Dietary Specialization in Mammals: Constraints and Costs of Detoxification of Plant Secondary Compounds

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Mammals that consume plants run the risk of being poisoned by naturally occurring toxins produced by plants. Little experimental work exists on the mechanisms mammals employ to deal with plant toxins, or how specialized species such as the koala are capable of consuming plants that are toxic to other species. The proposed research interfaces with ecology, chemistry, physiology and pharmacology to address mechanisms by which herbivores detoxify plant toxins as well as the costs of detoxification. The PI will investigate how woodrat herbivores that specialize on one species of toxic plant differ in detoxification physiology from generalist woodrats that consume a variety of plant species. Woodrats are a model system for this problem because the diversity of specialists and generalists woodrats is unparalleled by any other genus of mammalian herbivores. Thus far, the PI has found that specialist herbivores are capable of tolerating higher doses of toxins than generalists. Contrary to conventional wisdom, specialists do not appear to have unique detoxification pathways compared to generalists but rather systems with a greater capacity. Preliminary results suggest that specialists are able to deal with high toxin loads because they can eliminate toxins faster than generalists. These studies will further investigate how specialists and generalists differ in detoxification abilities and will address issues on costs and constraints of dietary specialization. For example, are specialists more efficient than generalists in processing toxins? Do specialist herbivores trade-off the ability to detoxify a wide range of toxins in exchange for enhanced processing of a subset of toxins? In this century, mammalian herbivores will confront profound detoxification challenges. Novel toxins (eg dioxins, PCBs) are being added to the environment at unprecedented rates. Moreover, in the next 50 years, levels of CO2 in the atmosphere are predicted to double due to extensive burning of fossil fuels. Many species of plants grown under elevated levels of CO2 are substantially inferior in nutritional quality in that they are lower in protein and contain up to twice the toxin concentration as plants grown under current CO2 concentrations. Will mammalian herbivores be aversely impacted by such a radical change in the nutritional quality of their food, particularly the increase in toxins? The results of this research on detoxification processes of wild herbivores will provide vital insights on how herbivores will be affected by this change in nutritional quality of food.