Abstract
Why dominant males experiencing intense sperm competition sometimes show low investments in sperm production is not always obvious. One well-documented example is that of the external fertilizing teleost, the Arctic charr (Salvelinus alpinus), where individuals becoming dominant reduce sperm production and sperm swimming speed in water compared to subordinates. Here we report how ovarian fluid differentially influences sperm velocity of dominant and subordinate male Arctic charr. That is, sperm from dominant males increase their velocity in water diluted ovarian fluid compared to that observed in water, while sperm from subordinates, on the other hand, decrease velocity in ovarian fluid compared to that observed in water. Thus, subordinates, who invest more resources in their sperm and usually show the highest sperm velocity in water, have lower gains from their investment than dominant males when sperm are swimming in ovarian fluid. In sum, our result suggests that ovarian fluid increase sperm velocity more in dominant males than in subordinate males. Although this finding could partly be caused by cryptic female choice exerted by the ovarian fluid for sperm from dominant males, an alternative and more parsimonious explanation is that sperm from dominant males may simply be better designed for swimming in ovarian fluid compared to sperm from subordinate males. Thus, sperm production in the two reproductive roles seems to be adaptively tailored to different external environments.
Highlights
Polyandry leads to conflict between males over fertilizations resulting in both pre- and postcopulatory male adaptations (Birkhead and Møller, 1992, 1998; Andersson, 1994; Andersson and Iwasa, 1996; Eberhard, 1996; Alonzo and Warner, 2000; Simmons, 2001; Chapman et al, 2003)
Sperm velocity is documented to be of major importance for fertilization success under sperm competition in Arctic charr (Liljedal, 2005; Egeland et al, 2015) and carefully controlled in vitro sperm competition trials, including a realistic time-lag to subordinates ejaculation, have shown that subordinate males may fully compensate for disadvantages in their unfavorable mating role by having more and faster sperm than dominants (Egeland et al, 2015)
Dominant and subordinate Arctic charr have different sperm velocity when measured in water, but whether this difference in velocity is maintained when sperm is swimming under the influence of ovarian fluid is not known
Summary
Polyandry leads to conflict between males over fertilizations resulting in both pre- and postcopulatory male adaptations (Birkhead and Møller, 1992, 1998; Andersson, 1994; Andersson and Iwasa, 1996; Eberhard, 1996; Alonzo and Warner, 2000; Simmons, 2001; Chapman et al, 2003). Sperm velocity is documented to be of major importance for fertilization success under sperm competition in Arctic charr (Liljedal, 2005; Egeland et al, 2015) and carefully controlled in vitro sperm competition trials, including a realistic time-lag to subordinates ejaculation, have shown that subordinate males may fully compensate for disadvantages in their unfavorable mating role (i.e., ejaculating out of synchrony with the female) by having more and faster sperm than dominants (Egeland et al, 2015). Dominant and subordinate Arctic charr have different sperm velocity when measured in water, but whether this difference in velocity is maintained when sperm is swimming under the influence of ovarian fluid is not known
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