Sperm competition is a form of post-copulatory sexual selection whereby male ejaculates simultaneously physically compete to fertilize a single ovum. Sperm competition occurs between sperm from two or more rival males when they make an attempt to fertilize a female within a sufficiently short period of time. This results primarily as a consequence of polyandrous mating systems, or due to extra-pair copulations of females, which increases the chance of cuckoldry, in which the male mate raises a child that is not genetically related to him. Sperm competition among males has resulted in numerous physiological and psychological adaptations, including the relative size of testes, the size of the sperm midpiece, prudent sperm allocation, and behaviors relating to sexual coercion, however this is not without consequences: the production of large amounts of sperm is costly and therefore, researchers have predicted that males will produce larger amounts of semen when there is a perceived or known increase in sperm competition risk.
Sperm competition is not exclusive to humans, and has been studied extensively in other primates, as well as throughout much of the animal kingdom. The differing rates of sperm competition among other primates indicates that sperm competition is highest in primates with multi-male breeding systems, and lowest in primates with single-male breeding systems. Compared to other animals, and primates in particular, humans show low-to-intermediate levels of sperm competition, suggesting that humans have a history of little selection pressure for sperm competition.
Physiological adaptations to sperm competition
Physiological evidence, including testis size relative to body weight and the volume of sperm in ejaculations, suggests that humans have experienced a low-to-intermediate level of selection pressure for sperm competition in their evolutionary history. Nevertheless, there is a large body of research that explores the physiological adaptations males do have for sperm competition.
Testis size and body weight
Evidence suggests that, among the great apes, relative testis size is associated with the breeding system of each primate species. In humans, testis size relative to body weight is intermediate between monogamous primates (such as gorillas) and promiscuous primates (such as chimpanzees), indicating an evolutionary history of moderate selection pressures for sperm competition.
Ejaculate volume
The volume of sperm in ejaculates scales proportionately with testis size and, consistent with the intermediate weight of human males' testes, ejaculate volume is also intermediate between primates with high and low levels of sperm competition. Human males, like other animals, exhibit prudent sperm allocation, a physiological response to the high cost of sperm production as it relates to the actual or perceived risk of sperm competition at each insemination. In situations where the risk of sperm competition is higher, males will allocate more energy to producing higher ejaculate volumes. Studies have found that the volume of sperm does vary between ejaculates, and that sperm produced during copulatory ejaculations are of a higher quality (younger, more motile, etc.) than those sperm produced during masturbatory ejaculates or nocturnal emissions. This suggests that, at least within males, there is evidence of allocation of higher quality sperm production for copulatory purposes. Researchers have suggested that males produce more and higher quality sperm after spending time apart from their partners, implying that males are responding to an increased risk of sperm competition, although this view has been challenged in recent years. It is also possible that males may be producing larger volumes of sperm in response to actions from their partners, or it may be that males who produce larger volumes of sperm may be more likely to spend more time away from their partners.
Size of sperm midpiece
The size of the sperm midpiece is determined in part by the volume of mitochondria in the sperm. Sperm midpiece size is tied to sperm competition in that individuals with a larger midpiece will have more mitochondria, and will thus have more highly motile sperm than those with a lower volume of mitochondria. Among humans, as with relative testis size and ejaculate volume, the size of the sperm midpiece is small compared to other primates, and is most similar in size to that of primates with low levels of sperm competition, supporting the theory that humans have had an evolutionary history of intermediate levels of sperm competition.