Antipredator adaptation

A lone bison is standing its ground against a pack of wolves, thereby increasing its chance of survival

Antipredator adaptations are evolutionary adaptations developed over time, which assist prey organisms in their constant struggle against their predators.

A predator's acquisition of a food source can be divided into four stages: detection, attack, capture and consumption. At every stage in this predatory sequence, adaptations that maximize the prey's chance of survival have evolved.

Animal adaptations

Avoiding detection

Avoiding being noticed

Main article: Camouflage

The butterfly Kallima inachus masquerades as a dead leaf to avoid detection

For a predator to locate a potential meal, it must first identify an organism as prey. Prey, however, have many adaptive characteristics which make such a task difficult. Camouflage, making animals hard to identify by sight, is common in both terrestrial and marine animals. It can be achieved through crypsis or mimesis.

Crypsis makes an animal, whether predator or prey, hard to see. Visual crypsis is achieved through a variety of methods which are often combined. Many camouflage methods such as background matching, countershading, and disruptive coloration make use of patterns of pigmentation or structural coloration. Other methods include resting immobile and silent on an appropriate substrate, as do Egyptian Nightjar. Many marine animals rely on countershading or counterillumination to reduce their visibility from above and below, and transparency or silvering to reduce their visibility from their sides. Many cephalopods such as octopuses, squids and cuttlefish can rapidly change their pattern and colour for both camouflage and signalling. Some animals such as decorator crabs provide structures on their bodies for sessile animals such as sponges and sea anemones to grow, camouflaging their hosts.

Mimesis or masquerade leaves an animal, either predator or prey, in plain sight but hard to recognize, whether because it resembles another animal, or because it looks like an object of no interest to the observer, such as a leaf or twig. Some animals such as the Bird Dropping Spider and Stick insects closely resemble specific, inedible objects. Predators and parasites may resemble their prey in aggressive mimicry.

Polymorphism in the Grove snail is partly a response to predation by Song Thrushes

Visual Polymorphism, the existence of different forms within a single species, can reduce predation risk because some predators make use of search images to identify edible objects. This causes frequency-dependent selection in which forms with rarer colours and patterns are less likely to be killed. The striking polymorphism of the Grove snail may be an example of this, as predators like the Song Thrush search for snails of patterns that they recognize, though other explanations exist. There is evidence that polymorphic insect prey suffer less predation than single-morph species at a particular density, and can maintain higher population densities for a given rate of predation. This effect is clearer in tropical than in temperate areas, as predation in the tropics is stronger and more consistent throughout the year.

Avoiding predators in space and time

Animals alter the period in which they are awake in order to avoid predators. Generally, animals are either diurnal (active during the day), nocturnal (active during the night), or crepuscular (active during twilight) depending on food availability and predator prevalence. For example, Dipodomys merriami kangaroo rats become crepuscular instead of nocturnal on full moon nights, when predators could see them more easily.

Anolis sagrei lizards are more arboreal (living in trees) on islands where ground-living predators exist.

Highly mobile creatures such as seabirds migrate to avoid predators during the period in which they are most vulnerable, the breeding season, for example moving to offshore islands to establish colonies far from the reach of land predators.

Avoiding attack

Further information: Aposematism and Mimicry

Startling deimatic display of the phasmid Peruphasma schultei suddenly exposes the bright red hindwings.

Having been detected by a predator, many animals attempt to signal to a predator that they are not worth eating. Some animals make use of aposematic signals, for example bright warning coloration, or sounds and smells, advertising that they are poisonous. These patterns are often convergent, with red or yellow coupled with black being widely recognised as dangerous. Animals with strong defences, such as the ability to sting, often mimic each other in Mullerian mimicry, such as that between bees and wasps.

The eyespots of Automeris io

Batesian mimicry is the imitation by a harmless species of the warning signals of a harmful species directed at a common predator, as can be seen in a hoverfly mimicking a wasp. The coloration is effectively a bluff, attempting to deceive the predator. A different form of bluff is deimatic behaviour, where an animal puts on a display that may startle a predator, often with bright colours, eyespots, sudden movement, or making itself seem larger than it is. Thanatosis is the form of bluff in which an animal mimics its own dead body, feigning death to avoid being attacked by predators seeking live prey, or in the case of a predator to lure prey into approaching.

Some animals such as gazelles stot, jumping high with stiff legs, which appears to advertise their unprofitability to predators. The moth Automeris io possesses eyespots hidden on its hindwings. When under threat, the moth suddenly reveals these spots, aiming to startle its predator. Fearful of their own consumption, predators often retreat when startled. Eyespots are also used by some animals to trick a predator into thinking it has been spotted (when in fact, it may not have been), and so is less likely to succeed in pursuit.

Avoiding capture

Many animals have highly developed senses of sight, smell, and hearing so that they can detect danger and escape. By frequently scanning and monitoring their surroundings, especially when in the open, prey can avoid attack by hoping to see a predator before it reaches the 'critical distance' for an attack. This is a standard defense mechanism for animals in open grasslands and prairies. It is also common for arboreal animals to scan both the ground around them for terrestrial predators, and the sky for aerial predators. Smaller animals may not venture too far from cover in burrows or the undergrowth, where they can quickly hide when danger approaches. Flight is of huge importance in the avoidance of predators in those species that possess it.

