shape and plumage; several caterpillars and butterflies have eye-like spots that make them look much bigger than they really are. By changing its colour and body shape, the mimic octopus can imitate several dangerous animals, such as sea snakes and lionfish.

In mullerian mimicry, several dangerous species adopt the same aposematic coloration, strengthening each other's defense. For example, most bees and wasps have the same yellow and black pattern; the heliconians butterflies have a similar black, yellow and red colour scheme.

In the somewhat unusual martensian (or emsleyan) mimicry, a lethal species (say, the coral snake) imitates the pattern of a less dangerous one: the lethal species does not allow the predator to survive, and therefore to learn to avoid it, but the other one does, and makes an aposematic warning actually useful, so that both species can survive without being attacked.

Surviving the attack
If the prey is sighted and attacked by the predator, it can flee, fly off, hide in a den, or find safety in number: adult musk oxen arrange themselves in a circle with their horns pointing outwards, and the calves at the centre; hamadryas baboons assume a similar arrangement; some gazelles and deer resort to stotting, a particular leap with stretched legs and arched backs that is believed to be a signal of the individual's high fitness; mayflies and anchovies simply live in groups so large that no predator can ever kill more than a very small fraction of them.

Even if the attack is successful, the prey can try to avoid consumption. Many animals, such as opossums, pretend to be dead and decomposing, swelling their abdomen and secreting a scent of rotting meat; horned lizards make their eye capillaries burst, squirting blood on the predators (autohaemorrhaging). Others resort to autotomy (self-amputation), often of parts that regrow: lizards lose their tail, crabs their legs, octopi their tentacles, sea cucumbers vomit their own gastrointestinal tract; several species of ants and termites bring this phenomenon to the extreme with autothysis, where organs or venom glands inside some individuals burst, killing them and spilling noxious chemicals outside.

Other behaviours focus on distracting the predator: many birds pretend to be injured to lure the predator away from their nest, while plovers incubate a fake nest distant from the real one; blackbirds, corvids and gulls (and some mammals such as meerkats) attack or harass a predator in large groups (mobbing), even though none of them can harm it alone, at least to prevent it to hunt effectively. Most cephalopods can expel ink, a dark melanine-rich liquid, opaque and probably irritating, that hides their escape; in some cases, if the ink is particularly rich in mucus, it can form pseudomorphs, semi-solid shapes that can resemble the animal.

Deimatic behaviour is a threatening or startling behaviour meant to scare or distract the predator: cephalopods and chameleons turn bright red or other aposematic colours; leaf insects and mantises spread their legs and reveal coloured wings; several butterflies and moth reveal false eyes painted on their wings; many mammals bristle their fur to appear larger; snakes and cats hiss, tarantulas raise their front legs, frilled lizard open their frill, pufferfish engorge their body and so on.