Biological Journal of the Linnean Society, 2007, 90, 509–516. With 1 figure © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 509–516 509 Blackwell Publishing LtdOxford, UKBIJBiological Journal of the Linnean Society0024-4066© 2007 The Linnean Society of London? 2007 903 509516 Original Article IMMUNE FUNCTION AND FORCEPS SIZE M. J. RANTALA ET AL . *Corresponding author. E-mail: marrant@cc.jyu.fi Forceps size and immune function in the earwig Forficula auricularia L. MARKUS J. RANTALA 1,2 *, DEREK A. ROFF 1 and LIISA M. RANTALA 3 1 Department of Biology, University of California, Riverside, CA 92521, USA 2 Department of Biology, Section of Ecology, University of Turku, FIN-20014 Turku, Finland 3 Department of Biological and Environmental Science, University of Jyväskylä, PO Box 35, FIN-40351, Jyväskylä, Finland Received 6 October 2005; accepted for publication 20 June 2006 Females of many species select their mates on the basis of the size or intensity of sexual ornaments, and it has been suggested that these provide reliable signals of a male’s ability to resist parasites and pathogens. European earwigs, Forficula auricularia, are sexually dimorphic in forceps shape and length. Male forceps are used as weapons in male contests for access to females, but recent findings suggest that females also choose males on the basis of their forceps length. In the present study, we tested the hypotheses that in the European earwig, F. auricularia, the size of forceps is correlated with immune function and that immune function differs between the sexes. We found that encapsula- tion rate was not correlated with the length of forceps in either sex, but was negatively correlated with body size. By contrast, lytic activity of the haemolymph increased with overall body size in both sexes but, in females, lytic activity increased with relative forceps length whereas, in males, it decreased with relative forceps length. After accounting for effects of body size, there was no remaining significant correlation in females but the negative correlation in males remained. Furthermore, we found that males had higher encapsulation rate and higher lytic activity than females, suggesting that males have stronger immune defence. The results of the study indicate that the size of forceps in male earwigs does not reliably reflect male immune defence. © 2007 The Linnean Society of London, Biological Journal of the Linnean Society, 2007, 90, 509–516. ADDITIONAL KEYWORDS: encapsulation rate – immunocompetence – lytic activity – sexual ornament – sexual selection. INTRODUCTION Indicator models of sexual selection propose that sec- ondary sexual ornaments preferred by females may reliably signal male health and vigour by acting as handicaps (i.e. characters that impose costs to their bearer) (Zahavi, 1975, 1977; Heywood, 1989; Grafen, 1990; but see also Getty, 2002). It has been suggested that one such possible cost is an increased susceptibil- ity to parasites and pathogens (Folstad & Karter, 1992), which could influence the development and maintenance of secondary sexual ornaments through two major mechanisms. First, in cases where the acquisition of important metabolites for ornamental development results in increased exposure to para- sites and pathogens, ornaments may reveal an indi- vidual’s ability to tolerate parasite exposure (Folstad et al., 1994). Second, because development and/or maintenance of secondary sexual ornaments may impose costs to a male’s immune defence, they may honestly signal a male’s parasite resistance (Folstad & Karter, 1992). For example, if resources must be diverted away from immune function to maximize the expression of a trait, males may suffer increased susceptibility to parasites and pathogens (Folstad & Karter, 1992; Sheldon & Verhulst, 1996). Thus, only males in good condition or with high immunocompe- tence, or both, can afford to allocate resources to large ornaments, which should produce a positive correla- tion between immune defence and the expression of costly secondary sexual characters (Westneat & Birkhead, 1998; Rantala, Kortet & Vainikka, 2003b).