DEFENSE MECHANISMSrypes of animal science: Behavior, ecology, evolutionFields of study: Biochemistry, ecology, entomology, ethology, invertebrate biology, marine biology

Principal TermsAPOSEMATIC COLORATION: brightly colored

warning coloration that toxic species useto advertise their distastefulness towould-be predators

AUTOTOMY: the self-induced release of abody part

M~~MICXY: a type of defense in which an or-ganism gains protection from predatorsby looking like a dangerous or distaste-ful species

PREDATION : broadly defied, any interac-tion in which one organism consumesanother living organism, includingherbivory (predation on plants), parasit-ism (predation by small organisms), andfamiliar predation (where one animalkills and eats another animal)

SECONDARY METABOLITE: a biochemical thatis not involved in basic metabolism, of-ten of unique chemical structure and ca-pable of serving a defensive role for theorganism

SEQUESTER: to store a material derived fromelsewhere. In defenses, some predatorssequester defensive properties fromtheir prey to defend themselves fromtheir own predators

SYMBIOSIS : “living together”; a term that de-scribes the association between two spe-cies in which one species typically livesin or on the other species. Parasitism is acommon type.

A.11 organisms are composed of fixed carbon,blomolecules, and mineral nutrients, and

therefore represent energy and nutrient resourcesfor consumers. To be successful in life, animalsmust avoid, tolerate, or defend themselves againstnatural enemies such as predators, parasites, andcompetitors. The term “defense” can be attributedto any trait that reduces the likelihood that an or-ganism, or part of an organism, will be consumedby a predator. There are several categories of de-fenses that have evolved in animals, includingstructural defenses, chemical defenses, associa-tional defenses, behavioral defenses, autotomy,and nutritional defenses. Animals often possessmore than one type of defense, thereby havingbackup plans in cast the first line of defense fails.The number of defenses devised by organisms is areflection of the strong selective pressure exertedby predators.

Structural Defenses: Being Hard and SharpStructures that defend animals can act as externalshields: sharp spines located externally or inter-nally, skeletal materials that make tissues too hardto bite easily, or weaponry such as horns, teeth,and claws. External structures that protect vulner-able soft tissues include the chitonous exoskeletonof crustaceans, the calcareous shells of corals, mol-lusks, and barnacles, the tests (skeletal plates) ofechinoderms, the tough tunic of ascidians, and thehard plates of armadillos. The pretty shells thattourists collect along beaches were once used toprotect a soft, delicate animal that lived inside theshell. Hard, protective shells remain after the ani-mal dies and can be used by other animals for pro-

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Sequestration

The production of chemical defenses is often as-sumed to be expensive because it requires resourcesthat might otherwise have been used for growth andreproduction. One way for a species to avoid suchcost is to sequester compounds produced by its prey.

Sequestration was first discovered in the mon-arch butterfly. Monarch caterpillars feed on milk-weeds and sequester the plant’s cardenolides. Milk-weeds produce toxic cardenolides that deter mostvertebrate herbivores, but monarchs have evolvedthe ability to tolerate and sequester these com-pounds. When blue jays were fed monarchs, the

‘birds soon regurgitated and learned to associate thisunpleasant response with eating monarchs. To helpadvertise their chemical nastiness and perhaps to in-crease the learning response of birds, monarchsevolved aposematic coloration. Other species of but-terflies,suchastheviceroyand thequeenbutterflies,

tection. For example, small fishes will retreat intoempty conch shells when they feel threatened bypredators, and hermit crabs live inside empty snailshells to protect their soft, vulnerable abdomens.

Some animals cover their bodies with sharpstructures that puncture predators that try to bitethem. The porcupine is a good example of a mam-mal that uses this defensive strategy. Porcupinesare covered with tens of thousands of long,pointed spines, or quills, growing from their backand sides. The quills have needle-sharp ends con-taininghundreds ofbarbs that make the quills dif-ficult to remove. Sea urchins are also covered withlong, sharp spines that deter would-be predators.Urchins can move their spines, and will directthem toward anything that comes in contact withthem, such as a predator. While porcupines andurchins are covered with multiple spines, sting-rays defend themselves from enemies by inflict-ing a wound with a single barbed spine. Thewound is extremely painful, giving these raystheir common name.

Predators have sharp claws and teeth that helpthem grasp, subdue, and consume their prey.

mimic monarchs, affording them protection frompredators that have already learned to avoid orangeand black butterflies.

