Non-invasive hormone monitoring in free-ranging Weddell seals (Leptonychotes weddellii): a tool for remote population assessment

The presence and the activities of humans can have negative effects on other species, the impacts of which can be seen as changes in biomarkers such as steroid hormones. However, a detailed understanding of endocrine function is lacking in many species, especially wild populations living in remote localities. We provide the first evidence of concurrent and progressive hormone changes from the winter pre-breeding to the summer breeding period for Weddell seals (Leptonychotes weddellii) in East Antarctica. To do so we validated a non-invasive approach to quantify faecal metabolites of testosterone (fTM), progesterone (fPM), and estradiol (fEM). Minimum fTM concentrations were at least four times higher in samples from adult males than from a male pup, and female fPM concentrations were significantly higher when pregnant females were present, indicating that the assays can detect biologically relevant changes in circulating hormones. There were clear seasonal patterns for both sexes. In females, fEM and fPM levels increased during the pupping period and declined to baseline after the last female gave birth. In males, fTM increased during the breeding season while they were defending territories and mating with females. Our results suggest faecal hormone analysis can be a useful monitoring tool for Weddell seals and provide baseline information about temporal changes in hormone expression. In the future, this may be a useful tool for monitoring the effects of increased human activity on Weddell seal populations and applicable to other species.