Baseline glucocorticoids alone do not predict reproductive success across years but in interaction with enzymatic antioxidants

Glucocorticoids are known to adjust organismal functions, such as
metabolism, in response to environmental conditions. Therefore, these
hormones are thought to play a key role in regulating the metabolically
demanding aspects of reproduction, especially in variable environments.
However, support for the hypothesis that variation in glucocorticoid
concentrations predicts reproductive success is decidedly mixed. Two
explanations may account for this discrepancy: a) glucocorticoids might
not act independently but could interact with other physiological traits,
jointly influencing reproduction, and b) such an association could become
apparent primarily in challenging environments when glucocorticoid
concentrations increase. To address these two possibilities, we determined
natural variation in circulating baseline glucocorticoid concentrations in
parental great tits (Parus major) alongside two physiological systems
known to be related to an individual's metabolism: oxidative status
parameters (i.e., concentrations of pro-oxidants, dietary, and enzymatic
antioxidants) and body condition. These systems interact with
glucocorticoids and can also influence reproductive success. We measured
these variables in two breeding seasons that differed in environmental
conditions. When accounting for the interaction of baseline
glucocorticoids with other physiological traits, we found a positive
relationship between baseline glucocorticoids and the number of fledglings
in adult great tits. The strength of this relationship was more pronounced
for those individuals who also had high concentrations of the enzymatic
antioxidant glutathione peroxidase. When studied independently,
glucocorticoids were not related to fitness proxies, even in the year with
more challenging environmental conditions. Together, our study lends to
support the hypothesis that glucocorticoids do not influence fitness
alone, but in association with other physiological systems