Script:
Listen to part of a lecture in a zoology class. The professor is talking about animal defenses.
One of the most important relationships between different animal species is predation—the predator-prey interaction, in which a predator eats a prey. Predator species have several adaptations that help them catch prey species. Prey species have adaptations, too—physical and behavioral adaptations that enable them to elude predators and avoid being eaten. These defensive adaptations evolved in prey species through repeated encounters with predators over evolutionary time.
Some animal defenses are passive, such as hiding. Some defenses are active, such as escaping. Fleeing—running away—is the most direct anti-predator response, but it requires the animal to expend a lot of energy. A rabbit uses up a lot of energy running away from a lynx. Many animals avoid expending too much energy by escaping into a shelter.
Several prey species have some sort of vocalization—an alarm call—to announce the presence of a predator. The alarm call often triggers a behavioral defense called mobbing. During mobbing, the prey turns the tables and attacks the predator. For example, when a chickadee spots a threat—say, an owl—it calls out the alarm. The chickadee starts scolding the owl, sometimes actually striking it from behind. Birds of other species may fly in to investigate, and often participate in the mobbing. The other birds chase, dive-bomb, or surround the owl, usually vocalizing loudly. Their intent is to encourage the “enemy” to move on to another area.
Some animals rely on defensive coloration. A well-known example of defensive coloration is camouflage, which makes prey difficult to spot against a background of similar color. All a camouflaged animal has to do is remain still to avoid being seen. Thus, camouflage is probably the most effective passive defense. Incidentally, some predators also use camouflage, especially predators who lie in wait for prey and have to blend into their environment.
Moths that camouflage themselves to match a leaf stand a good chance of not being seen by moth-eating birds, unless, of course, the moths become so populous that a bird is likely to come across one by accident. Once the bird learns the moth’s identity, it has a search image for it, and the moth’s disguise is useless. Looking like a leaf is then no advantage because the bird will start pecking at leaves in the hope that they are moths, and it will keep doing it as long as a sufficient number do turn out to be moths.
Another animal defense is the use of chemical weapons. We’re all familiar with the chemical weapon of the skunk. Some animals-—like poisonous toads and frogs—can synthesize toxins that attack the nervous system of predators. Other animals acquire chemical defenses passively. For example, some caterpillars acquire poison from the plants they eat. Then, when a bird eats the caterpillar, the bird quickly vomits. After that, the bird will avoid eating that kind of caterpillar. Some birds can remember bad-tasting meals a year later.
Another defensive adaptation is warning coloration. Animals with effective chemical defenses are often brightly colored, and there is evidence that predators are more cautious in dealing with bright color patterns in potential prey. This is probably because a lot of poisonous or bad-tasting animals are colored brightly, with black and yellow, or black and red stripes.
Warning coloration quickly trains predators to avoid eating these brightly-colored animals. Some birds have an instinctive tendency to avoid eating insects with warning coloration. For ′ example, young warblers leave wasps alone because the birds recognize the danger in the black and yellow stripes of the" wasp. However, warning coloration is not an absolute defense, as there is also evidence showing that many birds occasionally feed on bees and wasps, perhaps after learning to cope with their chemical defenses.