Physiological validation of a non-invasive method to evaluate adrenocortical activity and the time course for the excretion of stress hormones in the feces of three species of desert gerbils

Abstract

We evaluated the suitability of a corticosterone enzyme immunoassay (EIA) to monitor excretion of fecal glucocorticoid metabolites (FGM) in response to Adrenocorticotropic hormone (ACTH) and saline injections in three desert rodent species (Gerbillus andersoni allenbyi (GA), Gerbillus nanus (GN), and Gerbilis piridium (GP). We exposed 24 gerbils (N = 9 for GA, N = 7 for GN, N = 8 for GP) to an ACTH and a saline injection at different times. Fecal samples were collected hourly for 24 hours after injection. The average starting concentration (baseline) FGM concentration was 797 ng/g for GA, 183 ng/g for GN, and 749 ng/g for GP. The average peak concentration was 2377 ng/g for GA, 589 ng/g for GN, and 1987 ng/g for GP. We were able to provide a physiological validation for the chosen assay in GAs and GPs, however, our results for GNs were less clear. We found an increase in FGM concentrations on average after 5.5 hours in GA, 3.1 hours in GN, and 3.8 hours in GP. We found a peak in FGM concentration on average after 8.8 hours in GA, 5.6 hours in GN, and 10.3 hours in GP. We determined that FGM concentration returned to starting value on average after 14.4 hours in GA, 9.1 hours in GN, and 15.1 hours in GP. The outcomes of this study can help establish trapping protocols and time frames for FGM monitoring of these wild small mammal populations. The time course for excretion of FGM is similar between the species in this study, and comparable to some non-desert rodents. We found high variation in the time course of excretion within species. This variation needs to be taken into account when monitoring stress responses in the field. By assessing adrenocortical activity using FGM monitoring, stress responses to varying ecological and environmental factors can be reliably examined in the field.