Abstract The evolution of alternative male mating phe- notypes inevitably involves variation in risk or intensity of sperm competition. Males that typically mate in disfa- voured roles (i.e. sneakers/parasitic males) are predicted to experience higher magnitudes of sperm competition than males mating in favoured roles (preferred by fe- males/exhibiting mate monopolisation). Here, we inves- tigated whether two distinct male phenotypes in cork- wing wrasse (Symphodus melops) differ with respect to adaptations for sperm competition in terms of sperm quantity, sperm motilityand fertilising capacity. We found that males exhibiting female mimicry (disfavoured role) had both absolutely and relatively larger gonads than males with typical secondary sexual characters (fa- voured role). Moreover, a higher proportion of sperm from female mimics was motile 5 min after activation compared to sperm of territorial males. There were, how- ever, no differences in sperm concentration or in in vitro fertilisation ability between the two male morphs. We also examined whether the male phenotypes differed in some immune parameters and if any such variation is as- sociated with sperm traits. Current theories predict that ejaculate quality should be negatively correlated with the level of immunocompetence. Territorial males had rela- tively heavier spleens than female mimics, but whether this reflects differing immunocompetence is uncertain, since the spleen is also an important organ for erythro- cyte storage. Furthermore, there were no differences in the densities of circulating lymphocytes or granulocytes, and the immune parameters did not covary with the spermtraits.Differences in immunocompetence are therefore unlikely to provide a major proximate explana tion for variations in sperm traits in corkwing wrasse. Keywords Symphodus melops · Corkwing wrasse · Alternative mating phenotypes · Male dimorphism · Sperm competition · Immunology Introduction Competition for partners and mate choice may lead to t evolution of alternative reproductive phenotypes, which theoretically be maintained through two types of select (Gross1996). First,by frequency-dependent selection, which implies that the different reproductive forms maintained at an evolutionary equilibrium with equ ness, and second, by status-dependent selection. The la implies that expression of the different tactics depends individual status or condition. Alternative reproductive notypes are found in most major taxa, with at least 140 species belonging to 28 different families exhibiting alt native mating patterns (Taborsky 1994, 1998; Gross 19 Sperm competition occurs when sperm from differen males compete for fertilisation (Parker 1970). The evolu tion of alternative male mating phenotypes inevitably in volves within-sex variation in the magnitude of spe competition (Parker 1998). Theoretical models regardin sperm competition in organisms with external fertil tion predict that males typically mating in disfavou roles (i.e. males exhibiting mating behaviours not i volving mate monopolisation or utilisation of female preferences) experience higher risk or intensity of sper competition than males mating in favoured roles (Parke 1998; Petersen and Warner 1998). The disfavoured males should consequently be selected to offset this dis advantage by investing relatively more into sperma genesis (Parker 1982, 1990a, 1990b, 1993; Gage e 1995). More precisely, males exhibiting disfavoured mating patterns are predicted to have relatively lar gonads for their size than males mating in favoure Communicated by J. Krause I. Uglem ( ✉ ) · T.F. Galloway · G. Rosenqvist Department of Zoology, Norwegian University of Technology and Science, 7491 Trondheim, Norway e-mail: ingebrigt.uglem@chembio.ntnu.no Tel.: +47 73-596278, Fax: +47 73-591309 I. Folstad Department of Ecology, Faculty of Science, University of Tromsø, 9037 Tromsø, Norway Behav Ecol Sociobiol (2001) 50:511–518 DOI 10.1007/s002650100392 O R I G I N A L A RT I C L E Ingebrigt Uglem · Trina Falck Galloway Gunilla Rosenqvist · Ivar Folstad Male dimorphism, sperm traits and immunology in the corkwing wrasse ( Symphodus melops L.) Received: 15 December 2000 / Revised: 26 May 2001 / Accepted: 6 June 2001 / Published online: 27 July 2001 © Springer-Verlag 2001