The complexity of biochemical, physiological, energetic and mechanical systems that make up an organism are extremely vulnerable to threats from the environment, and the greatest danger comes from other organisms. Since the beginning of evolution, each lifeform has been a vital resource for other ones, and it has has te need to defend itself, and to fight back.
Also see: List of camouflage methods
The most secure way to survive predation is avoiding to be recognised as a prey. Camouflage involves an organism modifying its own appearance for this purpose (it's called mimicry when it occurs in such a way to make it similar to another organism).
CrypsisIn crypsis, an organism blends itself in the environment, effectively becoming invisible (of course, it's also useful to predators to approach the prey). A disruptive pattern is among the simplest way to do this: the spot on a leopard's fur, or the mottled feathers of a nightjar break up the outline, as well as a complex system of appendages; zebras' stripes meld their shape with the other individuals in the herd. Generally, animals that live in the tall grass have vertical stripes, while those that live among bushes or in the underbrush have broad spots. Often, these animals have dark stripes or marks that hide the eyes, like giant pandas, raccoons, killer whales or common frogs.
The shadow is a primary mark of a three-dimensional shape: for this reason, many animals have a darker back, or generally an upper side darker than the lower side; several squids and fish resort to counter-illumination, that is, a ventral source of light. For low and squat organisms, lateral flanges that make the body wide and flat on the ground help to hide the shadow, especially if they have lighter fringed sides.
Some animals use self-decoration to hide themselves: the decorator crab covers itself with pebbles, sponges and seaweed, while caddisfly larvae use sand, twigs, or broken shells. Octopi, squids and some flatfish and lizard can actively change colour; foxes, hares and ermines change their coat with the season.
A very pecular form of crypsis is found in transparent species, such as comb jellies, young glass frogs, glass knifefish and others (see here). Jellyfish's main tissue, mesoglea, is devoid of cells and rich in water, and thus often transparent, but muscle and other tissues can be made so if the proteins are organized in fibrils smaller than the wavelength of visible light (the effect works best under water; outside, a transparent organism would look like a bag of water). The ice fish lacks haemoglobin in its blood, making it completely clear, though this requires a greater heart strain and forces it to live in oxygen-rich water. Anyway, some functions (protection from harmful radiations, photosynthesis, sight) require light absorption and their organs can never be invisible. Some fish, such as herrings and marine hatchetfish, can imitate transparency with a flat and thin body covered in guanine-rich silvery, highly reflective scales.
AposematismAposematism is the opposite of crypsis: rather than hiding, the organisms makes itself stand out in the environment signalling itself as an unappetizing prey. It's most common in toxic species (frogs and salamanders, bees and wasps, beetles, caterpillars, snakes and Heloderma lizards); it's usually a combination of black and a vivid colour such as red or yellow (or, rarely, blue). Green and brown are avoided, as they're common background colours, but of course this would depend from the dominant flora and geology: in a planet where most plants are red, green could be a common warning colour. In the (venomous) Hapalochlaena octopi, blue rings appear immediately before the bite (see "deimatic behaviour", below). In actually lethal species, though, aposematism is rare, since its success depend from the learning (and therefore, survival) of the predator.
MimesisMimesis, or true mimicry, involves an organism trying to be as similar as possible to anther one. Examples of cryptic mimicry include stick insects and leaf insects, the leafy seadragon and the mata-mata turtle, or the orchid mantis. Species that imitate visible but unattractive objects include several butterflies that look like dry leaves, as well as the spider Celaenia excavata, that mimics bird droppings.
Batesian mimicry is a form of mimicry in which a non-dangerous organism mimics a dangerous organism. Several snakes, beetles, butterflies, etc. have a false aposematic coloration, for example the moth Sesia apiformis, which has the yellow-black stripes and the transparent wings of a wasp; the hawk-cuckoo imitates true hawks in shape and plumage; several caterpillars and butterflies have eye-like spots that make them look much bigger than they really are. By changing its colour and body shape, the mimic octopus can imitate several dangerous animals, such as sea snakes and lionfish.
In mullerian mimicry, several dangerous species adopt the same aposematic coloration, strengthening each other's defense. For example, most bees and wasps have the same yellow and black pattern; the heliconians butterflies have a similar black, yellow and red colour scheme.