Predator Recognition

Learned Predator Recognition is influenced by pre-learning growth rate. That is the conclusion that was reached from the research done by Maud Ferrari, Grant Brown, Gary Bortolotti and Douglas Chivers on whether information learned about predators was influenced by prey growth trajectory before and after learning.

Safety in numbers

Animals that are the frequent target of predation often make use of 'safety in numbers'. This results in a situation where any one herd member is unlikely to be preyed upon, and in high populations, predator satiation is likely to occur. Grazing mammals often feed in social groups, also known as herds. Working as a group, a stalking predator is likely to be detected earlier, and when it attacks, the herd scatters, causing difficulty for the predator and allowing most, if not all, of the prey animals to escape. Prey animals may use alarm signals to alert other herd members when a predator is sighted or sensed. Animals usually have a breeding season, where all the members of the species spawn at the same time, in order to maximise their young's chance of survival. This is particularly pronounced in insects such as Magicicada and mayflies, where millions of individuals emerge from pupation on the same day.

Fighting off predators

Muskoxen in defensive formation, horns ready, and highly alert

Once an animal has failed at avoiding a predator or warding it off at a distance, self-defense is necessary. Many animals use horns, claws, and teeth to fight off predators. Some can inject venom and toxins, and skunks and bombardier beetles spray noxious chemicals to deter attackers. Mobbing, the harassing of a predator by many prey animals, is common in birds, and is usually done to protect the young in social colonies. The Eastern Honeybee mobs invading hornets, vibrating their flight muscles in order to raise the temperature around the hornet scout to lethal levels, rather than allowing the scout to bring others to their beehive.

Confusing the predator

A single zebra is hard to catch amongst a herd

Some animals have evolved dazzle camouflage, whereby instead of attempting to conceal themselves, they are patterned to cause motion dazzle, confusing a predator during an attack, and making it harder to select and track a target. An example is zebras, which stand out in the savannah when stationary, but when moving rapidly en masse, their stripes create a confusing, flickering mass in the eye of a predator such as a lion.

Avoiding consumption

This fishing spider has shed two of its legs to escape a predator

Having been captured, an animal must prevent the predator from killing and eating it. Mechanical defenses in those that possess them, such as armour and spines prevent access to softer edible parts. Distraction displays are used to direct the attention of the predator away from some vital area, such as the head, or a nest of chicks. These can be visual, as in misdirecting eyespots, behavioural, such as a mother-bird feigning injury, or chemical, as seen in the production of ink clouds by squid and octopuses.

Some animals (e.g. possums) freeze in cover or play dead when seen. Often this is accompanied by foul smells, as if their corpse is in an advanced state of decomposition, one which many animals would avoid consuming.

Some animals take a more drastic approach to defending themselves. Autotomy, the shedding of a non-vital bodypart, is utilised by some species to escape the grasp of a predator. Many lizards shed their tails when clasped, and arthropods will readily give up several legs if it allows their escape. When under threat, sea cucumbers rapidly eviscerate, ejecting part or all of their digestive tract. This is done to either anchor the cucumber into a rock fissure, or to eject toxins at the predator. Horned lizards, when threatened, increase the pressure in their sinus cavities until the blood vessels in the corners of the eyes burst, squirting blood at the attacker. Armoured crickets and many other insect species also use this method, known as autohaemorrhaging. These animals are able to survive the loss of these tissues, and later regenerate them.

The soldier ant caste of the Malaysian species Camponotus saundersi undergoes a process known as autothysis to defend their ant colony. The soldier ants have two large glands that run the entire length of their body, and when stressed during battle, abdominal muscles contract, causing the glands to explode, killing the ant, but spraying poison in all directions.

Plant adaptations

Aphid feeding on plant sap

Main article: Plant defense against herbivory

Many plant species have, over the course of their evolutionary history, developed physical and chemical defense mechanisms to deter herbivores. Thorns, spines, and prickles are examples of physical mechanisms. Stinging nettles for instance are covered in hollow hairs that can inject irritant chemicals. Acacias and roses are among the many plants protected by spines and thorns. Prickles and spines are not effective against small insects such as aphids however, so some plants, such as some of the Solanaceae, supplement a bristly surface with sticky secretions that trap and kill small pests. Other plants such as holly have thick tough leaves covered with slippery way, making feeding difficult for small herbivores.

Many Ericas have a sticky perianth, and some Moraeas have sticky peduncles that deter ants from plundering the flowers' nectar without contributing to their pollination. Many plants deter their enemies with repellent tastes or irritating or dangerous poisons in sap or in latex. The herbivores in turn developed wide ranges of counter-adaptations.

Symbiosis

Main article: Symbiosis

Organisms of different species may form symbiotic relationships whereby one or both organisms deter predators of the other in return for food, shelter, protection, or other useful resources. Examples include myrmecophytism such as in the Bullthorn Acacia, which provides food and shelter to the Acacia ant in return for active deterrence of predators; aphids that are tended and protected by ants in exchange for nectar produced from their bodies; and the relationship between Clownfish and sea anemomes which cooperate to protect each other from predators.^{[citation needed]}

See also

• Ultrasound avoidance
• Predator avoidance in schooling fish

Bibliography

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