Effects of Immune Activation and Glucocorticoid Administration on Feather Growth in Greenfinches MARJU MA ¨ NNISTE AND PEETER HO ˜ RAK Ã Department of Zoology, Institute of Ecology and Earth Sciences, Tartu University, Tartu, Estonia 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. J. Exp. Zool. 315:527–535, 2011. & 2011 Wiley Periodicals, Inc. How to cite this article: Ma ¨nniste M, Ho ˜rak P. 2011. Effects of immune activation and glucocorticoid administration on feather growth in greenfinches. J. Exp. Zool. 315:527–535. Immune defences belong to the most complicated and resource- demanding organismal functions (Schulenburg et al., 2009). Detection and destruction of parasites and pathogens involves construction of sophisticated recognition and memory pathways, release of harmful substances and increased metabolism. All those impose costs, which eventually lead to physiological trade- offs in allocation of resources between immunity and other components of fitness. A nascent discipline—immunoecology— proposes that such trade-offs have major impact on the evolution of physiological and life-history strategies (Sheldon and Verhulst, ’96; Lochmiller and Deerenberg, 2000; Lee, 2006; Schulenburg et al., 2009). With the advancement of immunoecology, the initially naı ¨ve contention of many animal ecologists that immunocompetence can be considered as an ubiquitous com- modity (the stronger the response, the better) has been gradually replaced with the appreciation of the importance of proper downregulation and termination of immune responses (Ra ˚berg et al., ’98; Graham et al., 2005, 2010; Day et al., 2007; Martin, 2009; Sorci and Faivre, 2009). Those conceptual advancements, however, have not yet been caught up with sufficient empirical work. Important role in downregulation and termination of the immune responses belongs to glucocorticoid (GC) hormones (Perretti and D’Acquisto, 2009), cortisol (in humans), or Published online 6 September 2011 in Wiley Online Library (wileyonline library.com). DOI: 10.1002/jez.701 Received 5 April 2011; Revised 25 May 2011; Accepted 20 July 2011 Grant Sponsor: Estonian Science Foundation; Grant number: 7737; Grant Sponsor: Estonian Ministry of Education and Science; Grant number: 0180004s09; Grant Sponsor: European Union through the European Regional Development Fund (Centre of Excellence Frontiers in Biodiversity Research). Ã Correspondence to: Peeter Ho ˜rak, Department of Zoology, Institute of Ecology and Earth Sciences, Tartu University, Vanemuise 46, 51014 Tartu, Estonia. E-mail: horak@ut.ee ABSTRACT J. Exp. Zool. 315:527–535, 2011 & 2011 WILEY PERIODICALS, INC. RESEARCH ARTICLE