Anti-predator adaptation

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Anti-predator adaptation in action: the seal shark Dalatias licha (a–c) and the wreckfish Polyprion americanus (d–f) attempt to prey on the hagfishes. First, the predators approach their potential prey. Predators bite or try to swallow the hagfishes, but the hagfishes have already projected jets of slime (arrows) into the predators' mouths. Choking, the predators release the hagfishes and gag in an attempt to remove slime from their mouths and gill chambers.[1]

Anti-predator adaptation refers to mechanisms developed over time through evolution, assisting prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle in order to maximize prey survival.

A class of anti-predator adaptations is predator deterrence, which can be divided into two major categories: morphological and behavioral defenses. Both of these types of defenses have evolved through natural selection because they increase the fitness of the prey. The increase of fitness leads to a better reproductive success of the individual possessing the favorable trait, and thus results in the persistence of the trait in the population over time.

Morphological defenses involve structural adaptations such as horns, spikes, stingers, claws, fangs and toxins. Some morphological defenses utilize aspects of the prey's appearance to avoid detection. These strategies include camouflage and mimicry.

Behavioral defenses involve acts performed by the prey to avoid predation. These defenses include actions such as pursuit deterrent signals, nocturnality, and group living.

Morphological strategies[edit]

Many species use morphological strategies to avoid being detected or consumed by predators. Morphological strategies come in a variety of forms and include both the structure and outward appearance of the animal. The most commonly seen morphological strategy is the use of structural features to deter and defend against predators. Examples of structural adaptations include horns, claws, spikes, spines and teeth. Other animals use their appearance as a type of morphological defense to avoid being noticed by predators. This type of strategy includes camouflage and mimicry.

Structural adaptations[edit]

Stinging Limacodidae slug moth caterpillars

Structural adaptation defenses are the most common type of morphological defense. This type of defense uses certain structural features of the species to avoid being consumed by the predator.

One common example of a structural adaptation to deter predators is a spine. A spine is a sharp, needle-like anatomical 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. A swipe from a surgeonfish spine can produce a deep cut in a predator’s flesh. Additionally, the area around the spines is often brightly colored to warn predators of danger.[2] Because of this, predators often avoid eating the Sohal surgeonfish to prevent injuring themselves. Some types of spines are only incidentally used for defense however, as research has shown that animals like the hedgehog may use theirs as shock absorbers based on their design when compared to animals like the porcupine [3]

Experiments have directly shown evidence for this decrease in predation of prey possessing this type of morphological defense. Many species of slug caterpillars, Limacodidae, have numerous protuberances and stinging spines along their dorsal surfaces. Studies show that species of limacodid larvae that possess these stinging spines suffer from less predation than larvae that do not possess the spines. In fact, studies have also shown that a predator of the Limacodidae, the paper wasp, experiences a learned aversion to the spined species and prefers to consume larvae without spines when given a choice. Thus, the limacodid larvae that are heavily armed with spines are more likely to survive predator encounters.[4]

Many of the structural adaptations animals use for defense also contain toxins, poisonous small molecules such as peptides that are produced in a living organism. The use of toxins by the prey further contributes to injuring the predator. For example, the bombardier beetle has specialized glands on the tip of its abdomen that allows it to direct a toxic spray towards predators. The spray is generated explosively through oxidation of hydroquinones and is sprayed at a temperature of 100 °C. This defense mechanism is highly successful at deterring predators.[5]

There are a variety of other types of morphological features that species use in a similar way as spines to either deter predators or to defend against predators once under attack. These include teeth, claws, fangs, spikes and horns.

Crypsis[edit]

Main article: crypsis

Crypsis is the ability of an organism to avoid observation or detection by other organisms. Methods of visual crypsis include camouflage and mimicry, and methods of behavioral crypsis include nocturnality. Both the morphological and behavioral crypsis mechanisms likely evolved due to predator and anti-predator adaptations.[6][7][8][9] For a predator to locate a potential meal, it must first identify an organism as prey. Prey, however, have many adaptive characteristics which make this task difficult.

Camouflage[edit]

Main article: camouflage
Camouflage defense illustrated by Flat-tail Horned Lizard
Flat-tail Horned Lizard with flattened body, eliminating shadow to successfully camouflage into their surrounding.

Camouflage is a type of visual crypsis that uses any combination of materials, coloration, or illumination for concealment to make the organism hard to identify by sight. This 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, self-decoration, cryptic behavior, or changeable skin patterns and color. Most methods contribute to camouflage by helping the animal hide against a background in order to avoid predation.