Nudibranchs are a class of sea slugs that lack theprotective shell that mostof theirgastropodrelativespossess. Because they lack the physical defense ofa shell, they must protect themselves from othermechanisms, and have ingenious ways of doing so.Most nudibranchs do not produce their own de-fenses. Rather, they sequester the defenses of theirprey. For example, aeolid nudibranchs are famousfor sequestering functional nematocysts from theircnidaian prey. They transfer nondischarged nemato-cysts from their gut to their skin, thereby protectingthemselves from predators such as fish. Nudibranchsalso sequester secondary metabolites from their in-vertebrate prey, which effectively protect them frompredatory fishes.

These same structures, used offensively in hunt-ing, can also be used to protect themselves fromtheir own predators. Small predators such asbadgers, raccoons, and foxes can fend off largerpredators such as wolves and mountain lions withtheir weaponry. Rather than risk injury, the largerpredators will avoid a fight with the smaller pred-ator and seek a less risky meal, such as a rabbit orrn”“X

Chemical Uefenses: Poor Taste, Bad Smell, orToxic ChemicalsBoth plants and animals defend themselves by us-ing compounds that are distasteful, toxic, or oth-erwise repulsive to consumers. Most defensivecompounds are secondary metabolites of uniquestructures, but can also include more generic com-pounds such as sulfuric acid or calcium carbonate.Secondary metabolites get their name becausethey are not involved in basic metabolic pathwayssuch as respiration or photosynthesis (that is, pri-mary metabolic reactions), not because they are 01secondary importance. Indeed, many organismsprobably could not sur\?ve in their natural env-

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This badger bares his sharp and powerful teeth to ward of a threateningpresence. (Digital Stock)

ronment without the protecrmn of their secondarymetabolites.

Stink bugs get their names because of thesmelly secondary metabolites they release frompores located on the sides of their thorax. Thesesmelly compounds repel predators, and may evenindicate toxicity to the predator. These insects arecommon garden pests that are usually controlledwith chemical pesticides. However, it appears thatthe eggs of stink bugs are not defended againstrely-poly pill bugs, which can control stink bugnumbers (and hence, garden damage) by preyingon eggs.

Bombardier beetles take chemical defenses astep further, erupting a boiling hot spray of chemi-cals in the direction of a predator. To accomplishthis, the bombardier beetle has a pair of glandsthat open at the tip of its abdomen. Each gland hastwo compartments, one that contains a solution ofhydroquinone and hydrogen peroxide, and theother that contains a mixture of enzymes. Whenthreatened by a predator, the bombardier beetlesqueezes the hydroquinone and hydrogen perox-ide mixture into the enzyme compartment, wherean exothermic reaction that produces quinone

takes place. The large amountof heat generated brings thequinone mixture to its boilingpoint, and it is forcefully emit-ted as a vapor toward the threatAn average bombardier beetlecan produce about twenty louddischarges of repulsive, hotchemicals in quick succession.

Chemical defenses are com-mon among small, slow animalssuch as insects, sponges, cni-darians, and sea slugs, whichmight be limited in their abilityto flee from predators. However,chemical defenses are ratherrare among large, fast animals.One of the few mammals thatuses chemical defenses is theblack-and-white-striped skunk.Most people are familiar with

the smelly chemical brew emitted from these ani-mals, as it is distinctly detectable along roadswhen skunks get hit by cars, and can be detectedup to a mile from the location where a skunksprays. These mammals hold their smelly musk inglands located below their tail, and squirt the liq-uid through ducts that protrude from the anus.When threatened by a predator, the skunk raisesits tail and directs its rear end toward the predator.A predator that has had prior experience with askunk might retreat from this display, but if thepredator is persistent at harassing the skunk, thestriped mammal will deliver a spray of smellychemicals that usually sends the predator running.The musk also causes intense pain and temporaryblindness if it gets in the eyes of the predator.

Associational Defenses: The Guard DogApproachAssociational defenses occur when a species gainsprotection from a natural enemy by associatingwith a protective species, such as when humansgain protection from enemies by keeping a guarddog on their property. Types of protection pro-vided to the defended species through this co-

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evolution can be structural, chemical, or aggreb-sivc.

