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Predator deterrence is an anti-predator adaptation developed over time through evolution, which assists 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.

Predator deterrence 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
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 Adaptions
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 (zoology) 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. Because of this, predators will often avoid eating the Sohal surgeonfish to prevent injuring itself.

Empirical evidence directly shows this decrease in predation for 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 the larvae that do not possess the spines. In fact, studies have also shown that one of the limacodidae predators, the paper wasp, experiences a learned aversion to the spined limacodid species and prefer to consume spineless larvae compared to the spined larvae when given the choice. Thus, the limacodidae larvae that are heavily armored with spines are more likely to survive predator encounters.

Many of the structural adaptations animals use for defense also contain toxins. Toxins are poisonous small molecules, peptides, or proteins 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.

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.

Defenses using Appearance
Crypsis is the ability of the organism to avoid observation or detection by other organisms. Methods of crypsis include camouflage, mimicry, and nocturnality. These morphological and behavioral defense mechanisms most likely evolved due to predator and anti-predator adaptations. 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 task difficult.

Main article:crypsis Camouflage is a type of visual crypsis using any combination of materials, coloration, or illumination for concealment, making animals hard to identify by sight. This is common in both terrestrial and marine animals. Camouflage can be achieved in many different ways, such as resemblance to surroundings, disruptive coloration, eliminating shadow, self-decoration, cryptic behavior, or changeable skin pattern or 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. Their bodies are flattened, with the sides thinning to an edge. The animals habitually press their bodies to the ground and their sides are fringed with white scales, which effectively hide and disrupt any remaining areas of shadow there may be under the edge of the body.

Main article: camouflage

Mimicry involves an organism (the mimic) which simulates signal properties of another organism (the model) so that the two are confused by a third living organism and the mimic gains protection, food, mating advantage as a consequence of the confusion. There are two classical examples of defensive mimicry: Batesian and Mullerian. Both involve aposematic or warning signals to a predator, with the favored response to not be attacked. In Batesian mimicry, a palatable prey species mimics the appearance of another species noxious to predators, thus reducing its risk of being attacked. This form is characteristic in many insects, where harmless species have evolved to imitate the aposematic signal of a harmful species directed at a common predator. Thus, predators that have tried to eat the unpalatable model species will learn to associate its colors and markings with unpleasant taste, learning to avoid the time and energy to catch the model and consequently the mimic. In Mullerian mimicry, two aposematic noxious forms conform to the same warning signal in order avoid a common predator. This natural phenomenon of a common warning signal is evident in viceroy and monarch butterflies. In both butterflies, the physical similarities of the wings and their unpleasant taste deter birds from eating both species.

Main article: mimicry

Unique Morphological Defenses
A few species have developed some noteworthy morphological defense mechanisms against predators that stand out because of their bizarreness. For example, the Texas Horned Lizard  is able to shoot squirts of blood from its eyes if it feels threatened Texas horned lizard. Because an individual may lose up to 53% of blood in a single squirt, this rare example of autohemmorhaging is only against persistent canid predators, canidae , as a last defense. Some example of canid predators are foxes, wolves, or coyotes. Texas Horned Lizards can rapidly increase the blood pressure within the thin-walled sinus of the eye sockets. The sinus walls then break due to increased pressure, sending a spray of blood usually aimed at the mouth or eyes of the predator. The canine will drop a horned lizard after being squirted and attempt to wipe or shake the blood out of its mouth, suggesting the fluid has a foul taste .Studies have identified this as specifically a canid predator defense. In an experiment, canids were less likely to eat Texas horned lizards than other lizard species when presented under the same circumstances. This suggests a learned aversion to horned lizards as prey and is a testament to its evolutionary effectiveness. Although other reptiles use cloacal discharges and external glandular secretions to deter predators, this bizarre example of blood squirting is only observed in three species of horned lizard.

Another notable defense mechanism can be observed in the primitive hagfish. Slime glands all along the body of a hagfish secrete enormous amounts of ‘mucus’ when they are provoked or stressed. The common predators of hagfish include seabirds, pinnipeds and cetaceans but most fish are missing from this list. Scientists hypothesize that over time, fish have become aware of the gill-clogging properties of hagfish slime and thus have evolved to avoid them. Because the slime has dramatic effects on the flow and viscosity of water, gills become entangled in the gelatinous substance and most fish that do attempt to capture a hagfish are chocked and will let go within seconds.

Behavioral Strategies
Many species have adapted behavioral strategies to avoid consumption and detection by predators. These strategies have evolved over time because they increase the lifespan, and thus the reproductive fitness of animals.

Pursuit-Deterrent Signals
Pursuit-deterrent signals are behavioral signals used by prey that apparently 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. This behavior is thought to be a honest signal of the gazelle's fitness in terms of its ability to outrun the predator that the predator evaluates when selecting which prey animal to pursue.

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, or in the case of a predator to lure prey into approaching. For example, white-tailed deer fawns 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 all movement called "tonic immobility". Tonic immobility is a reflex response that causes the fawn to enter a low body positioning that simulates the positioning of a dead corpse. Upon discovery of the fawn, the predator loses interest in the "dead" prey. As well, other symptoms of alarm bradycardia including salivation, urination, and defecation can cause the predator to lose interest entirely.

