Age-related differences in baseline and stress-induced corticosterone in Florida scrub-jays

In physiological studies of free-living species, it is essential to consider the context of the life history stage at which an individual was observed in order to link measures of physiology with ecological parameters. One such measure that is important to consider is the age of an individual. We tested whether baseline or stress-induced corticosterone levels vary with age in free-living Florida scrub-jays (Aphelocoma coerulescens) during the pre-breeding period. Corticosterone (CORT), the primary avian stress hormone, is released in response to stressful stimuli, and stimulates gluconeogenesis; however, it also serves as a chemical messenger that can influence other physiological processes, reproduction, and behavior. We monitored both baseline CORT levels longitudinally throughout a five-year period and stress-induced CORT responses over a shorter two-year period. We predicted that older jays would have lower baseline CORT levels and a dampened stress response compared to younger birds, as has been shown in other avian species. We found no significant differences in baseline CORT levels with age. We found a decrease in total corticosterone responses to a stressor with age, however, the oldest birds in the population showed greater total corticosterone responses to a stressor. These results may be a product of age-related changes in physiological processes related to the stress response or a result of selection acting on the population, resulting in only the most responsive individuals surviving to old age.