In the somewhat unusual martensian (or emsleyan) mimicry, a lethal species (say, the coral snake) imitates the pattern of a less dangerous one: the lethal species does not allow the predator to survive, and therefore to learn to avoid it, but the other one does, and makes an aposematic warning actually useful, so that both species can survive without being attacked.
Surviving the attack
If the prey is sighted and attacked by the predator, it can flee, fly off, hide in a den, or find safety in number: adult musk oxen arrange themselves in a circle with their horns pointing outwards, and the calves at the centre; hamadryas baboons assume a similar arrangement; some gazelles and deer resort to stotting, a particular leap with stretched legs and arched backs that is believed to be a signal of the individual's high fitness; mayflies and anchovies simply live in groups so large that no predator can ever kill more than a very small fraction of them.
Even if the attack is successful, the prey can try to avoid consumption. Many animals, such as opossums, pretend to be dead and decomposing, swelling their abdomen and secreting a scent of rotting meat; horned lizards make their eye capillaries burst, squirting blood on the predators (autohaemorrhaging). Others resort to autotomy (self-amputation), often of parts that regrow: lizards lose their tail, crabs their legs, octopi their tentacles, sea cucumbers vomit their own gastrointestinal tract; several species of ants and termites bring this phenomenon to the extreme with autothysis, where organs or venom glands inside some individuals burst, killing them and spilling noxious chemicals outside.
Other behaviours focus on distracting the predator: many birds pretend to be injured to lure the predator away from their nest, while plovers incubate a fake nest distant from the real one; blackbirds, corvids and gulls (and some mammals such as meerkats) attack or harass a predator in large groups (mobbing), even though none of them can harm it alone, at least to prevent it to hunt effectively. Most cephalopods can expel ink, a dark melanine-rich liquid, opaque and probably irritating, that hides their escape; in some cases, if the ink is particularly rich in mucus, it can form pseudomorphs, semi-solid shapes that can resemble the animal.
Deimatic behaviour is a threatening or startling behaviour meant to scare or distract the predator: cephalopods and chameleons turn bright red or other aposematic colours; leaf insects and mantises spread their legs and reveal coloured wings; several butterflies and moth reveal false eyes painted on their wings; many mammals bristle their fur to appear larger; snakes and cats hiss, tarantulas raise their front legs, frilled lizard open their frill, pufferfish engorge their body and so on.
Also see: Exoskeletons
Many organism prevent predator from consuming them through an armour that protects them from harm. Often, rigid structural elements are employed (wood in arboreal plants, the exoskeleton on arthropods); a protective external shell can be developed by organisms that otherwise lack rigid support (the lorica of some protozoans, the frustule of diatoms, the shell of mollusks) or that have an internal skeleton (the armour of ostracoderms, placoderms, ankylosaurs and pangolins, the carapace of turtles and armadillos, the scutes of crocodiles). Several plants, such as the holly, have a waxy cuticle on their leaves that makes them harder to chew and digest, and injuries at the stem release resin, a thick hydrocarbon-rich fluid that traps insects and seals open wounds. Another option to mitigate damage is ablative armor, that is, a structure which will come off leaving the rest unharmed. This can range from an actual coating ( such as scales on moths, silverfish, and herring) to an approach like shedding a tail in order to escape a predator (see "autotomy" above).
Even relatively soft armours can be made more effective with spines that can injure the attacker, as with sea urchins, Murex snails, cacti, horned lizards, hedgehogs, porcupines, etc. Trichomes (surface hollow cells, similar to hairs) are quite common in plants and protozoans; they take the shape of harpoons, arrowheads, hooks or antlers that block or even injure smaller herbivores; often, they release toxic chemicals (as in nettles; see below). Note that, in botany, a distinction is made between thorns (part of the stem or branches), spines (part of leaves) and prickles (hair-like protrusions).
Examples of mutualistic defense are known: the hollow spikes of Acacia collinsii host ants that attack any herbivore; clownfish inhabit sea anemones whose venom does not harm them thanks to a layer of mucus; some species of crabs bring sea anemones on their back.
