Ecological factors influencing the evolution of insects' chemical defenses

How insect defense chemicals have evolved has remained relatively understudied, compared with the evolution of aposematic signals of such defenses. Because there is mounting evidence that chemical defenses can generally be expected to be costly, understanding the evolution of such defenses and their maintenance in the face of the potential for automimicry (signaling by individuals that do not invest in defense) is nontrivial. One potential explanation is that chemically defended insects suffer less from predation than those that do not invest in chemical defenses. Here, we use a series of models to explore aspects of the evolution of such costly chemical defenses. Our models predict that investment in costly defenses can occur across a wide range of predation intensities; however, if predation intensity is low, then the defense has to be very effective to be selected, unless the defense is very cheap. Furthermore, the evolution of antipredatory defenses will be relatively insensitive to the severity of any mechanism, whereby prey pay a cost every time they use their defense against an attacking predator even if they survive the attack, but sensitive to the form of the relationship between initial investment in constituting the defense and survival benefit. Once defense becomes common in the prey population, prey may get a frequency-dependent benefit if predators learn to avoid prey of this type after several attacks. Finally, we predict that increasing the rate of avoidance learning by predators encourages reduced investment in antipredatory defenses by prey. The potential for these predictions to be tested empirically is discussed. Copyright 2008, Oxford University Press.