How does diet affect fecal steroid hormone metabolite concentrations? An experimental examination in red squirrels

A growing number of longitudinal studies in free-ranging animals are measuring fecal steroid hormone metabolite concentrations (FHM). Free-ranging animals can exhibit major seasonal changes in their diet, yet we know relatively little about how diet affects FHM. We experimentally manipulated the diets of female and male North American red squirrels (Tamiasciurus hudsonicus) to determine how diet affected fecal cortisol (FCM) and androgen (FAM) metabolite concentrations. We measured FCM using an enzyme immunoassay (EIA) that we have previously validated and measured FAM using an assay we have previously validated for use in females and validate for males herein. We validated our EIA to measure FAM in males by identifying that 44.5±0.05% of recovered radiolabeled testosterone was excreted in the feces, our EIA antibody detected the fecal testosterone metabolites, and males with scrotal testes had significantly higher FAM (3.02±0.06 lnng/g dry feces) than those with abdominal testes (2.73±0.06). We initially fed all squirrels the same diet, but then switched one group of squirrels to a diet consisting of conifer seed (n=4 squirrels) whereas the other group was switched to peanut butter (n=7). FCM and FAM in squirrels fed conifer seed significantly increased from 0 to 94 h after their diets were changed. FCM in squirrels fed peanut butter significantly declined, whereas FAM declined but not significantly. This demonstrates that change in dietary fiber consumption (peanut butter versus conifer seed) or even slight differences in diet (conifer versus sunflower seeds) can strongly influence FHM.