The Cost of Roaming Free: Assessing the Effects of Plant Secondary Metabolites on Diet Selection and Nutritional Condition in a Free-Ranging Generalist Herbivore

Large vertebrate herbivores have a wide variety of browsing options available. However, most plants contain a suite of plant secondary metabolites (PSMs) that can have toxic effects when ingested. Herbivores must therefore make dietary choices that minimize the potentially harmful effects of PSM ingestion and maximize the use of available nutrients and protein. During winter months, in northern latitudes, climatological factors restrict browsing options and many populations of herbivores must subsist primarily on forage that is relatively poor in nutritional quality and high in PSMs. Many species of herbivores have developed a suite of behavioral and physiological adaptations to cope with increased PSM exposure, including selective foraging and increased detoxification efficiency of potentially harmful PSMs. However, detoxification of PSMs may be energetically costly, exacerbating the effects of winter nutritional deficits, which in turn further decreases nutritional condition of free-ranging populations. As a result, PSMs may directly influence diet selection and winter nutritional condition of free-ranging herbivores. We used moose (Alces alces) on Isle Royale National Park, Michigan, to test how PSMs influence the diet selection, rates of intake and the physiological consequences of diet selection in a large mammalian herbivore during winter. We identified browsing patches and bite marks on balsam fir (Abies balsamia) trees and examined the PSM chemical profile and protein content of browsed and unbrowsed trees. We found that both diet selection and rate of intake by moose was directly influenced by PSMs. However, environmental conditions (e.g. snow depth) were the primary governing factor of intake rate. We also examined the effect of PSMs on nutritional condition of island moose using urine collected from snow. We quantified the concentrations of three metabolites found in urine: Glucuronic Acid (GA) as a biomarker of PSM exposure, Urea Nitrogen (UN) as a biomarker of overall nutritional condition, and Creatinine (C) to correct for individual sample dilution from snow volume or relative hydration of individual animals. We found that UN:C was positively correlated with increased GA:C, indicating higher PSM exposure which may reduce nutritional condition of moose. Furthermore, we found that increased ingestion of monoterpenes, a primary class of PSMs in fir, also directly correlated with decreased nutritional condition of moose. By employing techniques that can be applied generally to test the consequences of PSMs on nutritional ecology of herbivores, this research contributes to our understanding of herbivore foraging ecology and the potential nutritional consequences of “bottom-up” regulation of natural populations of herbivores by plants. The methods detailed here-in are applicable to testing consequences of PSMs in multiple species and habitat types. Specifically, measuring GA from urine collected snow provides managers and wildlife scientists with a simple, effective, non-invasive, and relatively inexpensive means by which to monitor the link between diet quality, behavioral and physiological adaptations to PSMs, and the nutritional consequences of diet quality. Finally, increased ability to utilize biomarkers of diet quality and nutritional condition may considerably contribute to the understanding of the health and population dynamics of difficult to manage or economically important herbivores