15.12: Predation produces adaptation in both predators and their prey.

Some words of advice in case you ever think about quickly approaching a horned lizard: be afraid. Be very afraid. Here’s why: as you get close to the lizard, it may zap you with streams of blood squirted from its eyes. In all likelihood, you will flinch. And as you flinch, the horned lizard will scurry away. The display is shocking, but the fact that evolution has produced extreme and effective anti-predator adaptations is not.

Predation—an interaction between two species in which one species eats the other—is one of the most important forces shaping the composition and abundance of species in a community. Predation, though, isn’t restricted to the obvious interactions involving one animal chasing down and killing another. Herbivores’ eating of leaves, fruits, or seeds is a form of predation, even though it doesn’t necessarily kill the plant. And predators are not necessarily physically imposing. Each year more than a million humans die as a result of disease from mosquito bites, compared with fewer than a dozen from shark attacks.

Predators are a potent selective force: organisms eaten by predators tend to have reduced reproductive success. Consequently, in prey species, a variety of features have evolved (and continue to evolve)—including the blood-squirting-eyeball effect—that reduce the organisms’ predation risk. But as prey evolve, so do predators. This coevolution is a sort of arms race with ever-changing and escalating predation-effectiveness adaptations causing more effective predator-avoidance adaptations, and vice versa. In this light, it may seem unexpected that exotic species often flourish when released into novel habitats, even though natural selection has not adapted them to their new environment. As it turns out, just as these species are not fully adapted to their new environment, they also have few predators there. And with low predation risk, they often can flourish. We’ll examine some common adaptations of both predators and prey.

Prey adaptations for reducing predation There are two broad categories of defenses against predators: physical and behavioral.

Physical defenses include mechanical, chemical, warning coloration, and camouflage mechanisms (FIGURE 15-24).

1. Mechanical defenses. Predation plays a large role in producing adaptations such as the sharp quills of a porcupine, the prickly spines of a cactus, or the tough armor protecting an armadillo or sow bug. These, as well as claws, fangs, stingers, and other physical structures, can reduce predation risk.

2. Chemical defenses. Further prey defenses can include chemical toxins that make the prey poisonous or unpalatable. Plants can’t run from their predators, so chemical defenses are especially important to them. Almost all plants produce some chemicals to deter organisms that might eat them. The toxins can be severe, such as strychnine from plants in the genus Strychnos, which kills most vertebrates, including humans, by stimulating nonstop convulsions and other extreme and painful symptoms leading to death. At the other end of the spectrum are chemicals toxic to some insects but relatively mild to humans, such as those found in cinnamon, peppermint, and jalapeño peppers. Ironically, many plants that evolved to produce such chemicals to deter predators are now cultivated and eaten specifically for these chemicals. One organism’s toxic poison is another’s spicy flavor.

Some animals can also synthesize toxic compounds (or perhaps, more commonly, can safely acquire them from the organisms they eat). The poison dart frog has poison glands all over its body, making it toxic to the touch. The fire salamander is also toxic, with the capacity to squirt a strong nerve toxin from poison glands on its back. Some animals, including milkweed bugs and monarch butterflies, can safely consume toxic chemicals from plants and sequester them in their tissues, becoming toxic to predators who try to eat them.

3. Warning coloration. Species protected from predation by toxic chemicals frequently have bright color patterns to warn potential predators, essentially carrying a sign that says “Warning: I’m poisonous—keep away.” To make it as easy as possible for predators to learn, different poisonous species often have the same color patterns.

In a clever twist on this, some species that are perfectly edible to their predators also have the same bright colors, in a phenomenon known as mimicry. As long as they are relatively rare compared with the toxic individuals they mimic—reducing the chance that predators might catch on to their trickery—the evolutionary ruse is quite successful.

4. Camouflage. An alternative to warning coloration, and one of the most effective ways to avoid being eaten, is simply to avoid being seen. An adaptation in many organisms is patterns of coloration that enable them to blend into their surroundings. Examples include insects that look like leaves or twigs and hares that are brown for most of the year but turn white when there is snow on the ground.

Behavioral defenses include both seemingly passive and active behaviors: hiding or escaping, and alarm calling or fighting back (FIGURE 15-25).

1. Hiding or escaping. Anti-predator adaptations need not involve toxic chemicals, physical structures, or special coloration. Many species excel at hiding or running, or both. With vigilance, it is possible to get advance warning of impending predator attacks, and then quickly and effectively avoid the predator. A variation of this strategy comes from safety in numbers: many species, including schooling fish and emperor penguins, travel in large groups to reduce their predation risk.

On many islands, animals show no fear of humans. Rather than encountering skittish lizards, for example, a human visitor to the Galápagos Islands must be careful not to step on them. The animals are not afraid because, given the small size of most islands, the number of predators is restricted. As a consequence, there has been no selection for skittishness and hypervigilance, traits that normally evolve in response to predation risk.

2. Alarm calling and fighting back. In many species, especially birds and mammals, individuals warn others with an alarm call. Although risky for the caller, such alarm calling can give other individuals—often close kin that are nearby—enough advance warning to escape (recall, from Chapter 9, the warning calls of Belding’s ground squirrels). Some prey species also turn the tables, mobbing predators to keep them from successfully completing their task. This category might also include the blood-squirting lizard described above, or the fulmar, a seabird that defends its nest from attacks with projectile vomiting aimed at the intruder.

Predator adaptations for enhancing predation Just as prey use physical and behavioral features to reduce their risk of predation, predators evolve in parallel ways to increase their efficiency. As milkweed plants have evolved to produce toxic chemicals, sequestered within the structures of the plant, milkweed bugs have evolved to be able to eat the toxic plants without suffering harm. And as prey have become better at hiding and escaping, predators have developed better sensory perception to help them detect hiding prey and faster running ability to catch them. Predators, too, make use of mimicry. The angler fish, tasseled frogfish, and snapping turtle all have physical structures that mimic something—usually a food item—of interest to potential prey. As a prey animal comes closer to inspect, the predator snaps it up (FIGURE 15-26).

Although natural selection leads to predators with effective adaptations for capturing prey, the adaptations are rarely so efficient that the prey species are driven to extinction. The explanation for this is referred to as the “life-dinner hypothesis.” Selection for “escape ability” in the prey is stronger than selection for “capture ability” in the predator. When, for example, a rabbit can’t escape from a fox, the cost to the rabbit is its life—and it will never reproduce again. On the other hand, when a fox can’t keep up with a rabbit, all it loses is a meal; it can still capture prey and reproduce in the future. In other words, the cost of losing in the interaction is much higher for the rabbit.