The detoxification limitation hypothesis: Where did it come from and where is it going?

The detoxification limitation hypothesis is firmly entrenched in the literature to explain various aspects of the interaction between herbivores and plant toxins. These include explanations for the existence of specialist and generalist herbivores and for the prevalence of each of these. The hypothesis suggests that the ability of mammalian herbivores to eliminate plant secondary metabolites (PSMs) largely determines which plants, and how much, they can eat. The value of the hypothesis is that it provides a clear framework for understanding how plant toxins might limit diet breadth. Thus, it is surprising, given its popularity, that there are few studies that provide experimental support either for or against the detoxification limitation hypothesis. There are two likely reasons for this. First, Freeland and Janzen did not formally propose the hypothesis, although it is implicit in their paper. Second, it is a difficult hypothesis to test, requiring an understanding of the metabolic pathways that lead to toxin elimination. Recent attempts to test the hypothesis appear promising. Results suggest that herbivores can recognize mounting saturation of a detoxification pathway and adjust their feeding accordingly to avoid intoxication. One strategy they use is to ingest a food containing a toxin that is metabolized by a different pathway. This demonstrates that careful selection of food plants is a key to existing in a chemically complex environment. As more studies characterize the detoxification products of PSMs, we will better understand how widespread this phenomenon is.