135 Views 10 Downloads
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 analyzed 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 leveled out any
possible difference. Our study also suggests that it is important to
identify biologically meaningful subsets of fastest sperm and cutoffs for
inclusions for assessing sperm competition via sperm speed
135 views reported since publication in 2020.