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Free keywords:
copulation order; Schistocephalus solidus; sperm competition; simultaneous hermaphrodite; sex allocation; sexual selection
Abstract:
Simultaneous hermaphrodites maximize their fitness by optimizing their investment into male or female functions.
Allocation of resources to male function (tissues, traits, and/or behaviours increasing paternity) is predicted to increase as
density, and the associated level of sperm competition, increases. We tested whether the simultaneous hermaphroditic
cestode Schistocephalus solidus uses cues of potential partner densities in its fish intermediate host to improve its male
reproductive success in the final host. We had two worms, one originating from a multiple infection in the fish intermediate
host and one from a single infection, sequentially compete to fertilize the eggs of a third worm. The fertilization rates of
the two competitors nearly always differed from the 50–50 null expectation, sometimes considerably, implying there
was a ‘winner’ in each experimental competition. However, we did not find a significant effect of density in the fish host
(single vs multiple) or mating order on paternity. Additional work will be needed to identify the traits and environmental
conditions that explain the high variance in male reproductive success observed in this experiment.
