Energetic consequences, trade-offs and elimination of plant secondary metabolites in a specialist (Neotoma stephensi) and generalist (Neotoma albigula) mammalian herbivore

For over 25 years, a fundamental objective in the study of plant-herbivore interactions has been to understand why the consumption of a single plant species is rare in mammals, as only a handful of mammalian herbivores (<1%) are dietary specialists. This dissertation provides the first comprehensive investigation of three distinct factors that may play a role in limiting dietary specialization. First, dietary specialization may be limited by the physiological challenge of consuming a single plant species. Second, specialization may be limited by dietary trade-offs, in that specialists have a reduced ability to consume novel species of plants. Lastly, dietary specialization may require specific physiological mechanisms that efficiently eliminate plant secondary metabolites (PSMs) from a single plant species. These predictions were tested by comparing the energetic consequences of ingesting PSMs, the dietary limitations of a specialist mammalian herbivore and the mechanisms of PSM elimination using a juniper specialist, Neotoma stephensi, and generalist, N. albigula, woodrat.; This thesis demonstrated that the energy budgets of specialist and generalist woodrats were negatively impacted by the intake of PSMs from juniper (i.e., naturally consumed PSMs) and novel PSMs (i.e., from creosote, which is not in the habitat of specialists and generalists). However, juniper specialists minimized the energetic costs associated with the intake of juniper through greater energy intake and lower energy expenditure than generalists and thus had more energy available for other energy dependent activities when consuming a juniper diet. One explanation for the high performance of specialists on juniper diet was that specialists absorbed fewer PSMs from juniper than generalist woodrats. Despite the high capacity to consume juniper, juniper specialists experienced a decreased ability to consume novel PSMs, suggesting a dietary traded-off associated with specialization. These data are the first to indicate that the energetic consequences of consuming PSMs, dietary trade-offs and absorption of PSMs may constrain dietary specialization in mammalian herbivores. Identifying these factors and their role in limiting and/or facilitating dietary specialization has provided a better understanding of the foraging ecology, physiology and evolution of mammalian herbivores.