Animal defense mechanisms

All organisms are composed of fixed carbon, biomolecules, and mineral nutrients, and therefore represent energy and nutrient resources for consumers. To be successful in life, animals must avoid, tolerate, or defend themselves against natural enemies such as predators, parasites, and competitors. The term "defense" can be attributed to any trait that reduces the likelihood that an organism, or part of an organism, will be consumed by a predator. There are several categories of defenses that have evolved in animals, including structural defenses, chemical defenses, associational defenses, behavioral defenses, autotomy, and nutritional defenses. Animals often possess more than one type of defense, thereby having backup plans in case the first line of defense fails. The number of defenses devised by organisms is a reflection of the strong selective pressure exerted by predators.

Structural Defenses: Being Hard and Sharp

Structures that defend animals can act as external shields: sharp spines located externally or internally, skeletal materials that make tissues too hard to bite easily, or weaponry such as horns or antlers, teeth, and claws. External structures that protect vulnerable soft tissues include the chitonous exoskeleton of crustaceans, the calcareous shells of corals, mollusks, and barnacles, the tests (skeletal plates) of echinoderms, the tough tunic of ascidians, and the hard plates of armadillos. The pretty shells that tourists collect along beaches were once used to protect a soft, delicate animal that lived inside the shell. Hard, protective shells remain after the animal dies and can be used by other animals for protection. For example, small fishes may retreat into empty conch shells when they feel threatened by predators, and hermit crabs live inside empty snail shells to protect their soft, vulnerable abdomens.

Some animals cover their bodies with sharp structures that puncture predators that try to bite them. The porcupine is a good example of a mammal that uses this defensive strategy. Porcupines are covered with tens of thousands of long, pointed spines, or quills, growing from their back and sides. The quills have needle-sharp ends containing hundreds of barbs that make the quills difficult to remove. Sea urchins are also covered with long, sharp spines that deter would-be predators. Urchins can move their spines, and will direct them toward anything that comes in contact with them, such as a predator. While porcupines and urchins are covered with multiple spines, stingrays defend themselves from enemies by inflicting a wound with a single barbed spine. The wound is extremely painful, giving these rays their common name.

Many predators have sharp claws and teeth that help them grasp, subdue, and consume their prey. These same structures, used offensively in hunting, can also be used to protect themselves from their own predators. Small predators such as badgers, raccoons, and foxes can fend off larger predators such as wolves and mountain lions with their weaponry. Rather than risk injury, the larger predators will typically avoid a fight with a smaller predator and seek a less risky meal, such as a rabbit or mouse.

Chemical Defenses: Poor Taste, Bad Smell, or Toxic Chemicals

Both plants and animals defend themselves by using compounds that are distasteful, toxic, or otherwise repulsive to consumers. Most defensive compounds are secondary metabolites of unique structures, but can also include more generic compounds such as sulfuric acid or calcium carbonate. Secondary metabolites get their name because they are not involved in basic metabolic pathways such as respiration or photosynthesis (that is, primary metabolic reactions), not because they are of secondary importance. Indeed, many organisms probably could not survive in their natural environment without the protection of their secondary metabolites.

Stink bugs get their names because of the smelly secondary metabolites they release from pores located on the sides of their thorax. These smelly compounds repel predators, and may even indicate toxicity to the predator. These insects are common garden pests that are usually controlled with chemical pesticides. However, it appears that the eggs of stink bugs are not defended against roly-poly pill bugs, which can control stink bug numbers (and hence, garden damage) by preying on eggs.

Bombardier beetles take chemical defenses a step further, erupting a boiling hot spray of chemicals in the direction of a predator. To accomplish this, the bombardier beetle has a pair of glands that open at the tip of its abdomen. Each gland has two compartments, one that contains a solution of hydroquinone and hydrogen peroxide, and the other that contains a mixture of enzymes. When threatened by a predator, the bombardier beetle squeezes the hydroquinone and hydrogen peroxide mixture into the enzyme compartment, where an exothermic reaction that produces quinone takes place. The large amount of heat generated brings the quinone mixture to its boiling point, and it is forcefully emitted as a vapor toward the threat. An average bombardier beetle can produce about twenty loud discharges of repulsive, hot chemicals in quick succession.

