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 competiti on in internally fertilizing organisms is the mating frequency of females, since it determines the extent of competition between ejaculates. In simult aneous hermaphrodites, energy spent on sperm has to be traded off against e nergy expended on ova production. By extending an existing model, we consid er how the number of matings affects sperm competition and, thus, sex alloc ation 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 s elects for increased sperm investment. As a result, the amount of sperm dig ested and male allocation can enter a coevolutionary cycle in which both wi ll 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 digest ion, 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 sp erm digestion should be less likely to display traits that increase the num ber of matings. Once ejaculates are large and costly, animals insisting on reciprocal sperm transfer will be favored since they receive some compensat ion for their investment. Under reciprocity, an even higher investment in s perm is favored. This is the result of the compensation received in the for m of the partner's ejaculate and a nuptial gift effect by increasing a part ner's production of eggs.