Sperm velocity and its relation to social status in Arctic charr (Salvelinus alpinus)

Sperm velocity has been shown to be a major determinant of fertilization success of external fertilizers. Yet, sperm velocity varies both within and between ejaculates and only a small number of fast sperm cells within an ejaculate are likely to have the potential of fertilizing the eggs. Having such fast cells should be of special importance during sperm competition, particularly for subordinate males that may release their sperm later or further away from eggs, than dominants. We examined ejaculates of dominant and subordinate male Arctic charr (Salvelinus alpinus), a species with sperm competition. Yet, rather than examining just average sperm velocity values, the aim was to examine whether the fastest fraction of sperm cell from dominant and subordinate males differed in velocity. While there was no difference in the average sperm velocity between dominant and subordinate males, analysis of the fastest swimming sperm cells show that subordinate males have significantly higher initial sperm velocity than dominant males within the 10, 5 and 1% fastest sperm cells. That is, the difference in sperm velocity between dominant and subordinate charr is most predominant among the fastest sperm cells. In sum, this study emphasizes the importance of studying the fastest sperm cells in the ejaculates, as status-dependent differences in sperm velocity may not be detected using average values.