SPERM COMPETITION AND THE EVOLUTION OF NONFERTILIZING SPERM IN MAMMALS

Nonfertilizing sperm with special morphologies have long been known to exist in invertebrates. Until recently, abnormal sperm in mammals were considered errors in production. Now, however, Baker and Bellis (1988, 1989) have proposed that mammalian sperm, like some invertebrate sperm, are polymorphic and adapted to a variety of nonfertilizing roles in sperm competition, including prevention of passage of sperm inseminated by another male. More specifically, their "kamikaze" sperm hypothesis proposes that deformed mammalian sperm are adapted to facilitate the formation and functioning of copulatory plugs (Baker and Bellis, 1988). Here I argue that most, maybe all, mammals are unlikely to produce nonfertilizing sperm. First, mammals might not be able to afford to evolve nonfertilizing sperm, given that a) fertilization is often unlikely despite the huge numbers of sperm produced; b) production of larger numbers of sperm is constrained, presumably because of metabolic costs, evidence for which includes the fact that in species in which sperm morphology and anatomy of the female reproductive tract increase the probability of fertilization, the numbers of sperm produced is lower than in others; and c) selection appears to act against the production of deformed sperm. Second, some of the evidence advanced for the existence of nonfertilizing sperm does not in fact support the idea. Third, accessory gland secretions are sufficient on their own to coagulate semen and produce fully functioning plugs; thus the male that used accessory gland secretions would be at a clear advantage over the male that diluted his fertilizing sperm with "kamikaze" sperm; and indeed, current evidence indicates selection on accessory glands, not sperm morphology, to enhance coagulation of semen. Fourth, predictions made on the basis of the "kamikaze" sperm hypothesis are not supported by quantitative comparisons of data from polyandrous and monandrous primates (i.e., those in which several males mate with a fertile female, and therefore in which sperm competition should be operating, and those in which only one male mates). Although sperm competition is almost certainly more intense in polyandrous genera than in monandrous genera (as indicated by, e.g., more frequent copulations and the production of more sperm per ejaculate from larger spermatogenic organs), polyandrous genera do not produce a greater proportion of deformed (i.e., nonfertilizing) sperm than do monandrous genera, or even necessarily a greater number of deformed sperm; nor a greater variety of sperm sizes-indeed they might produce fewer; nor fewer motile sperm (as might be expected if sperm are selected to stay behind and compete with sperm from subsequent males); and nor larger sperm (as might be expected if sperm are produced for functions other than to reach the egg). In sum, currently available evidence suggests that the function of all mammalian sperm is to fertilize, and that sperm competition in mammals occurs through scramble competition, not contest competition.