Validation and Application of Noninvasive Glucocorticoid and Thyroid 4 Hormone Measures in Free-Ranging Hawaiian Monk Seals

We validate fecal glucocorticoid (GC) and thyroid (T3) hormone metabolite measures in the Critically Endangered Hawaiian monk seal for the first time, and examine variation in the concentrations of these hormones in individuals across the species' range. We test hypotheses that monk seals from declining subpopulations have relatively high GCs and low T3 on average suggesting impacts of food limitation, and that this hormone pattern is more apparent in immature animals compared to adults, as food limitation is specifically indicated as a principal cause of poor body condition and survival of juvenile monk seals. We opportunistically sampled scat from 84 individually identifiable monk seals during the 2010 breeding season from two geographic regions, the main Hawaiian Islands and the Northwestern Hawaiian Islands. The main Hawaiian Islands subpopulation of monk seals is growing, whereas subpopulations at many sites in the Northwestern Hawaiian Islands are in decline. Best fit general linear models predicting variation in GCs and T3 (examined separately) were similar (after accounting for significantly elevated hormone concentrations associated with molt and possibly lactation); both included sample date, region, and monk seal age as predictors. GC concentrations were significantly lower in main Hawaiian Islands versus Northwestern Hawaiian Islands monk seals and decreased as the breeding season progressed. T3 concentrations were significantly lower in immature monk seals compared to adults. GC and T3 concentrations were positively correlated at 4 Northwestern Hawaiian Islands sites; prey may be adequate for physiological growth or maintenance at these sites but relatively stressful to acquire. GCs were highest at French Frigate Shoals, (a Northwestern Hawaiian Island site) while T3 was relatively low here, indicating a possible signal of food limitation. GCs were lowest in the main Hawaiian Islands. Disturbance associated with living near a high human population in the main Hawaiian Islands appears to impact monk seal physiology less than other stressors encountered in the remote and highly protected Northwestern Hawaiian Islands, where human presence is extremely low.

Gobush KS, Booth RK, Wasser SK. 2014. Validation and application of noninvasive glucocorticoid and thyroid hormone measures in free-ranging Hawaiian monk seals. General and Comparative Endocrinology 195:174–182.  https://doi.org/10.1016/j.ygcen.2013.10.020.