Marine lifestyle is associated with higher baseline corticosterone levels in birds

Because seawater is hyperosmotic relative to body fluids of most vertebrates, marine lifestyle is expected to strongly influence the physiology of marine tetrapods. Regulating the salt content of body fluids is energetically costly; and osmoregulatory organs may not totally overcome salt load and/or water loss. As a consequence, marine lifestyle should influence physiological systems involved in the maintenance of the physiological balance (homeostasis), in the mobilisation of energetic resources (e.g., to fuel salt excretion), or in the acquisition of resources (e.g., fresh water). Corticosterone (CORT) is one such generalist' mediator that is linked with energy expenditure, physiological stress and that activates osmoregulation. As a consequence, CORT is expected to be overall higher in marine tetrapods but this hypothesis has never been tested. Using comparative analyses, we tested this hypothesis in birds, a lineage for which available data on baseline CORT allow comparing marine versus terrestrial species, and species with or without salt glands. We found that marine species (and species with salt glands) display significantly higher baseline CORT during the wintering (but not the breeding) stage. Although salt glands' presence was tightly linked to phylogeny, our results suggest that marine lifestyle may impose a strong, but overlooked, influence on the allostasis-related physiology of marine birds. Such habitat-related variation in physiology is a major phenomenon to explore owing to its general implications for understanding the physiological basis of evolutionary transitions in habitat use.(c) 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115, 154-161.