Plants and animals need defenses to keep from being eaten. Nearly all animals have predators of one kind or another, and eluding capture usually means running away, hiding, or both. But when avoidance fails and the predator gets too near, most animals still have an effective last line of defense. Plants need defenses too, for protection from the many animals that feed upon them. Plants can’t run away or hide, but they have evolved a diverse armory of useful adaptations for fending off herbivores.
Teeth and claws, hooves and horns, talons and beaks make us wary around wild animals, even those much smaller than ourselves. Many of these are the tools these animals use for getting their food, but they are also important in defense. Showing these weapons is a good way to scare away a foe or avoid a close encounter. A dog bares its teeth, a porcupine raises its quills, and a snake pulls its head back to show its readiness to strike. All these are effective warnings that say, “I have weapons and I’m ready to use them.” Predators learn to weigh cost against benefit in any encounter, and the prey’s size, health, strength, and weaponry all come into consideration.
Looking bigger or fiercer is a ruse used often in the animal world. Many mammals can erect their fur, and birds can raise their feathers to increase their apparent body size. Toads inflate their bodies with air to look bigger, and owls spread their wings. Often looking scary is accompanied by a particular posture and warning sounds, for added effect. A rattlesnake gives a warning rattle as it coils for a strike, a dog growls as it bares its teeth, and a snapping turtle gives a fierce hiss before snapping its jaws. Cats arch their backs, raise their fur, hiss and whine, bare their teeth, and lash out with claws extended – an arsenal of warnings.
Even relatively defenseless creatures might have a surprise in store for a predator. Earthworms and slugs have unappetizing, slimy coatings. When alarmed, worms squirm and wiggle, and slippery fish flop about, making them hard to grab and hold. Some caterpillars twitch violently with similar effect. Playing dead is a hoax used most famously by the opossum and also by some insects, snakes, and spiders, among others. Another defense is to drop a body part, which often continues to twitch and distract the predator. Daddy-long-legs can detach a leg, and some lizards and salamanders can release the tip of the tail, then later regenerate all or part of it.
Many forms of camouflage offer protection for animals. Snowshoe hares grow a coat of white fur in winter. Mimics like stick insects look like twigs, katydids look like bright green leaves, and some caterpillars look a lot like bird droppings. Stripes and spots are helpful disguises, for they disrupt the outline of an animal, making it hard to pick out from its backdrop. A raccoon’s bandit mask and striped tail help it to blend in, as do the spots on a fawn.
Among plants, waxy coatings, sticky resins, and fuzzy coverings are part of the battery of physical defenses. Many leaf-eating or bark-piercing insects avoid conifers because of their tough leaf coatings and resinous sap that oozes out of any wound. Plants like mullein have thick coverings of hairs that protect their tender leaves from insects. Other common structural plant defenses are thorns, prickles, and spines. Thorns on roses, prickles on thistles, and spines on hawthorns and cacti are just a few examples. These daunting devices are modified leaves, stems or branches that have evolved over time to protect the plants from a host of herbivores, from grasshoppers to moose.
Some animals have prickles, too, most notably the porcupine, with its thousands of quills. Each quill is a modified hair, with a sharp tip covered with minute scaly barbs. These make it very easy for the quill to pierce skin (half the pressure needed for a hypodermic needle) and very hard to pull out backwards. Every muscle movement drives the quill deeper. A porcupine cannot throw or shoot its quills, but they are quite loosely attached. One swat with its tail leaves its attacker with a painful muzzle full of quills.
Prickles that sting are well known among bees, wasps, and hornets. Stingers have sharp points that can pierce the skin and inject poison. Some plants can inject venom too. Stinging nettles have tiny hairs on the leaves and stem that break off easily when touched and inject irritants that cause a burning sensation on the skin, effective deterrents for some grazing animals. Still, the caterpillars of several species of butterflies and moths can tolerate or avoid the stings and feed on them with little competition.
Having a hard covering is a good defense for plants and animals alike. Thick bark protects trees from insects and birds as well as weather and fire, but, as with any defense, animals have also evolved ways to penetrate this armor. Woodpecker beaks and insect jaws and saw-like ovipositors can pierce the bark, at least once it is starting to decay. The hard shells of nuts protect tree seeds from herbivores like squirrels. It’s hard work to gnaw through the thick, dense shell of a butternut. The American chestnut seed is protected by a very hard, very spiny shell – a double deterrent. Some animals have hard shells too, like turtles, snails, clams, and mussels. The hard outer skeleton of insects gives them some amount of protection from their predators.
Chemical defenses – tasting or smelling bad – are found in both plants and animals. Skunks have poor eyesight and short legs, so they can’t move fast. But they have a powerful weapon – two scent glands at their rear ends, from which they can spray a nasty-smelling liquid very accurately about ten feet. It can cause intense choking and even temporary blindness, and it is so strong that a human can detect the odor as much as a mile away. Mammalian predators gain great respect for skunks, so they rarely have to spray – just being black and white, hissing, stamping, and raising their tails when threatened is usually enough to make a predator back off. The great horned owl, however, has a poor sense of smell and is happy to dine on skunk.
Plants use chemical defenses more than any other kind. Some noteworthy examples are poison ivy and poison parsnip. Both produce rashes on humans, one that itches and the other that burns. These defenses probably evolved to deter insects; the effect on us is not immediate but takes a few days to develop. Many birds eat the berries of poison ivy with no ill effect. The bitter tastes of buttercup and jack-in-the-pulpit from toxic chemicals in the leaves are meant to deter herbivores of all kinds. Milkweed contains a milky juice that is poisonous and unpalatable to most animals. A few species, like monarch caterpillars and milkweed bugs and beetles, however, tolerate the poison and incorporate it into their own bodies – making these insects inedible as well. In the animal world, many amphibians have toxins in their skins, notably the American toad and the red eft, the adolescent form of the red-spotted newt. Having once picked up a toad or an eft, a dog is unlikely to bother either again.
How do protections like thorns, shells, or noxious odors evolve? The most effective defenses are passed on from generation to generation and constantly improved through natural selection. Nut shells get thicker, stronger, more dense. But then, squirrels’ teeth may get sharper and stronger in response. Every defense is eventually breached. However, it takes energy for plants to make thicker shells or for squirrels to grow stronger teeth. At some point the cost may outweigh any additional benefit, thus curbing the arms race between predator and prey, herbivore and plant.
In close encounters, plant and animal defenses come into play. Thorns and spines, stingers and armor, growling, coiling, or playing dead are all adaptations that give plants and animals protection from being eaten. In the struggle for survival, these daunting defenses give them the edge.
Alcock, John. Animal Behavior: An Evolutionary Approach. 10 ed. Sunderland, MA: Sinauer Associates, Inc., 2013.
Ingrouille, Martin. Diversity and Evolution of Land Plants. London: Chapman and Hall, 1992.
Mance, Dave III. “Avoiding Rash Decisions: A Guide to Plants You Shouldn’t Touch.” Northern Woodlands Magazine, 2009: http://northernwoodlands.org/articles/article/avoiding_rash_decisions_a_guide_to_plants_you_shouldnt_touch
Rezendes, Paul. Tracking and the Art of Seeing: How to Read Animal Tracks and Sign. 2nd edition. New York: Harper Collins, 1999.
Foster, Steven and Roger A. Caras. A Field Guide to Venomous Animals and Poisonous Plants. New York: Houghton Mifflin Co., 1994.