Small animals can avoid predators by using adefended species as habitat. For example, smallfishes defend themselves by associating with seaurchins, gaining protection by hiding among thesharp spines. Some species of shrimp inhabit thecavities and canals of sponges. Sponges areknown to be chemically and structurally de-fended against most predators, with the exceptionof angel fishes and parrot fishes. Finally, muchof the diverse coral reef fauna seeks protectionamong the cracks and the crevices in the reef.Reefs, slowly built by coral animals, are the largeststructures ever made by living organisms, andserve a protective role for thousands of speciesthat inhabit reefs.

Associational defenses can also be chemicallymediated. For example, bacteria that grow symbi-otically on shrimp eggs produce secondary me-tabolites that protect the egg from a parasitic fun-gus. The numerous examples of sequestration ofchemical defenses can be categorized as associa-tional defenses, as they involve associating withchemically defended prey.

An organism might even be defended by pro-tective species that aggressively attack would-bepredators, especially if the protected species is aresource for the aggressive defender. For example,humans are protected by guard dogs becausedogs view people as a resource that provides themwith food, water, and shelter. Stop feeding thedog, and it is likely to look elsewhere for some-body to protect. There are several nonhuman ex-amples of aggressive defensive associations, espe-cially among ants. Aphids are insects that feed onthe sugary phloem stream of plants. In the processof feeding and processing phloem, the aphids se-crete large amounts of honeydew, which the antsharvest and consume; that is, aphids provide antswith a resource. Ants tend to aphids in the sameway that dairy farmers tend to their cows. Theants carry aphids to prime feeding locations, de-fend aphids from predators, and periodically“milk” the aphids of their honeydew by strokingthem with their antennae.

Defensive Behaviors: Advertising and TrickeryBeing chemically defended does not protect ananimal from being accidentally eaten. Therefore,chemically defended animals often advertise thefact that they are nasty to avoid such accidents.This advertisement is often in the form of outland-ish colors and patterns that flaunt the animal’sdistastefulness to predators. Using bright warn-ing patterns is called aposematic coloration.

One problem with aposematic coloration is thetraining of predators: Bright coloration is onlyuseful if the predator understands the warning.Otherwise, the coloration simply makes the ani-mal a conspicuous prey item. An interesting waythat different species with aposematic colorationshare the cost of training naive predators isthrough mimicry. A predator that eats an individ-ual of species A (assume species A is bright redwith blue stripes) and vomits shortly thereaftermay learn to avoid things that are red with bluestripes, though at the cost of that first individual’slife. This educated predator will now avoid othermembers of species A, and any other organismthat looks like species A (the mimic), whether themimic is toxic or not. If the mimic is toxic, the sys-tem is termed Miillerian mimicry. If the mimic is apalatable species that looks like a toxic model, thesystem is termed Batesian mimicry.

Mimicry is common within groups of closelyrelated organisms (for example, snakes, butter-flies, and bees) which are already similar in ap-pearance. However, mimicry can also occur evenwhen the model and mimic are distantly related.For example, there are caterpillars that mimic thehead of a snake, moths that mimic the eyes of a cat,and beetles, moths, and flies that mimic stingingbees and wasps.

Autotomy: Throw the Predator a BoneSometimes, despite the best defenses, a predatorwill get hold of a prey. If this happens, some ani-mals are able to sacrifice a portion of their body tothe predator, with the hope that the remainingparts will survive, and perhaps even regrow thelost parts. This ability to lose a body part inten-tionally is called autotomy.

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Many lower animals, such as sponges, cnidari-ans, and worms, have great regeneration abilities,and can regrow body parts well. In fact, these ani-mals can even use regeneration as a form of asex-ual reproduction: Break the animal into four parts,and the parts will generate four complete individ-uals.

Sea cucumbers, in addition to being chemicallydefended, are able to eviscerate (autotomy of iw&tines) when harassed by a predator. These arenot fast animals, so this action does not allowthem to escape,but it might satisfy (or disgust) thepredator enough to make it lose interest in the seacuciunber. Losing a large portion of its digestivetract interferes with feeding, but the sea cucumbercan regenerate those parts of the gut that wereeviscerated, restoring itself to original function.Sea cucumbers also play an important role in a de-fensive association with the pearlfish. When thepearlfish feels threatened, it locates the anus of asea cucumber, then backs into its intestine, whereit hides until the danger has passed.

The regenerative ability of higher animals isgenerally less than that of lower animals. How-crcr, dutotomy does occur even in some v&e-bra@. Lizards are well known for their ability torelease the tips of their tails when grabbed by apredator. The predator is distracted, and perhapssatisfied, by the wiggling piece of flesh, and in the

meantime, the remainder of the lizard scampersoff to safety. Geckos release skin instead of tails.The part of the skin that is grabbed by the predatoris released, enabling the gecko to break free andIZGp.

