Given that carnivores almost always kill the prey they consume, the interests of both predator and prey are at stake: the best strategy for carnivores is to maximize their ability to capture and consume prey while the best strategy for prey is to minimize the likelihood of being eaten. We first consider the strategies carnivores use to obtain food and then examine how prey avoid being eaten.

What strategies do carnivores use to obtain food? The success of carnivores depends on balancing the cost of pursuing, capturing, and handling prey against the energetic benefit of consuming them, as discussed in Key Concept 52.4. At one extreme, carnivores use strength and swiftness to actively search for and capture high-quality prey. The “active pursuit” strategy is used by predators of all sizes: both Orca whales pursuing gray whales and foxes pursuing birds are fast, powerful predators and are equipped with strong jaws (Figure 55.4A). At the other extreme, an ability to look inconspicuous allows the stealthy predator to ambush prey unlucky enough to pass its way (Figure 55.4B). Various other predation strategies have evolved in predators that are smaller than their prey. For example, the jaws of some snakes open wide enough to allow them to swallow prey larger than their head. The tiny short-tailed shrew, among the smallest mammalian predators, produces venomous saliva that paralyzes earthworms and snails but also prey much larger than itself, including mice and small birds.

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Prey species have in turn developed a variety of defenses against predators. Some escape from predators simply by flying or running away. Others have morphological defenses; tough skin, shells, spines, or hair can foil even a determined predator (Figure 55.5A). Various other defenses that prey have evolved to avoid capture are described below.

AVOIDING DETECTION Prey species can often escape predators by hiding. One form of hiding is camouflage, also known as crypsis. Crypsis allows species to resemble objects that their predators consider inedible. The katydid in Figure 55.5B, for example, looks very much like a dead leaf, even down to the likeness of a spot of fungal decay.

Because the vision of many types of predators is adapted to spot moving prey, many prey species simply stop moving if they are being pursued. “Playing possum,” a term that is sometimes applied to this strategy, refers to the ability of opposums to simulate death.

CHEMICAL DEFENSES Many animals use chemical defenses to escape or repel their predators. Chemical defenses are generally used by animal prey that are small, weak, sessile, or otherwise unprotected. Many insects produce sprays, oozes, or froths when attacked. Bombardier beetles, for example, eject a hot (100°C) noxious chemical spray from the tip of their abdomen that can be fatal to attacking insects.

Predators may evolve adaptions to overcome their prey’s chemical defenses, as you saw in the case of the rough-skinned newt and the garter snakes that have become insensitive to the newt's protective toxin (see Figure 20.20). Some predators ingest and sequester their prey’s defensive chemicals and use them as defenses against their own predators. For example, some sea slug species incorporate toxic chemicals from their sponge prey, whereas others, which feed on hydrozoans, incorporate the stinging cells into their own bodies (Figure 55.5C). Likewise, poison dart frogs from Central and South America sequester toxins from their prey, which include ants, mites, and other small invertebrates (Figure 55.5D).

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WARNING SIGNALS Some prey species that defend themselves with toxic chemicals advertise that fact with a warning signal. Warning signals may be visual (many toxic species are brightly colored) or acoustical (the rattlesnake’s warning rattle, for example), depending on what sensory cues their predators use to find prey.

Many toxic prey, such as the nudibranchs and frogs in Figures 55.5C and D, sport bright colors or striking patterns to protect themselves against visually orienting predators. Such warning coloration increases the probability that a predator will learn to recognize and avoid the toxic species. Some vertebrate predators that rely on visual cues can learn quickly to associate certain color patterns with an unpleasant feeding experience.

MIMICRY SYSTEMS There are two general types of mimicry systems that have evolved repeatedly in nature: Batesian mimicry and Müllerian mimicry. In Batesian mimicry, a benign, edible species (the mimic) closely resembles a dangerous, toxic species (the model). The mimic benefits from the avoidance behavior of predators of the model (Figure 55.6A). In Müllerian mimicry, two or more species converge on a common warning signal; all benefit from providing a stronger recognition signal to predators. Many of the Neotropical longwing butterflies (Heliconius), which as caterpillars feed on toxic passionflower plants and incorporate the plant toxins into their bodies, are Müllerian mimics. Heliconius species living together in a particular geographic region are likely to have similar coloration and share a common warning pattern (Figure 55.6B). Genome sequencing of Heliconius Müllerian mimics has identified one gene, optix, that codes for a transcription factor that can, by changing gene expression patterns, create the same color patterns in Heliconius species that are not very closely related.

BEHAVORIAL MECHANISMS Prey often use behavioral mechanisms to avoid predation. For example, as you saw in Key Concept 52.6 and Figure 52.21, group behaviors such as flocking and alarm vocalizations can provide protection from predators. In the lionfish example introduced in the chapter opening story, the evolutionary change in behavior that predator and prey may experience has not yet had time to develop. In Investigating Life: The Lionfish King we describe research showing that the small reef-fish prey of lionfishes are likely experiencing a novel predatory behavior not previously encountered, and thus may be defenseless in the face of lionfish predation. Lionfish slowly approach their prey and direct jets of water toward them. Confused or distracted by the jets, the prey are often taken by headfirst capture. The highly efficient predatory behavior of lionfishes doubtless contributes to their success as invasive species in tropical Western Atlantic and Caribbean coral reefs.