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. Additionally, these lizards hide any remaining shadows by pressing their bodies to the ground.[10]

Mimicry[edit]

Main article: mimicry
Viceroy and monarch butterflies illustrate Mullerian mimicry
Viceroy butterfly (top) and monarch butterfly (bottom) exhibit Mullerian mimicry, being similar in color, and equally unpalatable to predators.

Mimicry is another type of visual crypsis. Mimicry occurs when an organism (the mimic) simulates signal properties of another organism (the model) to confuse a third living organism. This results in the mimic gaining protection, food, and mating advantages.[11] There are two classical examples of defensive mimicry: Batesian and Mullerian. Both involve aposematic coloring, 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.[12] 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 colors and markings, including the mimic displaying Batesian mimicry. In Mullerian mimicry, two aposematic noxious forms conform to the same warning signal in order avoid a common predator.[12] This natural phenomenon of a common warning signal is evident in viceroy and monarch butterflies. Birds avoid eating both types of butterflies because their visually similar wing patterns signify an unpleasant taste.

Morphological defenses[edit]

A few species have developed morphological defense mechanisms against predators. For example, the Texas Horned Lizard is able to shoot squirts of blood from its eyes if it feels threatened. Because an individual may lose up to 53% of blood in a single squirt,[13] this rare example of autohemorhaging is only used against persistent predators, such as a canidae, as a last defense.[14] Some examples of canid predators are foxes, wolves, and coyotes. Texas Horned Lizards are able to squirt blood from their eyes by rapidly increasing the blood pressure within the thin-walled sinus of their eye sockets. As a result, the sinus walls break, sending a spray of blood aimed at the mouth or eyes of the predator.[14] The canids often drop the horned lizard after being squirted and attempt to wipe or shake the blood out of its mouth, suggesting that the fluid has a foul taste.[15] Canids are less likely to eat Texas horned lizards compared to other lizard species when presented with one of the two under identical circumstances.[16] This suggests the presence of a learned aversion towards horned lizards as a prey.[17] Although other reptiles use cloacal discharges and external glandular secretions to deter predators, this rare example of blood squirting is only observed in three species of horned lizards.[18]

Another notable defense mechanism can be observed in the hagfish. Slime glands along its body secrete enormous amounts of mucus when it is provoked or stressed. Because the slime has dramatic effects on the flow and viscosity of water, gills of predators become entangled in the gelatinous substance. Most fish that attempt to capture hagfish choke and let go of the hagfish within seconds. Common predators of hagfish include seabirds, pinnipeds and cetaceans, but most fish are missing from this list.[19] Scientists have hypothesized that over time, fish have become aware of the gill-clogging properties of hagfish slime and thus have learned to avoid them.

A third morphological defense mechanism is the ability of some octopuses to mimic different animals. The octopus mimics the model by changing their skin color and pattern to match the model and moving in a motion similar to the model. An octopus can mimic several models, and it makes a decision by determining the best form to deter the predator. An example being when damselfish attack an octopus, the octopus will mimic a banded sea-snake by threading six arms down a whole and raising the other two in opposite directions. The two raised arms are banded, mimicking the pattern of the sea-snake. The sea-snake is a predator of the damselfish, therefor by mimicking the sea-snake the octopus deters the damselfish.[20]

Behavioral strategies[edit]

Many species have adapted behavioral strategies to avoid consumption and detection by predators. These adaptations have arisen over time because they increase an organism's ability to survive which indirectly allows them reproduce, thus increasing the overall fitness of the organism and eventually, over a number of generations, the fitness of the population.

Pursuit-deterrent signals[edit]

Pursuit-deterrent signals are behavioral signals used by prey that convince predators not to pursue them. A common example of this type of deterrence is seen in the gazelle. Gazelle stotting is when the animal jumps high with stiff legs and an arched back. It is thought that gazelles display this behavior as a signal to predators, to show 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.[21]

Another pursuit-deterrent signal is thanatosis. 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.[22] 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 dead corpse. Upon discovery of the fawn, the predator loses interest in the "dead" prey. Additionally, other symptoms of alarm bradycardia, including salivation, urination, and defecation, can cause the predator to lose interest.[23]

Nocturnality[edit]

Main article: nocturnality
Flying Fruit Bats at night, seen through a night camera

Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. This is a behavioral form of crypsis that can be 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.[8] Another nocturnal adaptation can be seen in kangaroo rats, which exhibit moonlight avoidance. These rodents 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. In controlled experiments, artificial moon-like illumination stimulates similar responses in their foraging behavior, suggesting that this behavior has evolved to reduce predation risk.[9]

Group living[edit]

Further information: Social animal

Individuals within a species often form groups despite the costs of greater competition for resources, risk of infection by pathogens and detection by predators. Yet one of the reasons group living has evolved in nature is because of the benefit this strategy has on predator deterrence. There are many advantages group living has on decreasing the risk of predation, some of which include the ones listed below.

