Physiological responses vary with molecular damage outcomes after recovery from constant light.

The Damage-Fitness model describes how stress is linked to damage and repair pathways that drive health and fitness outcomes across taxa. However, we lack an understanding of how variation in hypothalamic-pituitary-adrenal (HPA) responses (i.e., endocrine flexibility) affects damage, especially after recovery from stressors. In this study, adult female zebra finches were exposed to a normal photoperiod or constant light for 23 days followed by a recovery period. Using path analysis, we combined a suite of morphological and physiological traits to examine the mechanisms related to cellular damage outcomes. In control individuals, HPA axis reactivity was condition-dependent, such that birds with higher body mass had stronger HPA axis reactivity. HPA axis reactivity was associated with two specific relationships: a strong, positive, relationship with glucose reactivity and a slightly negative relationship with liver 4-hydroxynonaneal that covaried. Interestingly, this condition-dependency disappeared in birds recovering from constant light. While HPA axis reactivity was positively associated with glucose reactivity, this path relationship was not associated with any damage marker. Liver glucocorticoid receptor abundance was negatively associated with liver protein carbonyl damage in control birds, but this relationship was lost in birds recovering from light. These patterns indicate that long-term exposure to a stressor such as constant light can alter biologically linked relationships, even after cessation and recovery from that stressor. Yet, whether rewiring of physiological network connectivity is related to adaptive physiological outcomes, fitness-related traits, or performance remains unclear.