Abstract: Stress exposure and stress response are likely to vary according to life history strategies and across environmental contexts. If the release of glucocorticoids (e.g. corticosterone, CORT) in response to acute stress enables animals to rightfully cope with the situation, chronic exposure to high CORT levels can lead to deleterious cascading effects on animal’s physiology, behavior and fitness. Some species may be more adapted to cope with stressful events than others and might exhibit natural adaptations to limit the adverse consequences of prolonged high CORT levels. The aim of this study was to investigate the effects of experimentally elevated CORT levels on the growth, energetics and ageing of king penguin chicks (Aptenodytes patagonicus), using sub-cutaneous CORT implants. We investigated the effects of CORT both on the medium (23 days
after implant) and the long-term (fledging: ~ 200 days after the implant) on morphological (i.e. body mass and size), behavioral (i.e. aggressivity, physical activity) and physiological traits (i.e. heart rate, CORT response to acute stress, oxidative stress, mitochondrial density and telomere length). Although CORT-treated chicks were taller, more aggressive and displayed lower CORT response to acute stress on the medium-term, there was no significant impact of CORT on the body mass and whole-body energetics assessed through heart rate and physical activity. Moreover, CORT chicks displayed lower oxidized glutathione on the medium and long-term, thus indirectly preventing
oxidative damage, while telomere length and mitochondrial density were not influenced by CORT implants. Although chronic elevation of glucocorticoid levels is mostly known to result in negative physiological and behavioral outcomes, our results indicating some positive effects of CORT implants are consistent with the hypothesis that species who encounter several noxious stressors in their environment, could be naturally adapted to cope with them. The growth-enhancing effect of CORT might be mediated by a higher efficiency at which food is assimilated and/or used, and CORT chicks might benefit from aggressive behaviors during competitive interactions or predation
encounters. Through the antioxidant glutathione system, CORT treated chicks seem to have developed a resistance to oxidative stress that could last over time (i.e. possibly through a reduction
of ROS production). While CORT implants did not accelerate ageing or bring short and mediumterm costs, possible long lasting programming effects of CORT should be further investigated.