The alchemist Paracelsus once proposed that he had created a false human being through his science. Now the term "homunculus" has been associated with the "little man" that some of the leading spermists exhibit inside the head of the spermatozoon during the seventeenth century. The modern use of this term has though become redundant and obsolete as a result of its widespread notion of a comical and obscurantist episode in the evolution of natural sciences yet the new findings on the behaviour of sperm in the light of genetic research is bound to remind the creature within sperm.

One hundred years after Darwin considered how sexual selection shapes the behavioural and morphological characteristics of males for acquiring mates; Parker from UK realized that sexual selection continues after mating through sperm competition. Because females often mate with multiple males before producing offspring, selection favours adaptations that allow males to block sperm from previous males and to prevent their own sperm from pre-emption by future males. Since the 1970s, this area of research has seen exponential growth, and biologists now recognize sperm competition as an evolutionary force that drives such adaptations as mate guarding, genital morphology, and ejaculate chemistry across all animal taxa. The insects have been critical to this research, and they still offer the greatest potential to reveal fully the evolutionary consequences of sperm competition.

Many researchers are analyzing and extending theoretical and empirical work on different animals especially and human sperm competition. It considers both male and female interests in sperm utilization and the sexual conflict that can arise when these differ. It covers the mechanics of sperm transfer and utilization, morphology, physiology, and behaviour. Sperm competition is shown to have dramatic effects on adaptation in the context of reproduction as well as far-reaching ramifications on life-history evolution and speciation. Multiple mating is referred to copulation with multiple males, not repeated copulations with a single male. Repeated copulations involving a single pair are considered to increase the size of ‘ejaculate’ within a female’s reproductive tract, but not to constitute different ejaculates.

Sperm Competition, Interspecific and intraspecific

An ejaculate might behave as if it were a stampeding herd of spermatozoa, each sperm for itself in the race for an egg. ‘Sperm competition’ has been defined as competition within a single female between the sperm from two or more males (interspecific) for the fertilization of her ova. In effect, sperm competition is seen as a continuation of inter-male conflict within the female reproductive tract. A sperm’s fate is also determined by its own genetic constitution. Rivalry within ejaculates may limit cooperation among the members of an ejaculate when they compete with sperm produced by other males. Hence ‘sperm competition’ can also be referred to the potential for competition among the different sperm genotypes present in the ejaculate of a single male i.e. intraspecific. Therefore, selection favours genes expressed in males which suppress competition within ejaculates. In this process natural selection could favour traits that conferred a competitive advantage on certain sperm while reducing the competitive effectiveness of the ejaculate as a whole. A gene that gains an advantage in competition within an ejaculate (which is called a segregation distorter) may increase in frequency even if it is associated with significant costs to organism’s fitness.