A simple method to be unattractive to predators is concentrating toxins (harmful substances of organic origin) in the tissues. Poison is mainly employed as a defense in plants and mushrooms (toxic animals tend to use venom to kill preys or to strike back at the attacker; see below). See here and here a list of poisonous plants and fungi. Compare the lists of poisonous and venomous animals.
A skin cover of toxic mucus is common among amphibians such as poison dart frogs. Nettles are well-known for having stinging bristles formed by hollow trichomes, easily broken by any contact, that release formic acid; such structures have also appeared in at least eleven families of butterflies, in form of hair, down or spines that cover the caterpillars.
While animal toxins are usually a mixture of different chemicals (mostly proteins), poisonous plants use simpler substances that can easily be classified according to their chemical nature. For example:
- Alkaloids are strongly basic chemicals derived from aminoacids, and they can activate or inhibit enzymes, interfer with fat and sugar storage or with nervous transmission, bind to nucleic acids to block protein synthesis or outright destroy cells.
- Cyanogenic glycosides, such as the amygdalin found in bitter almonds, are stored in vacuoles and then released together with enzymes that turn them in highly toxic hydrogen cyanide.
- Terpenoids or isoprenoids are lypids made up by isoprene units; monoterpenoids (2 isoprene units) form essential oils such as menthol, capsaicin, anise and camphor, while diterpenoids (4 isoprene units) are toxic, and often founds in saps and resins.
- Phenols are hydrocarbons similar to alcohols that include lignin, cannabinoids and some neurotransmitters, and especially tannins, bitter and astringent chemicals that can interfere with protein digestion.
- Other defensive toxins employed by plants include formic acid (found in nettle hairs) and genistein, a substance found in clover that makes sheep temporarily sterile.
Attack and active defense
- The most common weapon in the animal kingdom is the mouth, as it's usually already provided with structures apt to demolish food. Most vertebrates have two hinged jaws lined with teeth, often specialized to cut, tear, gnaw, crush or chew; being made of dentin, a tissue rich in apatite, they're the hardest organic structure on Earth. Placoderms (an extinct group of armoured fish) lacked teeth, but had instead sharp bony plates called gnathal plates. Arthropods have a variety of rigid mouthparts to cut and chew, often derived from modified legs (and thus with vertical opening). Bird jaws are covered in a horny beak that can have a sharp edge; the beak is also horn-covered bone in turtles, sirens and ornithischian dinosaurs, chitine in cephalopods and fused teeth in pufferfish and parrotfish.
- Claws are another common mechanical weapon. They're horny (keratinous) structures at the extremity of the limbs of reptiles, birds and mammals (with the exception of the Xenopus laevis toad, they don't exist in amphibians); insects and spiders also have claw-like hooks called ungues. Nails are flat and broad claws used to dig or climb (as in primates); cats have muscles that pull the claws back in a sheath. Large claws are a common adaptation among myrmecophages, such as anteaters, to tear open anthills and hives. Sharp, curved claws called talons are the main weapon of several birds of prey such as hawks and owls; cassowaries have claws 13 cm long.
- A sharp bony blade is found on either side of the tail of surgeonfish. Billfish (such as swordfish and sailfish) have a long and pointed bony rostrum ("sword") that isn't used to stab preys but rather to slash at them with the edge. In a curious example of convergent evolution, sawfish and sawsharks have independently evolved a long, flat saw-like rostrum with the edge covered in pointy denticles.
- In several groups of crustaceans, the first pair of limbs is modified in pincers (or chelae; also called "claws") used to fight or to carry food; they have a mobile "finger", formed by the last segment (dactylus), that can be closed with another "finger" that extends from the previous segment (propodus); see here. Chelae-bearing limbs (chelipeds) are also found in scorpions, where they're developed from a different kind of appendage (chelicerae). A very particular use of pincers is found in Alpheidae shrimps. They can violently close one of their pincers, much larger than the other, creating a cavitation bubble that then collapses, creating a pressure of 80 kPa (enough to kill small fish), a sound intensity of 200 decibel (louder than a gunshot) and, through sonoluminescence, an extremely brief emission of light and a temperature of 5000 K.
