Sex allocation theory for simultaneous hermaphrodites tries to predict the optimal investment of limited
reproductive resources to male and female reproduction. It does so by assuming fitness gain curves, which are
functions that describe the relationship between investment into the male and female reproduction and the fitness
gained from a given investment. However, few studies have attempted to measure the actual shapes of fitness gain
curves and how they vary with changing environmental conditions. In internally fertilising simultaneous
hermaphrodites sperm competition is thought to influence the shape of the male gain curve, and as a consequence,
the optimal male allocation. In agreement with this idea we have recently shown that the marine flatworm
Macrostomum sp. increases male allocation when it is raised in a situation with high sperm competition. This
approach allows us to manipulate the male allocation of a worm and we can hence use it to determine the shape of
the male gain curve. First, we ask how male investment translates into sperm production. Second, we investigate
how male investment translates into sperm transferred to the mating partners and how it translates into male fitness.
By performing the latter experiments under different levels of sperm competition allows we can estimate how the
shape of the male gain curve changes with sperm competition.