Effects of immune activation and glucocorticoid administration on feather growth in greenfinches

Elevation of glucocorticoid (GC) hormone levels is an integral part of stress response (as well as its termination) and immunomodulation. These hormones are also responsible for mobilizing energy stores by stimulation of gluconeogenesis and inhibition of protein synthesis. Elevation of GCs is thus incompatible with other protein-demanding processes, such as moult. Previous studies have shown that chronic elevation of GC hormones suppresses feather growth. Here, we asked whether similar effect would also occur in the case of acute GC elevation and induction of an inflammatory response by foreign antigen. We performed an experiment on captive wild-caught greenfinches (Carduelis chloris) injecting birds with phytohaemagglutinin (PHA) and dexamethasone (DEX) in a factorial design. To assess the possible somatic impacts of these manipulations, we removed one of the outermost tail feathers before the experiment and measured mass and rachis diameter and length of the replacement feathers grown in captivity. Immunostimulation by PHA reduced rachis length, but did not affect feather mass or rachis diameter. Single injection of a synthetic GC hormone DEX significantly reduced all three parameters of feather size. Altogether, these findings demonstrate the sensitivity of feather growth to manipulation of immune and adrenal functions. Our results corroborate the somatic costs of immune activation and suggest that even a short-term elevation of GC hormones may induce long-term somatic costs with a potential impact on fitness. Our findings also imply that a single injection of DEX, frequently used as a diagnostic tool, can have lasting effects and researchers must consider this when designing experiments.