Functional Ecology 2006 20, 873–879 873 © 2006 The Authors. Journal compilation © 2006 British Ecological Society Blackwell Publishing Ltd Immune challenge reduces reproductive output and growth in a lizard T. ULLER,*‡ C. ISAKSSON† and M. OLSSON* *Institute for Conservation Biology, School of Biological Sciences, University of Wollongong, 2522 NSW, Australia, and †Department of Zoology, Göteborg University, Medicinaregatan 18, 405 30 Göteborg, Sweden Summary 1. A fundamental assumption in evolutionary immunology is that the immune system is costly to develop, maintain or activate. 2. Two plausible costs of activation of the immune system are decreased resources for growth and reproductive investment. However, few studies have estimated direct effects of an immune challenge, in particular in ectotherm vertebrates. We studied the con- sequences of an immune challenge in reproductive female dragons, Ctenophorus fordi, and in their offspring by exposing lizards to bacterial lipopolysaccharides (LPS). 3. The immune challenge led to decreased reproductive investment in terms of egg mass, but with no effect on probability of future reproduction. 4. Maternal immune challenge did not influence the response of their offspring to the same challenge. However, juveniles that were induced to mount an immune response had a higher thermal preference and showed reduced growth, but the magnitude of the growth effect was dependent on the date of oviposition, indicating maternal effects on offspring immunity. 5. Our results suggest that costs of immune activation may be important in shaping growth and reproductive strategies in ectotherms. Key-words: Cost, immune response, LPS, maternal effects Functional Ecology (2006) 20, 873–879 doi: 10.1111/j.1365-2435.2006.01163.x Introduction The immune system is of fundamental importance for an organism’s survival and therefore plays a central role in evolutionary ecology (Sheldon & Verhulst 1996; Lochmiller & Deerenberg 2000; Norris & Evans 2000; Schmid-Hempel 2003). If immunity is costly, life- history theory predicts that investment into the immune system should be traded off against other aspects of an individual’s fitness, such as reproductive investment (Sheldon & Verhulst 1996). Expenditure on immunity can be divided into three cost categories; production, maintenance and usage (reviewed in Klasing & Leshchinsky 1998; Lochmiller & Deerenberg 2000; Zuk & Stoehr 2002). Production costs imply that ontogenetic development of the immune system will interfere with resource allocation to growth and development (Klasing 1998; Soler et al . 2003; Brommer 2004; Jacot et al . 2005; Uller, Andersson & Eklöf 2006). Maintenance costs imply that immunity is paid for continuously and regardless of pathogen exposure (although high exposure to pathogens may lead to a higher up-regulation of the immune defence and therefore a higher cost; Lochmiller & Deerenberg 2000; Derting & Compton 2003). Finally, usage costs are paid only when the immune system is activated, and this magnitude should depend on the level of the immune response (Derting & Compton 2003; Martin, Scheuerlein & Wikelski 2003; Jacot, Scheuber & Brinkhof 2004). The relative importance of these costs will determine to what extent specific traits (e.g. reproductive investment) are traded off against immunocompetence. Documentation of costs of immune function is therefore central to our under- standing of the role of immunity in life-history evolu- tion (Ricklefs & Wikelski 2002; Schmid-Hempel 2003; Viney, Riley & Buchanan 2005). Providing strong evidence for direct costs of immunity can be difficult since pathogen effects on host health may confound costs of immunity per se . Furthermore, allocation of resources among functions other than the immune system may lead to similar pat- terns as would a direct cost of mounting an immune response. As a consequence, we still have a very limited understanding of the importance of costs of immunity at both the proximate (e.g. elevated metabolic rate) and the ultimate (e.g. life-history decisions) level. However, ‡Author to whom correspondence should be addressed. E-mail: uller@uow.ed.au