Plasticity of immune function and condition under the risk of predation and parasitism Gerrit Joop 1,2 and Jens Rolff 2 * 1 AG Ökologie, Zoologisches Institut, Technische Universität Braunschweig, D-38092 Braunschweig, Germany and 2 Department of Animal and Plant Sciences, University of Sheffield, Western Bank, Sheffield S10 2TN, UK ABSTRACT Ecological immunology attempts to elucidate the causes of the large variation in immunity and resistance observed in natural populations. Here we report on a novel experiment that investigated how the risks of parasitism and predation altered investment in immunity and condition in insects during larval development. The study organism is the damselfly Coenagrion puella, the parasite is a water mite and the predators are encaged Aeshna cyanea dragonflies. Our experiments show that females increase their investment in a cellular as well as a humoral component of the immune system in the presence of natural enemies. By contrast, males do not show such alteration. However, males show altered condition under the risks of parasitism and predation. Our results highlight the importance of species interactions for the plasticity of immune function. Keywords: dragonflies, ecological immunology, differences between the sexes, natural enemies, plasticity, water mites. INTRODUCTION In the wild, organisms face multiple natural enemies, which are often present simultaneously (Crawley, 1992; Sih et al., 1998). Facing multiple enemies can result in trade-offs, such that avoiding one enemy results in a higher encounter rate with another enemy (Decastaeker et al., 2002). However, only a few studies have examined the impact of multiple enemies simultaneously. These have focused on behavioural as well as life-history changes (e.g. Baker and Smith, 1997; Eklöv and Werner, 2000; Decastaeker et al., 2002; Lass and Bittner, 2002). Our aim was to examine the effects of multiple natural enemies on the plasticity of immune function and condition (see Rolff and Joop, 2002, for the use of condition) in larval insects. We concentrate on these traits because they are closely linked to fitness and to one another (Plaistow and Siva-Jothy, 1996; Rolff and Siva-Jothy, 2003; Schmid-Hempel, 2003). Furthermore, we included sex in our analysis. Differences between the sexes are often * Author to whom all correspondence should be addressed. e-mail: jor@sheffield.ac.uk Consult the copyright statement on the inside front cover for non-commercial copying policies. Evolutionary Ecology Research, 2004, 6: 1051–1062 © 2004 Jens Rolff