Abstract
How much of a fitness benefit is obtained by dominant males of external fertilizers from releasing ejaculates in synchrony with female egg-release when engaging in sperm competition, and what is the most important sperm trait for paternity in these situations? The Arctic charr (Salvelinus alpinus) is an external fertilizer experiencing intense male-male competition over reproductive opportunities including sperm competition. To compensate for their disadvantage the sneaker males, which often spawn out of synchrony with the female, produce more and faster sperm than the guarding males. We used controlled in vitro fertilization trials with experimentally produced dominant and subordinate, sneaker males to test what effect relative synchrony in gamete release, sperm quality (i.e., motility and velocity) and sperm quantity have on a male’s fertilization success in pair-wise sperm competitions. When the sneaker males released ejaculates after the guarding male there was no overall difference in fertilization success. The quality (i.e., motility and velocity) of a male’s sperm relative to that of the competing male was the best predictor of male fertilization success regardless of their mating tactic and spawning synchrony. The relative number of sperm cells also had an effect on fertilization success, but mainly when the dominant and sneaker male ejaculated synchronously. Our close imitation of natural sperm competition in charr shows that the sneaker males of external fertilizing species may fully compensate for their disadvantaged mating role by producing ejaculates of higher quality - an adjustment strangely not met by dominants.
Highlights
Sperm competition occurs when spermatozoa of two or more males have the opportunity to fertilize the same ovum (Parker, 1970) and it occurs amongst species practicing both internal- and external-fertilization (Birkhead and Møller, 1998)
No association was apparent between the proportion of eggs surviving and the relative paternity of the males as revealed by microsatellites. This suggests that it is unlikely that the actual fertilization success we measure was caused by differential mortality or differential ability to develop by the embryos sired by the two males
This is the first study to disentangle the effects of naturally occurring adjustments in sperm quality, sperm quantity and spawning synchrony and their interactions for paternity in sperm competition among subordinate and dominant individuals of an external fertilizing species
Summary
Sperm competition occurs when spermatozoa of two or more males have the opportunity to fertilize the same ovum (Parker, 1970) and it occurs amongst species practicing both internal- and external-fertilization (Birkhead and Møller, 1998). Following the pre-spawning site competition, males must synchronize ejaculation with female egg release in order to reduce the effect of sperm competition. This is documented in Atlantic salmon (Salmo salar), where a 2 s delay in sperm release under sperm competition decreased paternity by approximately 40% (Yeates et al, 2007). Among sneaker male Japanese medaka (Oryzias latipes) paternity dropped from 41 to 20% when spawning out of synchrony rather than in synchrony with the dominant male and the female The latter findings made the authors (Koya et al, 2013) suggest that the reproductive success in medaka was primarily determined by the timing of sperm release corresponding to egg release, something certainly possible given ejaculates of equal sperm numbers and a “fair raffle” (Parker, 1990)
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