Sperm competition models predict that males typically mating in disfavoured roles should be selected to compensate for their disadvantage by investing more into sperm. We studied the effect of rapid changes in social status on ejaculate investments during experimental trials with an externally fertilizing teleost--the Arctic charr (Salvelinus alpinus). We document that males becoming dominant produce less sperm with lower velocity, but have higher sex steroid concentrations than subordinate males. These differences in sperm characteristics seem mainly to result from a decreased investment in sperm among fish that become dominant compared to pre-trial levels. Moreover, these adjustments of sperm production and sperm velocity seem not to be traded against sperm longevity. Our results support theoretical models of sperm competition, as males forced to mate in disfavoured roles seem to invest more into ejaculate quality than males in favoured roles. Additionally, we are the first to report that males, in a species with status-dependent shifts in reproductive tactics, have evolved rapid tactic specific adjustments of sperm production and sperm velocity corresponding to what could be predicted from their reproductive roles.

Little evidence of benefits from female mate choice has been found when males provide no parental care or resources. Yet, good genes models of sexual selection suggest that elaborated male sexual characters are reliable signals of mate quality and that the offspring of males with elaborate sexual ornaments will perform better than those of males with less elaborate ornaments. We used cod (Gadus morhua L.), an externally fertilizing species where males provide nothing but sperm, to examine the potential of optimal mate selection with respect to offspring survival. By applying in vitro fertilizations, we crossed eight females with nine males in all possible combinations and reared each of the 72 sib groups. We found that offspring survival was dependent on which female was mated with which male and that optimal mate selection has the potential to increase mean offspring survival from 31.9 to 55.6% (a 74% increase). However, the size of the male sexual ornaments and sperm quality (i.e. sperm velocity and sperm density) could not predict offspring survival. Thus, even if there may be large fitness benefits of mate selection, we might not yet have identified the male characteristics generating high offspring survival.

SummaryA mismatch in timing between the release of male and female gametes in external fertilizers may lead to failed fertilization or, under sperm competition, reduced paternity. To quantify the actual synchrony of gamete release and the level of sperm competition we placed video cameras on two spawning grounds of a naturally spawning population of Arctic charr (Salvelinus alpinus). Females release eggs in response to courting (quivering) in less than 1% of the cases, yet to both dominant and sneaker males. That is, females initiated spawning with dominant males in 73.3% of the recorded spawning events. Although the actual spawning seems to be largely under female control, 55.6% of spawning events occur under sperm competition. The average time delay between dominant and sneaker males milt release under sperm competition is 0.68 s. Thus, female reproductive decisions seem to be strongly influenced by male-male competition and this may have set the stage for the evolution and maintenance of the observed plasticity in ejaculate characteristics of male charr.

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For males, the reproductive advantage of joining a lek varies among leks; consequently, males should join the lek yielding the highest fitness. When males experience low reproductive opportunities at one lek, it may pay to move to another. By observing tagged male Arctic charr (Salvelinus alpinus L., 1758) at three different lek sites within one lake, we examined whether males moved between leks. Some movement was observed, especially between closely located leks, but fish length and spermatocrit, traits indicative of reproductive success, were unrelated to whether or not individuals moved between leks. Little to no movement was observed between more distantly separated leks, even though the costs associated with movements across the relatively short distances between these leks should be low. This suggests that individuals, rather than moving from leks where they have low reproductive success, are relatively stationary. The lek fidelity documented in the present study may be important for production of local genetic differences between Arctic charr leks. Our results suggest that males with low reproductive success may enhance their fitness by means other than dispersal, e.g., by associating with relatives to increase inclusive fitness.

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BackgroundThe adaptive significance of female polyandry is currently under considerable debate. In non-resource based mating systems, indirect, i.e. genetic benefits have been proposed to be responsible for the fitness gain from polyandry. We studied the benefits of polyandry in the Arctic charr (Salvelinus alpinus) using an experimental design in which the material investments by the sires and maternal environmental effects were controlled.ResultsEmbryonic mortality showed a strong paternal genetic component, and it was lower in polyandrously fertilized offspring (sperm competition of two males) than in monandrous fertilizations. We also found that high sperm velocity was associated with low offspring mortality, but not with the size of the offspring or their yolk volume. Although no male effect was found on the size of the offspring yolk reserves, yolk volume was higher in offspring from polyandrous matings than offspring of the either of the two males when mated monandrously.ConclusionsIn support of the "good sperm hypothesis, we found that sperm velocity was positively associated with offspring fitness. In addition, our results suggest that polyandrous females gain genetic advantage (higher offspring survival) from this behavior, but that some benefits of polyandry (larger yolk volume) may not be explained solely by the additive genetic effects. This suggests that sperm competition environment may intensify the selection on genetically superior sperm which in turn may produce offspring that have superior yolk reserves. However, as high sperm velocity was not associated with larger yolk volume, it is possible that also some other non-genetic effects may contribute to offspring fitness. The potential role of polyandrous mating in inbreeding avoidance is discussed.

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