The effects of supplemental feeding on stress hormone concentrations in elk

Abstract

On twenty-two feedgrounds in western Wyoming, elk (Cervus elaphus) are provided with supplemental feed throughout the winter. Brucellosis seroprevalence of feedground elk is 26% whereas other elk in the Greater Yellowstone Ecosystem have historically had a brucellosis seroprevalence of 2-3%. The aggregation of elk during peak transmission allows brucellosis to persist in the feedground populations. In addition to creating the opportunity for disease transmission, the aggregation of elk on feedgrounds may have detrimental physiological effects. Studies have shown that chronically high stress hormone concentrations can suppress the immune system and lead to increased disease susceptibility. Potential stressors on the feedgrounds include high densities, large group sizes and aggressive social interactions. In this study I investigated how factors associated with supplemental feeding affect stress hormone levels, as indexed by fecal glucocorticoid levels, in elk on feedgrounds and elk on native winter range. I also worked with managers to experimentally alter the feeding distribution on the feedgrounds to examine how feeding density affects stress hormone levels and aggression rates. Results show that elk on feedgrounds have stress hormone levels 31% higher than elk on native winter range (Welch's t₂₇.₂₃=2.39, p=0.024). Experimental reduction of feed density did not have an effect on stress hormone level or aggression rates. But note the relationship between fGCs and local densities here. Although the feeding treatments did appear to reduce local feeding densities,s this effect was not significant and was small relative to the large differences in density among sites. Regardless as to the cause of the high stress hormone levels seen in supplementally fed elk, the feedgrounds are creating an epidemiological setting for disease transmission and a physiological state that may increase susceptibility to disease. The impact of these stress hormone concentrations on disease susceptibility remains unknown, but may be an important driver of disease dynamics in these elk populations.