RESEARCH ARTICLE Christophe Pe´ labon Æ A ˚ sa A. Borg Æ Jens Bjelvenmark Iain Barber Æ Elisabet Forsgren Æ Trond Amundsen Do microsporidian parasites affect courtship in two-spotted gobies? Received: 31 March 2004 / Accepted: 20 May 2005 / Published online: 13 August 2005 Ó Springer-Verlag 2005 Abstract Courtship and body condition of male two- spotted gobies (Gobiusculus flavescens) harbouring nat- urally acquired microsporidian infections were com- pared with those of microsporidian-free males under standardized conditions in the lab. Although parasite infection had no apparent effect on individual condition, it significantly affected male courtship intensity. This effect, however, was affected by the intensity of the fe- male courtship. Our study is one of the first to demon- strate a sub-lethal behavioural effect of a microsporidian parasite that could negatively affect the reproductive success of infected individuals. Our results also suggest that secondary sexual traits like courtship may be more sensitive to moderately detrimental effects of parasite infection than classical condition indices. Introduction Parasite infections can dramatically affect the population dynamics of their hosts by reducing host recruitment or survival (Minchella and Scott 1991; Poulin 1999). How- ever, parasites may also negatively affect the mating success of their host either by castration mechanisms (Hurd 2001) or by the alteration of sexual signals such as ornaments or behavioural displays (Kennedy et al. 1987; reviewed in Barber et al. 2000). By utilising nutrients normally used by their hosts for maintenance or by deteriorating specific host tissues, parasite infections can negatively affect condition-dependent secondary sexual traits (Hamilton and Zuk 1982; Folstad and Karter 1992; Houde and Torio 1992; Andersson 1994; Thompson et al. 1997) including courtship display (e.g. Borgia et al. 2004). Parasite infections may have complex effects on courtship display. Because selection should favour individuals that identify and select against parasitized mating partners, conspicuous infections can negatively affect attractiveness and mating success even in the ab- sence of any dramatic debilitating effect on host condi- tion (Milinski and Bakker 1990; Dugatkin et al. 1994; Rosenqvist and Johansson 1995; Barber et al. 2000). By incorporating individual differences in attractiveness into theoretical models of condition-dependent sexual selection, Reynolds (1993) suggested that morphologi- cally less attractive individuals might increase their attractiveness by courting at a higher rate, thus render- ing courtship dishonest with respect to individual con- dition. Therefore, an increase in courtship rate by parasitized males may be expected in the case of con- spicuous parasite infections with low virulence. An increase in courtship rate by parasitized individ- uals may also be expected if parasite infections reduce host survival. Normally, investment in a reproductive attempt is made at a sub-maximum level due to the trade-off between current and future reproduction (Williams 1966). Individuals infected with parasites that reduce longevity, and hence the prospect of future reproduction, may be selected to invest more heavily in the current reproduction, a phenomenon referred to as ‘‘terminal investment’’ (Forbes 1993; Polak and Starmer 1998; Candolin 2000). Because parasites might affect the amount of energy available for mate sampling, or interfere with sensory performance, infections can also affect mating prefer- ences of parasitized individuals, hence reducing their choosiness (Poulin 1994; Poulin and Vickery 1996; Lo- pez 1999; but see Zuk et al. 1998). Also, any decrease in Communicated by M. Ku¨hl, Helsingør C. Pe´labon (&) Æ A ˚ sa A. Borg Æ E. Forsgren Æ T. Amundsen Department of Biology, Norwegian University of Science and Technology, 7491 Trondheim, Norway E-mail: christophe.pelabon@bio.ntnu.no J. Bjelvenmark Department of Marine Ecology, Go¨teborg University, Kristineberg Marine Research Station, Fiskeba¨ckskil, Sweden I. Barber Institute of Biological Sciences, University of Wales, Aberystwyth, UK Marine Biology (2005) 148: 189–196 DOI 10.1007/s00227-005-0056-8