Sperm Utilization Strategies in Nonsocial Insects

It is postulated that the sperm precedence characteristics of most insect populations have resulted primarily from selection on females (1) to optimize the genetic composition of their progeny; (2) to discourage or encourage multiple matings for reasons other than genetic considerations; (3) to optimize their sperm storage capacity and utilization. In addition, selection on males to maximize egg fertilization by reduced displacement of their sperm and increased displacement of other sperm to the extent that these can be achieved without sacrificing the optimal mating strategy (Parker 1970a) probably has been important in some species. Parker's (1970a) hypothesis that the amount of sperm displacement in a population should stabilize at the value which yields the optimal overall male fertilization rate is accepted as the optimal male strategy; however, it is suggested that females often largely determine, by their behavior and by the structure and functioning of their reproductive tract, the optimal male strategy which provides the optimal sperm displacement pattern for the females. Selection on males independent of selection pressures on females is postulated to exert a major influence on the sperm precedence pattern of a population only when selection on females is relatively lax or nondirectional. Evidence is cited that females often actively regulate sperm movements within their reproductive tracts. Also, they may influence the competitiveness of stored sperm which is nutritionally dependent upon them. A trend is shown toward monogamy or low percent displacement in species with spheroid or ovoid spermathecae contrasting with a high percent displacement in species with elongate or tubular sperm storage organs. Selection for female monogamy is proposed to be based partly upon a greater mean fitness of progeny sired by the first mate, and thus to represent one extreme of a spectrum of sperm utilization strategies. Predation pressure and time budgeting considerations acting on females offer alternative or supplemental explanations for female monogamy. Selection on males probably sometimes has been important in facilitating or antagonizing selection on females for monogamy. Selection pressures proposed as promoting polyandry and sperm displacement in those numerous species which do not require multiple matings for an adequate sperm supply are discussed, and it is concluded that documentation of all these proposals is inadequate to judge their merits. It is emphasized that several types of indirect male parental investments applicable to many species offer testable hypotheses. These include the transfer of nutriments in the ejaculate, and various means of reducing predation of females. The increased genetic diversity gained by sperm displacement may be important in colonizations, promoting utilization of different limited resources within a group of siblings, and reducing undesirable direct competition between relative equals in a brood. Either relatively low or high sperm displacements may provide maximal genetic diversity, depending on the relative frequencies of matings and ovipositions, and whether diversity within an egg batch or within the total progeny is being considered. It is stressed that the three inbred populations cited all have a low percent displacement in contrast to most outbred populations. Further investigations on strongly inbred populations may provide a better estimate of the importance of selection on females for an optimal generic composition in their progeny as a determinant of the sperm precedence characteristics of a population.