Carlos Esteban Lara, Helen Taylor, Benedikt Holtmann, Sheri Johnson, Eduardo S. A. Santos, Neil Gemmell, Shinichi Nakagawa

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 systematically evaluated ranked groups of sperm, 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 across all the ranked groups of sperm, but statistical support was conditional on the number of sperm cells included in the analysis. Surprisingly, we found no significant relationship between fitness and sperm speed, contrary to theory – it is possible that the differential mating opportunities across social status leveled out any possible difference. Our study also suggests that it is important to identify biologically meaningful rankings of fastest sperm and cutoffs for inclusions for assessing sperm competition via sperm speed.