Defenses can be permanent (constitutive) or inducible. Phenotypic plasticity in defensive traits enables prey organisms to express a particular defense only if a reliable cue for a future attack is present. Thereby, the organisms can minimize costs affiliated with the formation or maintenance of a defense when predation risk is low. Inducible defenses are an appropriate mechanism to cope with the variable hazard of a frequently changing predator spectrum. In the animal kingdom, inducible defenses cover a taxonomic range from protozoans to vertebrates. The defensive traits range from behavior, morphology, and life-history adaptations to the activation of the specific immune system of vertebrates. Daphnia show the most prominent examples of morphological plasticity triggered by chemical cues, so-called kairo-mones, released by predatory invertebrates and fish. For example, elongated helmets, tail spines, or crests have been shown to reduce predator-caused mortality (Figure 2).
Several factors have been identified that favor the evolution of inducible as opposed to permanent defenses: (1) The attacker has to have a variable but sometimes relevant impact; (2) the defense must be effective within a relatively short time, so lag phases can be avoided; (3) a reliable cue has to indicate the danger; and (4) costs or tradeoffs have to outweigh the benefit of the defense during relevant periods of time.
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