Assessing stress in a mammal from plasma and feces: A nutritional mismatch between the diet needed and the food -available

A lack of agreement between the diet an animal needs for reproduction and survival and the food available in its environment has major impact on its fitness. The complexity of their digestive system is critical and may affect stress axis function. The squirrel family (sciurids) have a very simple gut and do not have the microbiome to digest high fiber foods well. Thus, they forage preferentially on forbs, seeds, and fungi, and avoid hard-to-digest grasses. We compared two measures of their stress axis - plasma free cortisol, a glucocorticoid (GC), and fecal cortisol metabolites (FCMs) - in Columbian ground squirrels (Urocitellus columbianus) from two meadows as they were building up body reserves in July for their pending hibernation. One meadow was natural with an abundance of forbs and the other unnatural with an abundance of grasses that had been seeded as a horse pasture. Our two measures went in the opposite direction: GC levels were lower on the natural meadow but FCMs were higher, whereas GC levels were higher on the grass meadow but FCMs lower. The fecal fiber content was lower on the natural meadow. Thus, when interpreting FCM levels, it is critical to understand both the digestive system of the study mammal and its diet to interpret stress axis function.