Species experiencing dramatic changes in nutrient availability across seasons might be expected to adjust their secretion of glucocorticoids in order to appropriately manage energy use. Imagine having a banquet available in spring but then having your food covered by a thick blanket of snow all winter! This study hypothesizes that changes in female bison fecal glucocorticoid metabolite (FGM) concentrations are associated with nutrient availability across the year. FGM are measured monthly throughout the year in several bison herds on natural rangeland, and nutritional quality of pasture samples is also assessed. In addition, FGM are measured in experimental bison groups that transition from pasture to either high-energy corn and hay diets or to a hay-based diet. There are spring, summer, and fall experimental groups. Data to date show that FGM concentrations in each herd on pasture change in association with measured pasture nutritional quality indicators, including protein, total digestible nutrients, and acid detergent fiber. FGM are higher when pasture nutrition is high, likely enabling the bison to utilize the available energy for growth and lactation, and thereafter FGM gradually decrease to minimal concentrations in winter, likely enabling the bison to conserve energy as pasture quality diminishes from summer to winter. So far, FGM concentrations in the experimental bison groups increased upon switching from pasture to higher-energy feed in spring, but FGM in these groups still decreased in summer and fall, just like they do in pasture bison, even though nutritional quality of feed was maintained. Continued investigation of FGM in the experimental groups across the year will help reveal if this decrease in FGM might be related to rumen acidosis in those groups on the higher-energy feed, or if a preserved evolutionary mechanism for seasonal energy conservation might be the best explanation. These data highlight the role of glucocorticoids in normal metabolic function and underscore the importance of considering nutrition when studying glucocorticoids, which are sometimes viewed too narrowly as "stress hormones". In addition, findings of this study can shed light on how we can both monitor and interpret the metabolic response of exotic ungulate species to manufactured zoo diets. We are collaborating with the ranches of Turner Enterprises for this amazing study.
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