Inbreeding depression and male fitness in black grouse

The male lifetime lekking performance was studied, and related to inbreeding‐outbreeding in a wild population of black grouse ( Tetrao tetrix ) in central Finland between 1989 and 1995. Inbreeding was measured as the mean heterozygosity and mean d 2 of 15 microsatellite loci. We found a significantly positive relationship between mean d 2 and lifetime copulation success (LCS), while the relationship between heterozygosity and LCS was close to significant. We also found that males that never obtained a lek territory had significantly lower mean heterozygosity than males that were observed on a territory at least during one mating season in their life. Furthermore, among males that were successful in obtaining a lek territory, LCS and mean d 2 were highest for those males that held central territories. We suggest that inbred males have a disadvantage (or outbred males have an advantage) in the competition for territories that may explain the relationships with LCS and inbreeding. Furthermore, the fact that mean d 2 was positively correlated with LCS whereas heterozygosity was not when we restricted the analysis to territorial males, suggests that mean d 2 provides more information about levels of inbreeding‐outbreeding than heterozygosity alone, and potentially highlights the effects of heterosis. To our knowledge, this is the first time that measures of inbreeding and lifetime fitness have been linked in a non‐isolated population. This is important in establishing that the relationships found in previous studies are not artefacts of low gene flow created by limited dispersal but a general feature of wild vertebrate populations.