JOURNAL OF AVIAN BIOLOGY 32: 338 – 344. Copenhagen 2001 Extra-pair paternity among Great Tits Parus major following manipulation of male signals Ken A. Otter, Ian R. K. Stewart, Peter K. McGregor, Andrew M. R. Terry, Torben Dabelsteen and Terry Burke Otter, K. A., Stewart, I. R. K, McGregor, P. K., Terry, A. M. R., Dabelsteen, T. and Burke, T. 2001. Extra-pair paternity among Great Tits Parus major following manipulation of male signals. – J. Avian Biol. 32: 338–344. Female Great Tits are known to eavesdrop on the singing behaviour of males. It is unknown, however, whether manipulation of these signals is sufficient to influence extra-pair copulations, or whether such potentially costly reproductive decisions are unaffected by altering short-term signals of condition. Using interactive playbacks, we systematically engaged males in territorial contests in which we could control whether the focal male won or lost the interaction, regardless of the males’ true potential. We then determined the levels and patterns of extra-pair paternity among experimental and neighbouring pairs using DNA microsatellite analysis. Extra-pair young were found in equal frequency among the nests of males allowed to win interactions as among those who lost interactions. However, cuckolded males were significantly less variable in allele sizes across the five microsatellite alleles tested than males who were not cuckolded. As measures of genetic variability are increasingly being found to correlate with individual fitness, this may suggest that females are attentive to underlying measures of condition when making extra-pair decisions. Short-term modification of the males’ perceived quality may be insufficient to cause females to alter extra-pair decisions. K. A. Otter (correspondence), Biology Program, NRES, Uniersity of Northern BC, 3333 Uniersity Way, Prince George, BC, Canada. E-mail: otterk@unbc.ca. I. R. K. Stewart and T. Burke, Department of Animal and Plant Sciences, Uniersity of Sheffield, Sheffield, United Kingdom. P. K. McGregor, A. M. R. Terry, and T. Dabelsteen, Department of Animal Behaiour, Zoological Institute, Uniersity of Copenhagen, Denmark. Extra-pair behaviour is now a commonly described phenomenon among socially monogamous passerines (Petrie and Kempenaers 1998). In many species, extra- pair behaviour is largely female-driven (Smith 1988, Houtman 1992, Kempenaers et al. 1992, 1997, Otter et al. 1998), prompting hypotheses on the benefits females derive from these matings (Kempenaers and Dhondt 1993). Among these is the genetic benefits hypothesis; females seek males higher in quality than the females’ mates for the genetic material they can provide the females’ offspring (Smith 1988, Kempenaers et al. 1992, 1997, Graves et al. 1993, Weatherhead and Boag 1995, Hasselquist et al. 1996, Otter et al. 1998). However, failure to find correlations between physical characteris- tics of males and their success in extra-pair paternity has led to other hypotheses being proposed, such as: females assuring fertility of eggs (Wetton and Parkin 1991, Sheldon 1994, Krokene et al. 1998); previewing future mates (Wagner 1991); increasing genetic compatibility or heterozygosity (Brown 1997, Kempenaers et al. 1999, Weatherhead et al. 2000); and gaining material benefits from extra-pair males (Gray 1997). Great Tits Parus major engage in moderate levels of extra-pair fertilizations (Blakey 1994, Krokene et al. 1998, Strohbach et al. 1998, Lubjuhn et al. 1999). Two of these studies, however, failed to find correlations between physical attributes of the males and extra-pair success (Strohbach et al. 1998, Krokene et al. 1998), leading Krokene et al. to suggest that extra-pair be- haviour in this species may serve primarily to assure fertilization of the clutch. By contrast, studies on two related species, the Blue Tit Parus caeruleus (Kempe- © JOURNAL OF AVIAN BIOLOGY JOURNAL OF AVIAN BIOLOGY 32:4 (2001) 338