Molecular Ecology (2004) 13, 1625–1633 doi: 10.1111/j.1365-294X.2004.02172.x © 2004 Blackwell Publishing Ltd Blackwell Publishing, Ltd. Extra-pair paternity does not result in differential sexual selection in the mutually ornamented black swan ( Cygnus atratus ) K. KRAAIJEVELD,* P. J. CAREW, T. BILLING, GREG J. ADCOCK and RAOUL A. MULDER Department of Zoology, University of Melbourne, VIC 3010, Australia Abstract We studied patterns of parentage in 85 broods (332 cygnets) of black swans during three breeding seasons, using a set of eight polymorphic microsatellite markers. We detected both intraspecific brood parasitism (IBP; < 5% of cygnets per year) and extra-pair paternity (EPP). In these years, 10 –17% (mean = 15.1%) of cygnets resulted from EPP, and 27 – 40% (mean 37.6%) of broods contained at least one extra-pair cygnet. Compared with levels of EPP in closely related species with similar life histories, these values are unexpectedly high. EPP in black swans appears unrelated to ecological factors (breeding density and synchrony) or genetic factors (genetic similarity between pair members or genetic quality of the offspring). We found no evidence that a mutual sexual feather ornament known to play a role in social mate choice in black swans (curled wing feathers) is involved in extra-pair mate choice. EPP does not lead to greater variance in reproductive success in males, relative to females in this species. We therefore suggest that EPP does not result in differential sexual selection on males and females, explaining why they are ornamented to the same degree. Keywords: breeding density, breeding synchrony, Cygnus atratus, extra-pair paternity, mutual ornamentation, offspring viability Received 27 December 2003; revision received 9 February 2004; accepted 9 February 2004 Introduction During the past decade it has become clear that extra-pair paternity (EPP) is widespread among birds (Birkhead 1998). However, general explanations for the considerable variation in rates of EPP found between and within species remain elusive. Some of the factors apparently associated with EPP within species do not explain differences be- tween species, and vice versa. In an attempt to resolve this problem, a hierarchical explanation for variation in EPP has been proposed (Bennett & Owens 2002; Griffith et al . 2002). In this view, the different rates of EPP observed among distantly related species are due to differences in fundamental life history parameters, such as parental care and reproductive lifespan (Mauck et al . 1999; Arnold & Owens 2002). However, intraspecific variation in EPP is more likely to be determined by the genetic benefits of alternative reproductive strategies (Kempenaers et al . 1992; Hasselquist et al . 1996; Sheldon et al . 1997) and the ecolog- ical opportunities to engage in them (e.g. breeding density and breeding synchrony, Dunn et al . 1994; Stutchbury & Morton 1995; Thusius et al . 2001; Bennett & Owens 2002). For a detailed review of these and other hypotheses to explain variation in rates of EPP, see Griffith et al . (2002). Another aspect of EPP that is the focus of much debate is its adaptive function (Griffith et al . 2002). Females engag- ing in EPP may gain benefits such as fertility insurance and increased offspring fitness, for example, through increased genetic compatibility or ‘good genes’ (Kempenaers & Dhondt 1993; Jennions & Petrie 2000). The fact that females engage in EPP potentially intro- duces a sexual selection pressure in the population. If females are selective about which extra-pair male they mate with and if preferences among females coincide, the Correspondence: K. Kraaijeveld. *Present address Department of Biology, Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK. Tel.: +44 (0)20 7679 7425; Fax: +44 (0)203832048; E-mail: k.kraaijeveld@ucl.ac.uk