Chemical defenses are common among small, slow animals such as insects, sponges, cnidarians, and sea slugs, which might be limited in their ability to flee from predators. However, chemical defenses are rather rare among large, fast animals. One of the few mammals that uses chemical defenses is the black-and-white-striped skunk. Most people are familiar with the smelly chemical brew emitted from these animals, as it is distinctly detectable along roads when skunks get hit by cars, and can be detected up to a mile from the location where a skunk sprays. These mammals hold their smelly musk in glands located below their tail, and squirt the liquid through ducts that protrude from the anus. When threatened by a predator, the skunk raises its tail and directs its rear end toward the predator. A predator that has had prior experience with a skunk might retreat from this display, but if the predator is persistent at harassing the skunk, the striped mammal will deliver a spray of smelly chemicals that usually sends the predator running. The musk also causes intense pain and temporary blindness if it gets in the eyes of the predator.

Associational Defenses: The Guard Dog Approach

Associational defenses occur when a species gains protection from a natural enemy by associating with a protective species, such as when humans gain protection from enemies by keeping a guard dog on their property. Types of protection provided to the defended species through this coevolution can be structural, chemical, or aggressive.

Small animals can avoid predators by using a defended species as habitat. For example, small fishes defend themselves by associating with sea urchins, gaining protection by hiding among the sharp spines. Some species of shrimp inhabit the cavities and canals of sponges. Sponges are known to be chemically and structurally defended against most predators, with the exception of angel fishes and parrot fishes. Finally, much of the diverse coral reef fauna seeks protection among the cracks and the crevices in the reef. Reefs, slowly built by coral animals, are the largest structures ever made by living organisms, and serve a protective role for thousands of species that inhabit reefs.

Associational defenses can also be chemically mediated. For example, bacteria that grow symbiotically on shrimp eggs produce secondary metabolites that protect the egg from a parasitic fungus. The numerous examples of sequestration of chemical defenses can be categorized as associational defenses, as they involve associating with chemically defended prey.

An organism might even be defended by protective species that aggressively attack would-be predators, especially if the protected species is a resource for the aggressive defender. For example, humans are protected by guard dogs because dogs view people as a resource that provides them with food, water, and shelter. Stop feeding the dog, and it is likely to look elsewhere for somebody to protect. There are several nonhuman examples of aggressive defensive associations, especially among ants. For example, aphids are insects that feed on the sugary phloem stream of plants. In the process of feeding and processing phloem, the aphids secrete large amounts of honeydew, which certain species of ants harvest and consume; that is, aphids provide ants with a resource. Ants tend to aphids in the same way that dairy farmers tend to their cows. The ants carry aphids to prime feeding locations, defend aphids from predators, and periodically "milk" the aphids of their honeydew by stroking them with their antennae.

Defensive Behaviors: Advertising and Trickery

Being chemically defended does not protect an animal from being accidentally eaten. Therefore, chemically defended animals often advertise the fact that they are nasty to avoid such accidents. This advertisement is often in the form of outlandish colors and patterns that flaunt the animal’s distastefulness to predators. Using bright warning patterns is called aposematic coloration.

One problem with aposematic coloration is the training of predators: Bright coloration is only useful if the predator understands the warning. Otherwise, the coloration simply makes the animal a conspicuous prey item. An interesting way that different species with aposematic coloration share the cost of training naïve predators is through mimicry. A predator that eats an individual of species A (assume species A is bright red with blue stripes) and vomits shortly thereafter may learn to avoid things that are red with blue stripes, though at the cost of that first individual’s life. This educated predator will now avoid other members of species A, and any other organism that looks like species A (the mimic), whether the mimic is toxic or not. If the mimic is toxic, the system is termed Müllerian mimicry. If the mimic is a palatable species that looks like a toxic model, the system is termed Batesian mimicry.

Mimicry is common within groups of closely related organisms (for example, snakes, butterflies, and bees) which are already similar in appearance. However, mimicry can also occur even when the model and mimic are distantly related. For example, there are caterpillars that mimic the head of a snake, moths that mimic the eyes of a cat, and beetles, moths, and flies that mimic stinging bees and wasps.

