Sperm Utilization Strategies in Nonsocial Insects

Abstract

The sperm precedence characteristics of most insect populations have resulted primarily from selection on females to optimize the genetic composition of their progeny, to discourage or encourage multiple matings for reasons other than genetic considerations and to optimize their sperm storage capacity and utilization. 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) 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. Females often largely determine by their behavior, 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. Females often actively regulate sperm movements within their reproductive tracts. They may influence the competitiveness of stored sperm which is nutritionally dependent on 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 evidently based partly on a greater mean fitness of progeny sired by the 1st mate, and represents 1 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 documentation of 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. The 3 inbred populations cited have a low percent displacement in contrast to most outbred populations.