Anti-predator adaptation
Anti-predator adaptations are mechanisms developed through evolution that assist prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, namely by avoiding detection, warding off attack, fighting back, or escaping when found.
The first line of defence consists in avoiding detection, through mechanisms such as camouflage, masquerade, apostatic selection, living underground, or nocturnality.
Alternatively, prey animals may ward off attack, whether by advertising the presence of strong defences in aposematism, by mimicking animals which do possess such defences, by startling the attacker, by signalling to the predator that pursuit is not worthwhile, by distraction, by using defensive structures such as spines, and by living in a group. Members of groups are at reduced risk of predation, despite the increased conspicuousness of a group, through improved vigilance, predator confusion, and the likelihood that the predator will attack some other individual.
Avoiding detection
Staying out of sight
Animals may avoid becoming prey by living out of sight of predators, whether in caves, burrows, or by being nocturnal. Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. This is a behavioral form of detection avoidance called crypsis used by animals to either avoid predation or to enhance prey hunting. Predation risk has long been recognized as critical in shaping behavioral decisions. For example, this predation risk is of prime importance in determining the time of evening emergence in echolocating bats. Although early access during brighter times permits easier foraging, it also leads to a higher predation risk from bat hawks and bat falcons. This results in an optimum evening emergence time that is a compromise between the conflicting demands. Another nocturnal adaptation can be seen in kangaroo rats. They forage in relatively open habitats, and reduce their activity outside their nest burrows in response to moonlight. During a full moon, they shift their activity towards areas of relatively dense cover to compensate for the extra brightness.File:Phrynosoma mcallii.jpg |thumb |right |Camouflage illustrated by the flat-tail horned lizard, its flattened, fringed and disruptively patterned body eliminating shadow
Camouflage
uses any combination of materials, coloration, or illumination for concealment to make the organism hard to detect by sight. It is common in both terrestrial and marine animals. Camouflage can be achieved in many different ways, such as through resemblance to surroundings, disruptive coloration, shadow elimination by countershading or counter-illumination, self-decoration, cryptic behavior, or changeable skin patterns and colour. Animals such as the flat-tail horned lizard of North America have evolved to eliminate their shadow and blend in with the ground. The bodies of these lizards are flattened, and their sides thin towards the edge. This body form, along with the white scales fringed along their sides, allows the lizards to effectively hide their shadows. In addition, these lizards hide any remaining shadows by pressing their bodies to the ground.Masquerade
Animals can hide in plain sight by masquerading as inedible objects. For example, the potoo, a South American bird, habitually perches on a tree, convincingly resembling a broken stump of a branch, while a butterfly, Kallima, looks just like a dead leaf.Apostatic selection
Another way to remain unattacked in plain sight is to look different from other members of the same species. Predators such as tits selectively hunt for abundant types of insect, ignoring less common types that were present, forming search images of the desired prey. This creates a mechanism for negative frequency-dependent selection, apostatic selection.Warding off attack
Many species make use of behavioral strategies to deter predators.Startling the predator
Many weakly-defended animals, including moths, butterflies, mantises, phasmids, and cephalopods such as octopuses, make use of patterns of threatening or startling behaviour, such as suddenly displaying conspicuous eyespots, so as to scare off or momentarily distract a predator, thus giving the prey animal an opportunity to escape. In the absence of toxins or other defences, this is essentially bluffing, in contrast to aposematism which involves honest signals.Pursuit-deterrent signals
Pursuit-deterrent signals are behavioral signals used by prey to convince predators not to pursue them. For example, gazelles stot, jumping high with stiff legs and an arched back. This is thought to signal to predators that they have a high level of fitness and can outrun the predator. As a result, predators may choose to pursue a different prey that is less likely to outrun them.White-tailed deer and other prey mammals flag with conspicuous tail markings when alarmed, informing the predator that it has been detected.
Warning calls given by birds such as the Eurasian jay are similarly honest signals, benefiting both predator and prey: the predator is informed that it has been detected and might as well save time and energy by giving up the chase, while the prey is protected from attack.