- A very simple method to damage another organism through kinetic energy is the use of clubs: heavy and blunt bodyparts that are used to hit the enemy. Gorillas and polar bears often swing their heavy arms as clubs, while zebras, giraffes, ostriches and kangaroos kick with their feet (it's not uncommon to found in the african savanna lions with their skull or ribs cracked by a giraffe kick). Specialized clubs are not very common: stegosaurs and ankylosaurs used their spiny or knobby tail (thagomizer). Mantis shrimp can use their raptorial limbs to hit their prey (mollusks or crustaceans) with an acceleration of 10400 g, similar to a bullet, also generating a cavitation bubble, though not as strong as Alpheidae. The extinct flightless ibis Xenicibis xympithecus, that lived in Jamaica 10 000 years ago, had heavy bones in its wings that it might have used as clubs; the long "handle" before the mass centre helps making the impact stronger. Some Furahan predators use similar weapons.
- Developed several times in vertebrates, horns are, in a strict sense, structures of bones covered in keratin; they're found in Bovidae (cattle, goats, antelopes) and pronghorns, in horned lizards, in some chameleons and probably in Ceratopsidae. Spurs are claw-like strictures, also formed by horn-covered bone, found on bodyparts other than head or toes; they're typical of male landfowls, and they're also found in tortoises, monotremes and lemurs. Different structures are also called horns: they're made of keratin only in rhinos, ossified cartilage in giraffes, bone only in deer and chitin in Dynastes beetles. Tusks, derived from teeth (mouse deer, narwhals, warthogs, elephants, walruses) or maxillae (stag beetles) can have the same function of horns. It's been hypothesized that horns are developed by large and visible organisms in open environments. Often (e.g. rams and ibexes) large horns are used to fight between conspecifics; in this case, they usually point back and lack a sharp edge.
- In some cases, the tongue can also be used as a weapons. Cats have tongues covered in sharp and pointed papillae that help them licking every trace of flesh off bones; mollusks have a similar structure, the radula, a chitinous ribbon covered in teeth used as a cutting weapons in predatory gastropods such as the ghost slug and turned into a poisonous harpoon in cone snails (see below). Some vertebrates such as lungless salamanders, frogs and chameleons have a long and adhesive tongue propelled forward by elastic structure with an acceleration beyond 40 g; the salamander Bolitoglossa is able to launch its tongue in only 11 ms.
- Cephalopod limbs (specifically, all the length of the arms and the tip of the true tentacles of octopi, squids and cuttlefish) are lined with suckers; they're formed by circular muscles with a chitinous ring where a low pressure is produced by a muscular piston, and they're very useful to grapple preys or competitors; the chitinous ring is often sharp or hooked, as in the Humboldt squid. The deep-sea squid Octopoteuthis can detach its own limbs, in a unique form of "attack autotomy" (see above), that then keep attacking the enemy on their own.
- Constriction is an attack typical of large snakes, and especially of Boidae. The snake wraps its body around a smaller prey, crushing it with its coils. Constriction (which bends the body vertically in Boidae and horizontally in Colubridae) doesn't kill the prey by breaking its bones, but rather by suffocation; however, the quicker death of smaller preys suggests that other mechanisms are involved as well, perhaps a pressure increase that causes a cardiac arrest. Even the smallest constrictors can develop up to 3000 Pa of pressure.
- Interesting as it may be, projectiles are not actually unknown in biology. Liquid projectiles are varied, and include venom (also see here), venomous silk, and slime; the archerfish can spit water up to 3 metres far, causing insects to fall in the river; bombardier beetles expel scalding water (see below); some penguin species expel liquid feces, and a petrel vomits a mixture of wax and triglycerides (which is also food for its offspring). Solid projectiles are rarer; for the most part, they are limited to fecal matter - silver spotted skipper catterpillars can project fecal matter up to 1.4 meters. Theoretically, though, almost any disposable object could be used as a solid projectile; bits of bone coated in discarded tissue could be ejected at considerable speeds, and, due to increased weight, considerable force. Antlion larvae throw sand to their preys, while ground squirrels, elephants and apes can throw sand, stones and branches.
- Several fish have developed electric organs for a sensorial purpose, but there are at least three groups where an organic electric field is used as a weapon: electric catfish, electric eels and electric rays. The electric eel has specialized organs made up by hundreds of electrocytes, flat disc-shaped cells that pump out sodium and potassium ions, producing a tension up to 600 V between the inside and the outside. In a fraction of a second, the ions then return in the cells, becoming an electric current (albeit not a very strong one; usually below 1 A).
