Sperm digestion and reciprocal sperm transfer can drive hermaphrodite sex allocation to equality

The intensity of sperm competition determines how much reproductive effort should be invested in sperm. One important factor affecting sperm competition in internally fertilizing organisms is the mating frequency of females, since it determines the extent of competition between ejaculates. In simultaneous hermaphrodites, energy spent on sperm has to be traded off against energy expended on ova production. By extending an existing model, we consider how the number of matings affects sperm competition and, thus, sex allocation in internally fertilizing simultaneous hermaphrodites. We then go on to explore the consequences of two common characteristics of hermaphroditic mating systems, namely, sperm digestion and reciprocal insemination. Since sperm digestion reduces the competitive ability of a given ejaculate, it selects for increased sperm investment. As a result, the amount of sperm digested and male allocation can enter a coevolutionary cycle in which both will increase up to the point of equal investment in male and female gametes and in high rates of digestion. Because of this high degree of sperm digestion, fitness through the male function becomes more dependent on the amount of resources invested in sperm than on the number of matings. As a result, sperm digestion reduces Bateman's principle. Hence, hermaphrodites with sperm digestion should be less likely to display traits that increase the number of matings. Once ejaculates are large and costly, animals insisting on reciprocal sperm transfer will be favored since they receive some compensation for their investment. Under reciprocity, an even higher investment in sperm is favored. This is the result of the compensation received in the form of the partner's ejaculate and a nuptial gift effect by increasing a partner's production of eggs.