letters to nature between 0.2 and 1.0 in nestlings. As not all individuals were typed with the same set of pedigrees and (2) distinguishing full-sib pair members. Anim. Genet. 25, 37–44 (1994). microsatellite markers, we calculated standardized individual heterozygosity10 29. Pemberton, J. M., Coltman, D. W., Coulson, T. N. & Slate, J. in Microsatellites, Evolution and (proportion of heterozygous loci/mean heterozygosity of typed loci). This measure is used Applications (eds Goldstein, D. B. & Schlo¨tterer, C.) 151–164 (Oxford Univ. Press, Oxford, 1999). in all analyses and is referred to as ‘individual heterozygosity’ for simplicity. We also 30. Turelli, M. & Ginzburg, L. R. Should individual fitness increase with heterozygosity? Genetics 104, calculated two other, previously suggested measures of individual genetic diversity: 191–209 (1983). internal relatedness10 and mean d2-value29 where d 2 is the squared length difference between the alleles at a locus. Heterozygosity, standardized heterozygosity and internal Acknowledgements We thank D. Blomqvist, S. Griffith, D. Hasselquist, L. Keller, M. Milinski, relatedness were highly correlated (all r . 0.89, P , 0.0001) and all three measures led to A. Peters, B. Sheldon, C. Wedekind and D. Zeh for comments on the manuscript; K. Carter, similar conclusions (data not shown). Mean d 2-values were weakly correlated with the D. Kaulfuss, H. Kunc, K. Peer, A. Po¨sel and A. Tu¨rk for help with field and laboratory work; other measures (r ¼ 0.24–0.32, all P , 0.0001) and explained comparatively little S. Andersson for computing the colour variables; and H. Winkler (Konrad Lorenz Institute for variation in individual fitness. Comparative Ethology) and R.-T. Klumpp and A. Fojt (Institute of Silviculture, University of The assumption that heterozygosity measured at a limited number of loci reflects Agricultural Sciences, Vienna) for logistic support. genome-wide heterozygosity can be questioned30. Any relationship between average heterozygosity and fitness might be due to close linkage between a microsatellite locus and Competing interests statement The authors declare that they have no competing financial loci coding for fitness-related traits. To check whether the effect of overall heterozygosity interests. on fitness depended on one or a few of the microsatellite loci, we repeated all analyses for each locus separately (data not shown). Heterozygosity at six out of seven loci significantly Correspondence and requests for materials should be addressed to B.K. affected at least one fitness measure, and different fitness measures were predicted by ([email protected]). heterozygosity at different loci, so we only present data on overall heterozygosity. We calculated pairwise relatedness between partners using Relatedness 5.0 (http:// www.gsoftnet.us/GSoft.html). The average relatedness between adult males and females (expected r ¼ 0) in our study population was 20.0007 (s.e.m. calculated by ‘jack-knifing’ over loci ¼ 0.0009). In a random sample of 100 broods, the average relatedness among full siblings (expected r ¼ 0.5) was 0.47 (s.e.m. ¼ 0.02). The mean inbreeding coefficient of .............................................................. our population (F is), calculated from the microsatellite data using Fstat V2.9.3, equals 20.006. The observed level of inbreeding was 3.3%: two of 61 local recruits (with known RNA molecules stimulate prion parents) mated with close relatives (r ¼ 0.5 and 0.125). Received 21 March; accepted 25 July 2003; doi:10.1038/nature01969. protein conversion 1. Jennions, M. D. & Petrie, M. Why do females mate multiply? A review of the genetic benefits. Biol. Rev. 75, 21–64 (2000). Nathan R. Deleault, Ralf W. Lucassen & Surachai Supattapone 2. Tregenza, T. & Wedell, N. 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