Many of these advantages decrease the risk of attack for individuals living within the group. The evolutionary advantage of this decreased risk is that the fitness of the individual increases. Thus, even though a larger group may be more susceptible to predators, individuals within the group are less susceptible to attack by the predator. This has allowed group living to persist in evolutionary history, as seen in the following examples.

Dilution effect[edit]

A dilution effect is seen when animals living in a group "dilute" their risk of attack. The large group size serves as a type of concealment for any given individual in the group. If the animal is alone, a predator directly aims its attack at the single available target. However, if the prey animal is in a group, the chance that the predator attacks that particular individual is reduced.

George C. Williams and W.D. Hamilton first proposed that group living evolved because it provides benefits to the individual rather than to the group as a whole. The dilution effect supports their proposal because this effect refers to the attack rate per individual rather than attack rate per group. The benefit is to the individual, but not to the group, because a larger group size makes the group more visible to predators.[citation needed]

The dilution effect is seen in many instances in nature. One common example of the dilution effect is seen in the shoaling behavior of fish. Shoaling occurs when a large school of fish live together. One of the reasons we see this behavior in fish is because the larger the school of fish, the lower the probability that each individual fish is targeted. In fact, there are experiments that provide direct evidence for the decrease in individual attack rate seen with group living. One example of a study that provides evidence for the dilution effect is in the observation of the Camargue horse in Southern France. A type of parasitic fly, called the Horse-fly, often attacks these horses. These flies suck the horse's blood, carry diseases and are most common during specific weeks in the year. During these weeks, the Camargue horses gather in large groups. Though experimental evidence has shown that more flies are attracted to the large groups of horses, each horse has a lower attack rate.[24] Thus, the Camargue horses dilute their individual risk of attack.

A dilution effect is also seen in water striders. These insects reside on the surface of the water and are attacked from beneath by predatory fish. Experiments varying the group size of the water striders showed that the attack rate per individual water strider decreases as group size increases.[25] However the decrease in attack rate may not be entirely due to the dilution effect. Other predator deterrence effects of group living, as described below, may cause the overall attack rate for the entire group to decrease. If the entire group is attacked less, then it follows that each individual within the group would also be attacked less. Because there are multiple ways group living can help deter predators, whether on a group level or individual level, it is difficult to attribute the decrease in attack rate to just the dilution effect. The dilution effect is just one factor that plays a role in predator deterrence on the level of the individual prey.

In a group, prey seek central positions in order to reduce their domain of danger. Individuals along the outer edges of the group are more at risk of being targeted by the predator.

Selfish herd[edit]

Main article: Selfish herd theory

The selfish herd theory was proposed by W.D. Hamilton. It refers to the idea of reducing the individual's domain of danger. A domain of danger is the area within the group in which the individual is more likely to be attacked by a predator. The center of the group has the lowest domain of danger, so animals will constantly strive to gain this position.

In a study testing Hamilton's selfish herd effect, Alta De Vos and Justin O'Rainn (2010) studied Brown fur seal predation from great white sharks. Using decoy seals, the researchers varied the distance between the decoys to produce different domains of danger. The seals with a greater domain of danger had an increased risk of shark attack.[26] This effect explains why animals seek central positions in a group.

Predator confusion[edit]

A single zebra is hard to catch amongst a herd

Predator Confusion is another advantage of group living in terms of predator deterrence on the individual level. Individuals living in large groups may be safer from attack because the predator may be confused by the large group size. As the group moves, the predator has greater difficulty focusing in on one targeted prey.

An example of this type of predator deterrence may perhaps be seen in the zebra. When stationary, a single zebra stands out in the savannah because of its large size. To reduce this risk of attack, zebras often travel in herds. The striped patterns of all the zebras in the herd confuse the predator, making it harder for the predator to focus in on an individual zebra. Furthermore, when moving rapidly, the zebra stripes create a confusing, flickering movement in the eye of the predator.[27] This also makes a single zebra harder to catch amongst the herd.

Communal defense[edit]

In communal defense, prey groups actively defend themselves by attacking or mobbing a predator, rather than allowing themselves to be passive victims of predation.

