Sperm competition is a form of post-copulatory sexual selection[1] whereby male sperm simultaneously physically compete to fertilize a single ovum.[2] 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.[3] 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.[1][3][4] 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,[1][3][4][5] however this is not without consequences: the production of large amounts of sperm is costly[6][7] 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.[3][7]
Sperm competition is not exclusive to humans, and has been studied extensively in other primates,[5][8][9] as well as throughout much of the animal kingdom.[10][11][12] 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.[5][8] 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.[5]
^ abcShackelford, Todd K.; Goetz, Aaron T. (2007). "Adaptation to sperm competition in humans". Current Directions in Psychological Science. 16 (1): 47–50. doi:10.1111/j.1467-8721.2007.00473.x. S2CID 6179167.
^Pham, Michael N.; Shackelford, Todd K. (2014). “Sperm competition and the evolution of human sexuality”. In T.K. Shackelford & R.D. Hansen. The Evolution of Sexuality. Cham: Springer. pp. 257-275. ISBN 9783319093840.
^ abcdShackelford, Todd K.; Goetz, Aaron T.; LaMunyon, Craig W., Pham, Michael N; Pound, Nicholas. (2016). “Human sperm competition”. In D.M. Buss. The Handbook of Evolutionary Psychology (2nd ed.). New York: Wiley. pp. 427-443. ISBN 9781118755884.
^ abcdMartin, Robert D. (2007). “The evolution of human reproduction: A primatological perspective”. Yearbook of Physical Anthropology.50: 59-84.
^Dewsbury, Donald A. (1982). “Ejaculate cost and male choice”. The American Naturalist.119 (5): 601-610.
^ abBaker, R. Robin; Bellis, Mark A. (1993). “Human sperm competition: Ejaculate adjustment by males and the function of masturbation”. Animal Behaviour.46 (5): 861-885.
^ abMøller, Anders Pape. (1988). “Ejaculate quality, testes size and sperm competition in primates”. Journal of Evolution.17 (5): 479-488.
^Dixson, A. F. (1987). "Observations on the evolution of the genitalia and copulatory behaviour in male primates". Journal of Zoology. 213 (3): 423–443. doi:10.1111/j.1469-7998.1987.tb03718.x.
^Parker, G.A. (1970). “Sperm competition and its evolutionary consequences in the insects”. Biological Reviews. 55 (4): 525-567.
^Birkhead, T.R.; Møller, A.P. (1992). Sperm competition in birds: Evolutionary causes and consequences. London: Academic Press. ISBN 0121005402.
^Ginsberg, J.R.; Huck, U.W. (1989). “Sperm competition in mammals”. Trends in Ecology and Evolution. 4 (3): 74-79.
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