Sexual Selection and Insect Sperm

Abstract

Males produce the smaller gamete. The inconsequence of sperm in comparison to the enormity of eggs has dominated the evolution of male characteristics (Trivers 1972). Females, with their clutches of expensive ova, are limiting resources for which males compete in a variety of ways (Blum and Blum 1979). These different manifestations of masculinity have perhaps, in turn, influenced the radiation of insect sperm morphology and behavior. The following related questions arise from a consideration of sperm both as microorganisms and tools of male reproductive interests: i) Why are there more sperm than eggs? ii) Do sperm behave as individuals? iii) Do competition between ejaculates and conflicts of interest between females and the sperm they contain result in gametic adaptations? Notes on topics indirectly bearing on these problems are placed in an appendix. Why are sperm-to-egg ratios greater than 1? Some mites and insects transfer fewer than 2 sperm per ovum, but these are exceptions to the rule of tens or hundreds of sperm for every egg (Cohen 1971, 1975, 1977). Since only 1 of the multitude can genetically participate in the zygote, why go to the expense of producing such a mass? Males may swamp the female reproductive system with sperm to block the introduction of rival ejaculates (Parker 1970). A contrasting argument holds that gametic exuberance is largely symbolic, a display of a male's ability to obtain resources. Females may make decisions about which male's gametes to use based on the dimensions of their mates' ejaculates (Mary Willson 1979, discussing excess pollen per ovule). These hypotheses have 2 difficulties explaining the full range of insect sperm-to-egg ratios: i) Even in insects whose females mate only once, males transfer excess sperm. In the monogamous mosquito Aedes aegypti, for instance, the male passes 2000 sperm to a female who will lay about 85 eggs (Jones 1968, Christopher 1960). There are, in such cases, no rivals to block or postcopulatory choices between males to be made. ii) Neither specifies an advantage to females who accept excessive numbers of sperm. While the female sometimes expels or digests a portion of the male's ejaculate, the part she stores, and possibly maintains, is often still in excess of the number of ova. Of the 2000 Aedes sperm, about 1000 will make their way to the spermatheca (Jones 1968). Why, then, do males produce more sperm than a female keeps and why does she keep more than she needs? In some cases, males may pass large numbers of sperm as a nuptial gift, a contribution to the good health and fecundity of his offspring's mother (see Thornhill 1976, 1980). It is possible that many are digested in the female genital tract as a source of nourish-