Nutritional Defenses: Not Worth the EffortSome animals, such as corals, jellyfish, anemones,and gorgonians (phylum Cnidaria), possess atype of combined structural and chemical defensein the form of specialized stinging cells callednematocysts. When nematocysts are stimulated,they rapidly discharge a barb that punctures theskin of a predator, often releasing toxic chemicalsat the same time. The stinging sensation that peo-ple get when they swim into a jellyfish is causedby ncmatocysts. Some of these jellyfish stings areso potent that they can result in death.

Not only do many predators avoid jellyfish be-cause they posses nematocysts, but predatorsmay avoid jellyfish because they are jellylike, be-ing 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 isbasically a bag of seawater (as jellyfish are), thenpredators might not bother eating these nutrient-deficient animals. Thus, these animals are “nutri-tionally” defended. Nutritional defenses are alsoused by plants, but they are generally not an avail-

Acacia-Ant Mutualism

Acacia ants are found throughout Central Amer-ica and only inhabit the hollow thorns of acacia trees.The ants harvest and feed on nectar from extrafloralnectaries and special leaf tips, called Bcltian bodies,produced by the acacia trees. In a sense, these antsare herbivores of acacia trees. However, in return forthe shelter and food provided by the tree, the antsdefend the tree from other herbivores and competi-tars. When an herbivore begins feeding on the acaciatree, the ants release an alarm order that signals thecolony to attack the herbivore with painful bites andstings. Thebusyants also remove competingvegeta-tion that comes in contact with the tree or that grows

near the trunk of the tree. The weeding activity of theants results ina circleof cleared ground surroundingthe acacia tree that also protects the tree from thedamaging effects of fire. This mutualistic relation isobligate, meaning that each species is absolutely de-pendent on the other: The tree cannot survive with-out the ants and vice versa. Other species take ad-vantage of the acacia-ant mutualism: Some birdspreferentially nest in acacia trees, presumedly be-cause the ants deter egg predators as well as herbi-vores. How the birds avoid attack by the ants is notwell understood.

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able strategy for animals other than jellyfish, as Death and dying; Displays; Ethology; Exoskel-most animal tissue is relatively nutritious. etons; Food chains and food webs; Herbivores;

-Greg Cronin Instincts; Learning; Mimicry; Nutrient require-See also: Adaptations and their mechanisms; mats; Pheromones; Poisonous animals; Preda-Camouflage; Carnivores; Claws, nails, and hooves; tion; Regeneration; Shells; Teeth, fangs, and tusks;Coevolution; Communication; Competition; Territoriality and aggression; Vocalizations.

BibliographyCloud&y-Thompson, John L. Tooth and Claw: Defensive Strategies in the Animal World.

London: J. M. Dent & Sons, 1980. Areadablevolume that covers defensemechanismsin great detail and with many examples.

Edmunds, Malcolm. Defence in Animals. Burnt Mill, England: Longman, 1974. Technical,comprehensive guide to antipredator defenses, and the evolutionary arms race be-tween predator and prey. Contains photographs, illustrations, and quantitative re-sults from experiments.

Evans, David L., and Justin 0. Schmidt, eds. Insect Defenses: Adaptive Mechanisms andStrategies of Prey and Predators. Albany: State University of New York Press, 1990. Anedited volume that examines the many ways that the most successful group of organ-isms on earth deals with predators.

Kaner, Etta. Animal Defenses: How Animals Protect Themselves. Toronto: Kids Can Press,1999. A colorfully illustrated book aimed at adolescents. It places defenses of animalsin context of human behavior.

McClintock, James B., and Bill J. Baker, eds. Marine Chemical Em&y. Boca Raton, Fla.:CRC Press, 2001. This technical volume is the most current, comprehensive book onmarine chemical ecology. The book provides cellular, physiological, organismal, evo-lutionary, and applied perspectives creating a high-resolution snapshot of the field atthe start of the twenty-first century.

Owen, Denis. Survival in the Wild: Camouflage and Mimicry. Chicago: University of Chi-cago Press, 1980. Alook at animals that appear to be something other than what theyare. Some try to look like their background to avoid detection, others try to appearlike a dangerous animal: while others are brightly colored to advertise nastiness.Easy reading with numerous illustrations and photographs.