Background: The 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.

Results: Embryonic 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.

Conclusions:

In 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.