In contrast to plants, animals cannot regenerate completely after attack. Therefore, their defense often is to rely on constitutively expressed defensive compounds. Especially those animals which are too slow to escape their predators have developed very effective repellents or toxins. It can be distinguished between active and passive defense. For active defense, toxins are actively introduced into the attacker, for example, with the help of a sting (bees) or a poison fang (snakes). A form of passive defense is an unpleasant, often bitter taste of a prey that will teach the predator to avoid further feeding on such species.
Defensive compounds are either produced by the organism itself, or taken up from the food and stored (sequestered), or are obtained from associated symbiotic microorganisms.
This article can only present some exciting examples of chemical defense mechanisms of animals given the scope afforded, and consequently is far from being complete. However, it is still attempted to illustrate the diversity of defensive strategies. In particular, attention is drawn to general principles that are often followed in spite of structural variety.
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