Toxins used by animals as offensive weapons can be classified according to their effects, such as the kind of biological function they target:
- Hemotoxins destroy red blood cells (hemolysis), interfere with blood clotting and damage tissues. While the death from hemotoxins is not especially fast, so a predator might have to track the prey after the attack (and of cours eit wouldn't be very effective as a defense) it helps it to digest the prey's tissues by breaking down proteins. Viperid snakes such as rattlesnakes commonly use hemotoxins.
- Cytotoxins and necrotoxins kill cells, causing the death and decomposition of all living tissues affected by the toxin. The effects are slow, but devastating. The puff adder and the brown recluse spider are examples of predators that employ necrotoxins.
- Neurotoxins block synapses and sometimes destroy neurons; they can induce death in a fraction of a second. Simple chemicals such as lead, nitric oxide and ethanol can have limited neurotoxic effects; botulinum and tetanus are instead extremely powerful (in fact, they're the two most powerful toxins in the world). Widow spiders, most scorpions, cobras, box jellyfish and cone snails are predators that employ neurotoxins.
Besides the nature of the venom itself, there are different strategies to inoculate it into the enemy's body:
- Some venomous animals, such as Heloderma lizards, Komodo dragons, solenodonts and some shrews, bite their opponents while secreting venom in their saliva.
- The model of the syringe (a hollow needle connected to a gland that pumps venom directly inside a body) has arisen several times in the animal kingdom. The most obvious example is the snake fang; different groups of snakes can be recognised from the shape and disposition of teeth, in aglyphs (uniform teeth and no fangs, e.g. pythons), opisthoglyphs (posterior grooved teeth, eg. colubrids), proteroglyphs (a pair of hollow anterior fangs, e.g. cobras) and solenoglyphs (a pair of fangs that point backwards and a mouth that can be opened at 180°, e.g. vipers). Other examples of venom-bearing "syringes" include the spider chelicerae; centipede forcipules; the stinger of bees and wasps (derived from a modified ovipositor); those of scorpions; those of weevers, lionfish and stonefish (derived from fin rays); those of stingrays (derived from skin denticles); those of cone snails; the venomous spur of male platypodes; the quills of Lonomia caterpillars.
- Cnidocysts or nematocysts are structures found in specialized cells (cnidocytes) typical of the phylum Cnidaria (jellyfish, hydrae, sea anemones, etc.). Cnidocytes concentrate calcium ions that raise the osmotic pressure inside the cell, recalling water that quickly fills it and violently expels a sort of harpoon with an acceleration up to 40 000 g. The nematocyst is used to inoculate venom, but other cnidocytes have structures to stick to them with an adhesive coating (ptychocysts) or to wrap around them with a string (spirocysts). Nematocyst venom can be extremely painful and quickly lethal even to large-sized organisms.
- Some New World tarantulas have developed nettle-like urticating bristles that they can detach and throw at the opponent by rubbing their legs against their abdomen. The bristles, often barbed, can embed themselves in the skin and the eyes of large vertebrates, causing painful and sometimes fatal rashes and edemas (swelling due to fluid accumulation).
- Toxic secretions exist in a huge variety of forms, but the most impressive example is perhaps the bombardier beetle's boiling spray. These beetles have two abdominal chambers, one of them filled with hydroquinone (C6H4(OH)2) and hydrogen peroxide (H2O2), and the other with water and catalytic enzymes catalase and peroxidase (see here). When a muscular contraction mixes their content, hydroquinone and hydrogen peroxide react producing water, oxigen and paraquinone (C6H4O2); the reaction greatly raises the temperature, and the expansion of oxygen forces out from the abdomen the mixture of paraquinone and scalding water.
- Several small carnivores (badgers, wolverines, etc.) have anal glands that secrete a foul-smelling liquid. This feature is refined by skunks, which produce sulfur compounds, thiols or mercaptans, that can be expelled up to 3 metres far. Skunk thiols, similar to butanethiol, can cause severe irritation to eyes and mucous membranes, and their smell can be perceived at 1 km away.