Nocturnality
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 enhance predation. Predation risk has long been recognized as critical in shaping behavioral decisions. For example, this predation risk is of prime importance in determining time of evening emergence in echolocating bats. Nocturnality may have evolved in these bats because of predation risk. Although early access during brighter times permits more food, it also leads to higher predation risk from Bat Hawks and bat falcons. Therefore, they show an optimum evening emergence time that is a compromise between the conflicting demands.

Another adaptation of nocturnality can be seen in the kangaroo rats, which exhibit moonlight avoidance. These nocturnal rodents that usually forage in relatively open habitats and reduce activity outside their nest burrows in response to moonlight. During a full moon, they shift their nocturnal activity towards areas of relatively dense cover. In controlled experiments, artificial moon-like illumination has stimulated similar responses in foraging behavior. This behavioral response is assumed to reduce predation risk.

Main article: nocturnality

Group Living
Main article: 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 is 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 who spots the animal will directly aim its attack on the prey. However, if the animal is in a group, the chance that the predator aims its attack to the same 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.

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 provide evidence for this 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. Thus, the Camargue horses dilute their individual risk of attack.

Another common example of the dilution effect is 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. However it is important to note that 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.

Selfish Herd was proposed by W.D. Hamilton and 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 well-known 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. This effect explains why animals will seek central positions in a group.

Main Article: Selfish herd theory

Predator Confusion is yet 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 is 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. This also makes a single zebra harder to catch amongst the herd.

Communal Defense is when prey groups are not just passive victims but actively defend themselves by attacking or mobbing a predator.

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. Numerous animals display mobbing behaviors to protect themselves from predators.

For examples, Red colobus monkeys exhibit mobbing behavior when threatened by chimpanzees, a common predator. The male red colobus monkeys group together to place themselves between predators and the group's females and juveniles. The males act to jointly jump and bite the chimpanzees as an active form of defense.

Additionally, fieldfares nest either solitarily or in colonies. Within the colonies, fieldfares demonstrate communal predator defense by members mobbing and defecating on approaching predators including crows, resulting 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 fieldfares, predation was greatest in the nests near the colonies. However, with fieldfares present, nest predation was higher in the nests near solitary fieldfares thus demonstrating the reduced predation resulting from communal defense.

Improved Vigilance occurs in group living because groups are able to detect predators sooner than solitary individuals. For many predators, success is dependent on surprise and if the prey is alerted too soon in an attack then it has 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 piegeons than they are when attacking smaller ones. 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 will follow.

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 and are subject to predation by lions. As group size increases, the frequency that each individual raises it head to look for predators decreases. Ostriches are able to run at speeds that exceed those of lions as well as maintain the speed for greater distances. Thus, lions tend to attack a prey when its head is down and grouping by ostriches makes it difficult for the lion to determine how long the ostriches' heads will stay down. Thus although individual vigilance decreases, overall vigilance of the group increases with group size and each individual ostrich can spend more time with its head down feeding and still have increased protection.

Unique Behavioral Defenses
Defensive Regurgitation

Some birds and insects use defensive regurgitation to ward off their predators. In theory, a predator will be much less likely to pursue their prey when it smells unappetizing, like vomit. Some birds, such as the Northern Fulmar (an aquatic bird that lives in arctic regions), for example, will vomit a bright orange, oily substance when they feel threatened. The vomit oil of Northern Fulmars (made from fish oils in their aquatic diets) has a deadly effect on predator birds because it will cause their feather to matte, leading to a loss of flying ability and of water repellency. 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 mom and dad that danger is present. Instead of recognizing the vomit as a signal to come help their nest, mom and dad actually 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 lemon juice. Although Eruasian Roller chicks aren’t intending to call their parents for help, the adults have evolved this reactiveness to their chicks’ danger cues. In this species, parents have learned to stay away when there is danger to benefit their lifetime reproductive success. In other words, this type of predator defense has both direct and indirect benefits.

Numerous insects utilize defensive regurgitation as well. For example, the Eastern tent caterpillar will regurgitate a droplet of digestive fluid to repel attacking ants. . Similarly, larvae of the noctuid moth will regurgitate when disturbed by ants. Their vomit has repellent and irritant properties and is mostly effective at deterring predator attack.

Suicidal Alturism

Another extreme predator deterrence behavior is observed in the Malaysian exploding ant. Social hymenoptera rely on altruism to protect the entire colony. Self-destructive defensive behaviors function to increase the fitness of the entire colony and thus genes for self-sacrificial behaviors are passed from generation to generation. Ants use their exocrine glands and secretions for a number of reasons: communication, reproductive signaling, and colony defense. The Malaysian exploding ants are known for a pronounced hypertrophy of their glands and for their novel uses in territorial combat. Simply grasping a worker ant’s leg with forceps will cause them to suicidially expel the contents of hypertrophied glands. The adhesives mixed with these compounds may further magnify the effects of the irritant and corrosive irritant aromatic compounds are likely magnified by the adhesives. These sticky substances adhere to their targets and also send signals to additional enemy ants as these arrive to rescue their nest mates.