Sex‐Specific Early Life Corticosterone Stress‐Coping Capacity Is Linked to Mitochondrial Content and Telomere Maintenance

Glucocorticoids are upregulated during acute challenges, contributing to maintenance of energy balance. However, if this upregulation exceeds a threshold or becomes chronic, it can lead to harmful effects. Investigating the factors behind between-individual variation in the magnitude of the response to acute stress and how effectively individuals regulate this response to return to baseline glucocorticoid levels (recovery capacity), is therefore important. Glucocorticoid regulation may be influenced by mitochondrial content, as these hormones are produced in the mitochondria of the adrenal glands. Furthermore, early-life adversity can impair glucocorticoid regulation, and these effects may be mirrored by other mechanisms, such as telomere maintenance, suggesting possible correlations among these traits. In this study, we evaluated whether variation in corticosterone response to an acute challenge and recovery capacity in 27 Cocos Booby (Sula brewsteri) nestlings was related to mitochondrial DNA copy number (mtDNAcn), mass growth rate, and telomere dynamics during the first 30 days of life. We found sex differences in recovery capacity: females sustained higher corticosterone levels than males after an acute challenge. Higher mtDNAcn was associated with greater corticosterone increases and weaker recovery, suggesting that mtDNAcn reflects an individual's capacity to produce glucocorticoids. Finally, telomere dynamics, but not mass growth rate, were related to recovery capacity, a relationship only significant in females. Females with better telomere maintenance had weaker recovery capacity than those with poorer telomere maintenance. These results highlight links among glucocorticoid regulation, mtDNAcn, and telomere dynamics, suggesting that these traits are jointly shaped, likely through sex-specific pathways during early life.