Mimicry or mimesis is one form of camouflage, which refers to any trait that disguises an organism. Camouflage is common among various animals across the food chain, as it can serve predators to better surprise prey as well as function as a defense mechanism. The most well-known type of camouflage is the use of body shape, coloration, patterning, or materials to make an animal difficult to see. Examples include moths that blend in with the bark of trees, stick insects that look like twigs of the plants they inhabit, and fish with pale underbellies and dark upper bodies to blend in with the sky when viewed from below and with the water or ground when viewed from above. Some animals tolerate the growth of other organisms on parts of their bodies to provide better camouflage, as in the case of sloths with algae growing in their fur. Certain species, including types of octopuses and chameleons, have the ability to actively change the color and pattern of their skin in order to better blend in with their surroundings.

Autotomy: Throw the Predator a Bone

Sometimes, despite the best defenses, a predator will get hold of a prey. If this happens, some animals are able to sacrifice a portion of their body to the predator, with the hope that the remaining parts will survive, and perhaps even regrow the lost parts. This ability to lose a body part intentionally is called autotomy.

Many simpler animals, such as sponges, cnidarians, and worms, have great regeneration abilities, and can regrow body parts well. In fact, these animals can even use regeneration as a form of asexual reproduction: Break the animal into four parts, and the parts will generate four complete individuals.

Sea cucumbers, in addition to being chemically defended, are able to eviscerate (autotomy of intestines) when harassed by a predator. These are not fast animals, so this action does not allow them to escape, but it might satisfy (or disgust) the predator enough to make it lose interest in the rest of the sea cucumber. Losing a large portion of its digestive tract interferes with feeding, but the sea cucumber can regenerate those parts of the gut that were eviscerated, restoring itself to original function. Sea cucumbers also play an important role in a defensive association with the pearlfish. When the pearlfish feels threatened, it locates the anus of a sea cucumber, then backs into its intestine, where it hides until the danger has passed.

The regenerative ability of higher animals is generally less than that of lower animals. However, autotomy does occur even in some vertebrates. Lizards are well known for their ability to release the tips of their tails when grabbed by a predator. The predator is distracted, and perhaps satisfied, by the wiggling piece of flesh, and in the meantime, the remainder of the lizard scampers off to safety. Geckos release skin instead of tails. The part of the skin that is grabbed by the predator is released, enabling the gecko to break free and escape.

Nutritional Defenses: Not Worth the Effort

Some animals, such as corals, jellyfish, anemones, and gorgonians (phylum Cnidaria), possess a type of combined structural and chemical defense in the form of specialized stinging cells called nematocysts. When nematocysts are stimulated, they rapidly discharge a barb that punctures the skin of a predator, often releasing toxic chemicals at the same time. The stinging sensation that people get when they swim into a jellyfish is caused by nematocysts. Some of these jellyfish stings are so potent that they can result in death.

Not only do many predators avoid jellyfish because they posses nematocysts, but predators may avoid jellyfish because they are jellylike, being composed of more than 95 percent water. It takes time and effort for predators to locate, handle, ingest, and digest prey. If the prey item is basically a bag of seawater (as jellyfish are), then predators might not bother eating these nutrient-deficient animals. Thus, these animals are "nutritionally" defended by being of little value to most consumers. Nutritional defenses are also used by some plants, but they are generally not an available strategy for animals other than jellyfish, as most animal tissue is relatively nutritious.

Principal Terms

aposematic coloration: brightly colored warning coloration that toxic species use to advertise their distastefulness to would-be predators

autotomy: the self-induced release of a body part

mimicry: a type of defense in which an organism gains protection from predators by looking like a dangerous or distasteful species

predation: broadly defined, any interaction in which one organism consumes another living organism, including herbivory (predation on plants), parasitism (predation by small organisms), and familiar predation (where one animal kills and eats another animal)

secondary metabolite: a biochemical that is not involved in basic metabolism, often of unique chemical structure and capable of serving a defensive role for the organism

sequester: to store a material derived from elsewhere. In defenses, some predators sequester defensive properties from their prey to defend themselves from their own predators

symbiosis: "living together"; a term that describes the association between two species in which one species typically lives in or on the other species. Parasitism is a common type.

Bibliography

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