How can sperm competition work?



A random draw form of sperm competition has been suggested as a reproductive strategy, alternative to direct aggressive competition, in chimpanzee evolution. We note that individual reproductive competition involves complexly related social behavioral and physiological components, so that care must be exercised in setting forth logically distinct categories of reproductive strategies. We recognize the heuristic value of simplifying models but question the utility of assuming the female reproductive tract to be a neutral arena for intermale gametic competition. We review studies of sequence and timing in ejaculation and deposition of sperm, of the interactive aspects of sperm transport, and of the antigenicity of spermatozoa, and we conclude that nonrandom factors play too important a role in insemination and fertilization to be ignored. Spermatozoa are best viewed as cell complexes which vary within and between individuals in ways that provide a basis for prezygotic selection.