The Role of Plant Chemical Defenses in The Nutritional Ecology of Voles

Abstract

Although forage quality influences the individual performance, food preferences, habitat selection, and population density of microtine rodents (voles and lemmings), the impact of secondary plant compounds on forage quality is largely unknown. This thesis reports results of studies conducted to examine the effects of plant chemical defenses on the individual performance, food habits, and population dynamics of voles. Results from laboratory feeding trials (Chapter One) with prairie voles (Microtus ochrogaster) showed that plant phenolics (quercetin, tannic acid, and quebracho) can reduce survival and growth of voles. The effects of phenolics were related to molecular structure, and included toxicity, feeding deterrence, and reduction of protein digestibility. Mammalian herbivores detoxicate many phenolic compounds by conjugation with glucuronic acid, and this process elevates urinary excretion of components of the glucuronic acid pathway. Chapter Two verifies that measurement of uronic acid excretion by voles can provide a useful index of their detoxication loads. Chapter Three presents baseline data on the food habits of meadow voles (Microtus pennsylvanicus) in two grassland habitats, bluegrass oldfield and tallgrass prairie. Food habits shifted seasonally, and during summer were related to the availability and quality of forage. Chapter Four discusses my test of the hypothesis that cycles in the population densities of voles are caused by induction and relaxation of plant chemical defenses in response to grazing. Results showed that in some instances grazing can induce greater production of phenolics and lower the quality of forage, and that chemical defenses may limit peak population densities by influencing forage quality. However population cycles do not appear to be caused by changes in the levels of these compounds.