Biological Journal of the Linnean Society, 2003, 79, 503–510. With 3 figures © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 79, 503–510 503 Blackwell Science, LtdOxford, UKBIJBiological Journal of the Linnean Society0024-4066The Linnean Society of London, 2003? 2003 79? Original Article COURTSHIP SONG and IMMUNE FUNCTIONM. J. RANTALA and R. KORTET *Corresponding author. E-mail: marrant@dodo.jyu.fi Courtship song and immune function in the field cricket Gryllus bimaculatus MARKUS J. RANTALA* and RAINE KORTET Department of Biological and Environmental Science, University of Jyväskylä, PO Box 35, FIN-40351, Jyväskylä, Finland Received 27 August 2002; accepted for publication 3 January 2003 It has been assumed that sexual ornaments have evolved to reveal males’ health and vigour for females. Choosy females may indirectly use ornaments as an indicator of the presence and effectiveness of genes for resistance against parasites. In this study we tested whether females of the Mediterranean field cricket, Gryllus bimaculatus, can use courtship song as a cue for choosing males with high immunocompetence, measured as encapsulation rate of nylon implants and lytic activity of haemolymph. We found that female crickets preferred courtship songs from males with a high encapsulation rate. Female crickets also had a tendency to prefer courtship songs with high tick rate and long high-frequency tick duration. These preferred song components were positively correlated with encap- sulation rate, but negatively correlated with lytic activity of the male. In contrast to previous studies of crickets, there was no correlation between male weight and encapsulation rate or lytic activity. There is some evidence in another cricket species that the ability to encapsulate pathogens is heritable. Thus, in light of this study it seems pos- sible that by preferring males according to their courtship song, females might benefit by increasing the parasite resistance of their offspring. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 79, 503–510. ADDITIONAL KEYWORDS: immunocompetence – Phonotaxids – sexual ornaments – sexual selection. INTRODUCTION Hamilton & Zuk (1982) proposed that the sexual orna- ments on which a female bases her choice reveals a male’s health and vigour, which can be affected by par- asites. Females may therefore indirectly select for genetic resistance to parasites. Many observational and experimental studies support Hamilton & Zuk’s hypothesis (e.g. Møller, 1988, 1990; Clayton, 1990; Milinski & Bakker, 1990; Zuk et al., 1990a,b; Houde & Torio, 1992; Saino & Møller, 1996; Taskinen & Kortet, 2002; for reviews, see Clayton, 1991; Møller & Saino, 1994), but no such relationship was found in many other studies (for review see Hamilton & Poulin, 1997). The immunocompetence handicap hypothesis offers a mechanistic extension to Hamilton & Zuks’ theory (Folstad & Karter, 1992; Wedekind & Folstad, 1994). According to this hypothesis, sexual trait expression may be constrained through a trade-off with immune function. The hypothesis suggests that secondary sex- ual characters develop in response to circulating androgens (or other self-regulating biochemicals that increase the expression of secondary sexual characters but reduce the functioning of the immune system), which suppress the ability of individuals to raise an immune defence against parasites (Folstad & Karter, 1992). The immunocompetence handicap hypothesis argues that because the production of a secondary sex trait is costly, animals may have to shift energy and metabolites from other tissue to produce well- developed secondary sex characters. Sex hormones may accomplish this by shutting off energy from the immune system and other systems, so that it can be