Playing dead
Another pursuit-deterrent signal is thanatosis or playing dead. Thanatosis is a form of bluff in which an animal mimics its own dead body, feigning death to avoid being attacked by predators seeking live prey. Thanatosis can also be used by the predator in order to lure prey into approaching.An example of this is seen in white-tailed deer fawns, which experience a drop in heart rate in response to approaching predators. This response, referred to as "alarm bradycardia", causes the fawn's heart rate to drop from 155 to 38 beats per minute within one beat of the heart. This drop in heart rate can last up to two minutes, causing the fawn to experience a depressed breathing rate and decrease in movement, called tonic immobility. Tonic immobility is a reflex response that causes the fawn to enter a low body position that simulates the position of a corpse. Upon discovery of the fawn, the predator loses interest in the "dead" prey. Other symptoms of alarm bradycardia, such as salivation, urination, and defecation, can also cause the predator to lose interest.
Distraction
Marine molluscs such as sea hares, cuttlefish, squid and octopuses give themselves a last chance to escape by distracting their attackers. To do this, they eject a mixture of chemicals, which may mimic food or otherwise confuse predators. In response to a predator, animals in these groups release ink, creating a cloud, and opaline, affecting the predator's feeding senses, causing it to attack the cloud.Distraction displays attract the attention of predators away from an object, typically the nest or young, that is being protected, as when some birds feign a broken wing while hopping about on the ground.
Mimicry and aposematism
occurs when an organism simulates signal properties of another organism to confuse a third organism. This results in the mimic gaining protection, food, and mating advantages. There are two classical types of defensive mimicry: Batesian and Müllerian. Both involve aposematic coloration, or warning signals, to avoid being attacked by a predator.In Batesian mimicry, a palatable, harmless prey species mimics the appearance of another species that is noxious to predators, thus reducing the mimic's risk of attack. This form of mimicry is seen in many insects. The idea behind Batesian mimicry is that predators that have tried to eat the unpalatable species learn to associate its colors and markings with an unpleasant taste. This results in the predator learning to avoid species displaying similar colours and markings, including Batesian mimics, which are in effect parasitic on the chemical or other defences of the unprofitable models. Some species of octopus can mimic a selection of other animals by changing their skin color, skin pattern and body motion. When a damselfish attacks an octopus, the octopus mimics a banded sea snake. The model chosen varies with the octopus's predator and habitat. Most of these octopuses use Batesian mimicry, selecting an organism repulsive to predators as a model.
In Müllerian mimicry, two or more aposematic forms share the same warning signals, as in viceroy and monarch butterflies. Birds avoid eating both species because their wing patterns honestly signal their unpleasant taste.
File:Erethizon dorsatum du Québec.jpg |thumb |The porcupine Erethizon dorsatum combines sharp spines with warning coloration
Defensive structures
Many animals are protected against predators with armour in the form of hard shells, leathery or scaly skin, or tough chitinous exoskeletons.A spine is a sharp, needle-like structure used to inflict pain on predators. An example of this seen in nature is in the sohal surgeonfish. These fish have a sharp scalpel-like spine on the front of each of their tail fins, able to inflict deep wounds. The area around the spines is often brightly colored to advertise the defensive capability; predators often avoid the Sohal surgeonfish. Defensive spines may be detachable, barbed or poisonous. Porcupine spines are long, stiff, break at the tip, and in some species are barbed to stick into a would-be predator. In contrast, the hedgehog's short spines, which are modified hairs, readily bend, and are barbed into the body, so they are not easily lost; they may be jabbed at an attacker.
File:Stinging Rose caterpillars, Megan McCarty65.jpg |thumb |upright |Stinging Limacodidae slug moth caterpillars
Many species of slug caterpillar, Limacodidae, have numerous protuberances and stinging spines along their dorsal surfaces. Species that possess these stinging spines suffer less predation than larvae that lack them, and a predator, the paper wasp, chooses larvae without spines when given a choice.