Until now, physiological stress assessment of large whales has predominantly focused
on adrenal glucocorticoid (GC) measures. Elevated GC concentrations in feces (fGC)
are known to reflect stressful disturbances, such as fishing gear entanglement and
human-generated underwater noise, in North Atlantic right whales (Eubalaena glacialis).
However, there can be considerable variation in GC production as a function of sex
and life history stage, which may confound the interpretation of fGC levels. Additionally,
GC antibodies used in immunoassays can cross-react with other fecal metabolites (i.e.,
non-target steroids), potentially influencing fGC data. Here, aldosterone concentrations
(fALD; aldosterone and related metabolites) were measured in fecal samples from right
whales (total n=315 samples), including samples from identified individuals of known
life history (n=82 individual whales), to evaluate its utility as a complementary
biomarker to fGC for identifying adrenal activation. Concentrations of fALD were positively
correlated with fGCs in right whales (r=0.59, P<0.001), suggesting concurrent secretion
of these hormones by the adrenal gland. However, fALD levels were less influenced
by concentrations of reproductive steroids in feces, minimizing the potential confounder
of assay cross-reactivity in samples with highly skewed hormone ratios. Across different
life history states for right whales, fALD concentrations showed similar patterns
to those reported for fGC, with higher levels in pregnant females (35.9±7.6ng/g) followed
by reproductively mature males (9.5±0.9ng/g) (P<0.05), providing further evidence
of elevated adrenal activation in these groups of whales. The addition of fALD measurement
as a biomarker of adrenal activation may help distinguish between intrinsic and external
causes of stress hormone elevations in large whales, as well as other free-living
wildlife species, providing a more comprehensive approach for associating adrenal
activation with specific natural and anthropogenic stressors.