Significant covariates of non-invasive measures of glucocorticoids and thyroid hormone in free-ranging Hawaiian monk seals (Monachus schauinslandi)

Background/Question/Methods

Endocrine profiles of stress and nutrition in free-ranging wildlife provide crucial insight into how individuals respond to natural and human-induced disturbances.  Tracking these physiological measures over time may allow scientists and managers to understand species-specific response patterns with improved resolution and sensitivity and predict critical periods when intervention might be most beneficial.  This may be especially valuable for the critically endangered Hawaiian monk seal (Monachus schauinslandi).  We validated fecal hormone measures, glucocorticoids (GCs) and thyroid hormone (T3), in free-ranging monk seals for the first time by opportunistically sampling scat from 84 individually identifiable seals across 7 breeding sites during the 2010 breeding season. We then examined relationships between GCs and T3 and a number of covariates to promote an understanding of the natural variation in the stress response in the species. We investigated how hormone measures varied with molt and lactation because we expected these states to be associated with extreme endocrine profiles. Then, we examined relationships between age, sex, location (breeding site) and time (sample date) and the two hormone metabolites with general linear mixed models. 

Results/Conclusions

Molting seals had significantly greater GCs and T3 than those not in molt. The reproductive status of adult females influenced GCs, but not T3.  Lactation was associated with the highest GC measure, and pregnancy the lowest. Best fit models predicting variation in GCs and T3 (examined separately) were similar and both included time, location and age (molting and lactating seals excluded).  Hormone values decreased as the season progressed, and they were lower in immature seals than adults, though not significantly so for GCs.  Seals in the main Hawaiian Islands (MHI) had significantly low GCs compared to seals in 6 different breeding sites in the Northwestern Hawaiian Islands (NWHI).  Likewise, seals in the MHI, but also one NWHI site, had the lowest T3 measures; however differences in T3 between the sites were not significant. The MHI subpopulation of monk seals is growing, whereas many sites in the NWHI are in decline.  Higher stress hormones in NWHI seals compared to those in the MHI may reflect or contribute to this difference.