Repeatability of daily profiles of baseline glucocorticoid metabolites measured in the urine and faeces of eastern rock sengis (Elephantulus myurus)

Abstract

Repeatability of hormone concentrations is of great interest for studies investigating the evolution of hormonal traits. Particularly the repeatability of glucocorticoids (GCs) in response to a stressor is frequently investigated, but often only point (initial and/or response value), or single measures are used. A new method takes into account the entire individual hormone profile and generates an individual profile repeatability (PR) score. The method was developed for response profiles, but it may also be valuable for baseline values in species with diurnal changes in hormone concentrations. GCs are determined in a variety of matrices, and repeatability can vary considerably depending on the matrix. We investigated the repeatability of baseline GC metabolite (GCM) concentrations measured in urine (uGCM) and faeces (fGCM) of captive eastern rock sengis (Elephantulus myurus) using the more traditional linear mixed model approach and the PR method. GCMs were assessed over 24 h and measurements were repeated twice with two weeks between replicates. A diurnal rhythm in GCM concentrations associated with the activity period of the sengis was found in urine, but not in the faeces. Urinary GCM concentrations exhibited a moderate repeatability, whereas the repeatability of fGCM concentrations was low. Urinary GCM concentrations and their repeatability differed between the sexes; with higher concentrations and lower PR scores in females. No such sex differences were apparent for fGCM concentrations and the PR score was not able to characterise repeatability of fGCM concentrations, which were lacking a distinct profile. The PR score enabled a successful quantification of the repeatability of the diurnal uGCM profiles. Hormone profile, sex and sample matrix can affect hormonal traits considerably and the results may be obscured if these factors are not carefully considered.