Allomones are interspecific semiochemicals that primarily benefit the sender. Animals as well as plants defend themselves chemically against predators. Myriads of ways to deter predation have evolved. Many chemicals have more than one function, being aimed not only at predators but also at parasites, prey, or conspecific competitors. As common denominator of allomones we assume that the inclusive fitness of the sender – rather than the receiver – is enhanced. Chapter 10 deals with defenses by animals, and Chapter 11 with plant defenses against herbivores.

Numerous ingenious chemical defense mechanisms of vertebrates protect against predators, but also against competitors and fierce prey. These range from innocuous odors of prey animals or plants that signal that they are unpalatable to some of the most toxic secretions known in nature. Slower animals tend to rely more on chemical and mechanical defenses. Humans have known animal toxins since time immemorial and medical research has elucidated the mechanisms involved in great detail. However, ecologists have investigated why animals and plants have poisons and venoms in the first place only since the 1950s.

Fish

Among fish we find some of the most potent toxins in animals. We distinguish passively toxic fish from actively toxic fish. The former simply have toxins in their tissues, typically taken from some other source such as their diet.