Mobbing is the harassing of a predator by many prey animals. Mobbing is usually done to protect the young in social colonies, and numerous animals display mobbing behaviors to protect themselves from predators. For example, Red colobus monkeys exhibit mobbing behavior when threatened by chimpanzees, a common predator. The male red colobus monkeys group together and place themselves between predators and the group's females and juveniles. The males jump together and bite the chimpanzees as an active form of defense.[28]

Additionally, fieldfares nest either solitarily or in colonies. Within the colonies, fieldfares demonstrate communal predator defense by having members mob and defecate on approaching predators. This results in reduced predation. In an experiment, artificial nests were egg-bated and placed either near fieldfare colonies or near solitary fieldfares. In the absence of other fieldfares, predation was greatest in the nests near the colonies. However, with fieldfares present, nest predation was higher in the nests near solitary fieldfares, demonstrating the reduced predation that results from communal defense.[29]

Small birds, such as grackles will often harass red-tailed hawks in the sky while flying.

Improved vigilance[edit]

In the improved vigilance effect, groups are able to detect predators sooner than solitary individuals. For many predators, success depends on surprise. If the prey is alerted early in an attack, they have an improved chance of escape.

For example, wood pigeon flocks are preyed upon by goshawks. Goshawks are less successful when attacking larger flocks of wood pigeons than they are when attacking smaller flocks. This is because the larger the flock size, the more likely it is that one bird will notice the hawk sooner and fly away. Once one pigeon flies off in alarm, the rest of the pigeons follow.[30]

Another example of improved vigilance is within wild ostriches in Tsavo National Park in Kenya, which feed either alone or in groups of up to four birds. These ostriches are subject to predation by lions. As the ostrich group size increases, the frequency that each individual raises it head to look for predators decreases. Because ostriches are able to run at speeds that exceed those of lions for great distances, lions try to attack the ostrich when its head is down. By grouping with ostriches, the lions experiences greater difficulty in determining how long the ostriches' heads stay down. Thus, although individual vigilance decreases, the overall vigilance of the group increases with group size. Each individual ostrich can spend more time with its head down to feed but also receives increased protection.[31]

Behavioral defenses[edit]

A Northern Fulmar chick protects itself with projectile vomit

Defensive regurgitation[edit]

Some birds and insects use defensive regurgitation to ward off their predators. In theory, a predator should be much less likely to pursue its prey when the prey smells unappetizing. For example, some birds, such as the Northern Fulmar vomits a bright orange, oily substance when it feels threatened.[32] The vomit oil of Northern Fulmars is made from fish oils from their aquatic diets and has a deadly effect on predator birds. This is because the oily vomit causes the predator's feathers to mat, leading to the loss of flying ability and the loss of water repellency.[32] This effect is especially dangerous for aquatic birds because their water repellent feathers protect them from hypothermia when diving for food.

European Roller chicks also vomit a bright orange, foul smelling liquid when they sense danger. This behavior not only repels prospective predators, but it might also serve as a warning sign to parents that danger is present. Interestingly, instead of recognizing the vomit as a signal to come help their nest, the parents respond by delaying their return. In an experiment by Parejo and colleagues, Eurasian Roller parents approached the nest much more slowly and infrequently if the nest was covered in chick vomit, as opposed to when the nest was covered in lemon juice.[33] In this species, parents have learned to stay away from the nest when there is danger, in order to benefit their own lifetime reproductive success. In other words, this type of predator defense has direct benefits for the chick and indirect benefits for the parents.

Numerous insects utilize defensive regurgitation as well. For example, the Eastern tent caterpillar regurgitates a droplet of digestive fluid to repel attacking ants.[34] Similarly, larvae of the noctuid moth regurgitates when disturbed by ants. The vomit of noctuid moths has repellent and irritant properties that are mostly effective at deterring predator attacks.[35]

Suicidal altruism[edit]

Another unusual type of predator deterrence is observed in the Malaysian exploding ant. Social hymenoptera rely on altruism to protect the entire colony, so the self-destructive defensive behaviors benefit the fitness of all individuals in the entire colony. This contributes to the genes for self-sacrificial behavior being passed down to future generations over evolutionary time.[36] Ants use their exocrine glands and secretions for a number of reasons: communication, reproductive signaling, and colony defense.[37] The Malaysian exploding ants, however, are known for their novel use of the pronounced hypertrophy of their glands in territorial combat.[38] Simply grasping a worker ant's leg with forceps causes it to suicidally expel the contents of its hypertrophied glands.[38] Corrosive irritant compounds and adhesives are expelled from the glands and released onto the predator. These sticky substances adhere to the predators and prevent predation. Additionally, the substance serves as a signal to additional enemy ants to stop predation of the rest of the colony.[39]

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