The threat of predation can cause increased adrenal secretion of glucocorticoids that, if sustained, can result in chronic stress that might predispose animals to disease, reduced growth, or poor fertility. Fecal glucocorticoid metabolite (FGM) concentrations were measured between September 2011 and July 2012 in two Montana bison (Bison bison) herds, one herd living with and the other without wolves (Canis lupus) in their habitat. The relevance of FGM concentrations (as measured by the corticosterone enzyme immunoassay) and a confirmation of an acute adrenal response was demonstrated by transient increases (P<0.001) in FGM concentrations in bison herded through a chute system and following a wolf encounter. The FGM concentrations were higher in the herd with wolves than the herd without wolves overall (P=0.045), but the monthly differences between the herds were inconsistent (magnitude=0.3-3.5 mu g/g; P=0.011 monthxherd). The herd with wolves had higher FGM in April, June, July, September, and November, but the herd without wolves had higher FGM in January, February, and May. Seasonal changes in FGM concentrations (P<0.001) were measured across the year in both herds (magnitude=10.8 and 6.9 mu g/g, respectively), exceeding any differences between herds potentially attributable to wolf presence. Concentrations of FGM were highest in April-July and were lowest in January-February. Evidence of a chronic stress response was not found, and increased disease susceptibility in the bison living with wolves seems unlikely.

American plains bison ( Bison bison ) take advantage of naturally-available nutrients, especially protein, by shifting grazing among cool-season grasses, forbs, legumes, and warm-season grasses as plant species transition from growing to dormant states across the year. Bison also transition to a lower metabolic rate and critical temperature in winter to conserve energy. Seasonal patterns in fecal glucocorticoid metabolite (FGM) concentrations in bison have previously been described, and this study sought to investigate the role of seasonal nutritional variation on FGM. Fecal samples (n=30) were randomly collected monthly for ≥ 1 year from five herds of female bison maintained on grassland pastures at four ranches for FGM analysis. Grass clippings also were collected monthly and sent to a commercial lab for pasture nutrient analysis. Subsets of bison from two herds were removed from pasture and provided with concentrated grain- and hay-based diets, and fecal samples (n=17-30) were collected weekly or monthly for three to seven months. FGM were measured using two commercially available assays: corticosterone (CJM006) and 11-oxoetiocholanolone (72T), and an adrenocorticotropic hormone challenge was administered to a single male bison. An increase in FGM following the challenge was detected by the 11-oxoetiocholanolone but not the corticosterone assay, yet monthly FGM concentrations measured by the two assays were correlated (p ≤ 0.02, r ≥ 0.7). Both assays demonstrated a seasonal pattern in FGM concentrations in bison on pasture, which was correlated (p ≤ 0.001) with protein (r ≥ 0.7), total digestible nutrients (r ≥ 0.7), and acid detergent fiber (r ≥ -0.7) across two years. Removing bison from pasture and transitioning to concentrated grain and hay diets eliminated seasonal patterns in FGM concentrations. Changes in FGM concentrations associated with dietary quality should be considered in studies seeking to investigate bison nutrition, health, and adrenal responses to other stimuli.