Dunnock social status correlates with sperm speed, but fast sperm does not always equal high fitness

Sperm competition theory predicts that males should modulate sperm investment according to their social status. Sperm speed, one proxy of sperm quality, also influences the outcome of sperm competition because fast sperm cells may fertilize eggs before slow sperm cells. We evaluated whether the social status of males predicted their sperm speed in a wild population of dunnocks (Prunella modularis). In addition to the traditional analysis of the average speed of sperm cells per sample, we also analysed subsamples of the fastest sperm cells per sample. In other words, we systematically evaluated the effects of including different numbers of the fastest sperm in our analyses, ranging from the 5-fastest sperm cells to the 100-fastest sperm cells in a sample. We further evaluated whether fitness, defined here as the number of chicks sired per male per breeding season, relates to the sperm speed in the same population. We found that males in monogamous pairings (i.e. low levels of sperm competition), produced the slowest sperm cells, whereas subordinate males in polyandrous male-male coalitions (i.e. high levels of sperm competition) produced the fastest sperm cells. This result was consistent regardless of the number of fastest sperm included in our analyses, but statistical support was conditional on the number of sperm cells included in the analysis. Interestingly, we found no significant relationship between fitness and sperm speed, which suggests that it is possible that the differential mating opportunities across social status levelled out any possible difference. Our study also suggests that it is important to identify biologically meaningful subsets of fastest sperm and cut-offs for inclusions for assessing sperm competition via sperm speed.