Antipredator adaptations are evolutionary adaptations developed over time, which assist prey organisms in their constant struggle against their predators.
The act of a predator acquiring a food source can be divided into four stages: detection, attack, capture and consumption.[1][2] At every stage in this predatory sequence, adaptations that maximize the prey's chance of survival have evolved. This, in turn, has driven adaptation in their predators. This kind of interaction over long periods is known as co-evolution.
Many predatory animals are themselves preyed upon. To defend themselves, predatory animals often use their methods of attacking prey to inflict or threaten grievous injury to their own predators. For example, the electric eel uses the same electrical currents to kill prey, and to defend itself against predators (such as anacondas, caimans, jaguars, egrets, cougars) which prey on fish of similar size to the eels.
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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. Crypsis is the ability of prey to evade detection by predators (or vice versa).
Camouflage is one heavily utilised method, and involves the mimicking of the colour and patterns of the environment. This is achieved by external pigmentation patterns, and resting immobile and silent on an appropriate substrate, as do Egyptian Nightjar. Many marine animals use countershading or counterillumination to reduce their visibility from above and below. Some, such as the Mimic Octopus, can rapidly change their pattern and colour to provide a better defence. Some animals provide structures on their bodies for plant life to grow, camouflaging its host. Other animals masquerade as specific, inedible objects: the Bird Dropping Spider and the Phobaeticus Stick insects are notable examples.
Visual Polymorphism, the existence of different forms within a single species, is a strategy used to reduce predation risk. Predators make use of search images to differentiate edible objects from all other objects; this causes frequency-dependent "apostatic selection" in which morphs with rarer colours and patterns are less likely to be killed. The striking polymorphism of the Grove snail may be partly caused by apostatic selection by predators including the Song Thrush.[3] 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.[4][5]:11
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. Highly mobile creatures such as seabirds migrate to avoid predators during the period in which they are most vulnerable, the breeding season, moving to offshore islands to establish colonies far from the reach of land predators.
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 colouration, or sounds and smells, so as to advertise that they are poisonous.[6] These patterns are often convergent, with red or yellow coupled with black being widely recognised as dangerous. 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.
Some animals, particularly gazelles, are known to stot, which, among other things, may advertise their unprofitability to predators.[7] 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.
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 defence 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.
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 the 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 the Mayfly, where millions of individuals emerge from pupation on the same day.[8]
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.[9]
Some animals have evolved dazzle camouflage, whereby instead of attempting to conceal themselves, they are patterned to cause motion dazzle, confusing a predator and making it harder to select 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.
Having been captured, an animal must prevent the predator from killing and eating it. Mechanical defences 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. As an absolute last line of defence, many insects taste extremely bitter, so that once a predator has begun to feed they quickly stop, and, if this is a less vital area such as a leg or a wing, the insect survives the encounter.
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 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.[10] 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.[11] Armoured crickets and many other insect species also utilise this method, known as autohaemorrhaging.[12] 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.[13][14]
Many plant species have, over the course of their evolutionary history, developed physical and chemical defense mechanisms to deter herbivores. Prickles, spines, and thorns are examples of physical mechanisms. Stinging nettles are covered in small hairs that contain chemicals that are a skin irritant to many mammals, including humans.
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