DISSERTATION RESEARCH: The role of toxin complementation in herbivore defense

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Herbivores with specialized diets can co-opt the defensive chemicals of their food-plants for their own defense. This research provides a first test of the hypothesis that grazing herbivores do the same, by regulating their intake of multiple plant chemicals to optimize defense against enemies. Alternatively, grazing among toxic plants might benefit herbivores by reducing the harmful effects of over-ingesting a single toxin associated with a particular food-plant species. These alternative hypotheses will be tested through three experiments involving a caterpillar already known to ingest one type of plant toxin as a defense against lethal parasites. First, the researcher will investigate how parasitism and the type of parasite affect the caterpillar's intake of two types of toxins (found in different food-plants). Second, she will compare caterpillar resistance against parasites when they have consumed the two toxins singly or in combination. To address toxin dilution, this experiment will also determine the effects of these diets on the survival and growth of unparasitized caterpillars. Finally, she will test whether the ratio of toxins eaten by unparasitized caterpillars confers deterrency against ubiquitous ant predators. Taken together, these results will identify how this herbivore benefits by grazing on toxic plants. This work will connect the science of pharmacology with animal behavior and ecology. Humans, domesticated grazing herbivores, and wild grazing herbivores face the common problem of obtaining a healthy diet from a chemically diverse array of plant foods. Using grazing caterpillars as a model, this research will test and develop scientific theory that can be applied to other species, including livestock and humans. In addition, understanding the reasons for host plant selection by herbivores will inform the management and conservation of biodiversity in grasslands, which are imperiled ecosystems and home to the caterpillars, plants, predators and parasites under study.