User:JS.Chester/sandbox

Testis and penis size:

Evolution has caused sexually selected adaptations to occur in penis and testis size in order to maximize reproductive success and minimize sperm competition (22) (Moller, 1988).

Sperm competition has caused the human penis to evolve in length and size for sperm retention and displacement. To achieve this the penis needs to be a sufficient length to reach any rival sperm and to maximally fill the vagina (Mautz, Wong, Peters, & Jennions, 2013). In order to ensure that the female retains a male's sperm, the adaptations in length of the human penis have occurred so that the ejaculate is placed close to the female cervix (Masters & Johnson, 1966). This is achieved when complete penetration occurs and the typical penis pushes against the cervix (Weijmar Schultz, van Andel, Sabelis, & Mooyart, 1999). These adaptations have occurred in order to release and retain sperm to the highest point of the vaginal tract. As a result, this adaptation also leaves the male’s sperm less vulnerable to sperm displacement and semen loss. Another reason for this adaptation is due to the nature of the human posture, gravity creates a vulnerability for semen loss. Therefore, a long penis which places the ejaculate deep in the vaginal tract could reduce the loss of semen (Gallup & Burch, 2004).

Another evolutionary theory of penis size is female mate choice and its associations with social judgements in modern day society (Mautz). A study which illustrates female mate choice as an influence on penis size presented females were with life-size, rotatable, computer generated males. These varied in height, body shape and flaccid penis size, with these aspects being examples of masculinity (Mautz). Females ratings of attractiveness for each male revealed that larger penises were associated with higher attractiveness ratings. The relations between penis size and attractiveness have therefore led to frequently emphasized associations between masculinity and penis size in popular media (Lever, Frederick & Poplau). This has led to a social bias existing around penis size with larger penises being preferred and having higher social status. This is reflected in the association between believed sexual prowess and male penis size and the social judgement of penis size in relation to 'manhood' (Lever, Frederick & Poplau).

Like the penis, sperm competition has caused the human testicles to evolve in size through sexual selection (Moller, 1988). This means that large testicles are an example of a sexually selected adaptation. The human testicles are moderately sized when compared to other animals such as gorillas and chimpanzees, placing somewhere midway (Harcourt, Purvis & Liles, 1995). Large testicles are advantageous in sperm competition due to their ability to produce a bigger ejaculation (Simmons, Firman, Rhodes & Peters, 2003). Research has shown that a positive correlation exists between the number of sperm ejaculated and testis size (Simmons, Firman, Rhodes & Peters, 2003). Larger testes have also been shown to predict higher sperm quality, including a larger number of motile sperm and higher sperm motility (Moller, 1988).

Research has also found that testis size is dependent on breeding system in which the species resides (Harcourt, Harvey, Larson & Short 1981). Single-male breeding systems or monogamous societies tend to show smaller testis size than do multi-male breeding systems or extra pair copulation (EPC) societies. Human males live largely in monogamous societies like gorillas, and therefore testis size is smaller in comparison to primates in multi-male breeding systems such as chimpanzees. The reason for the differentiation in testis size is that in order to succeed reproductively in a multi-male breeding system, males must possess the ability to produce several fully fertilizing ejaculations one after another (Moller, 1988). This however is not the case in EPC societies, where a reduction in fertilizing ejaculations has no effect on reproductive success (link) (Moller, 1988). This is reflected in humans, as the sperm count in ejaculations is decreased if copulation occurs more than 3 to 5 times in a week (Freund, 1962). JS.Chester (talk) 12:41, 31 March 2016 (UTC)

References

Freund, M. (1962). Interrelationships among the characteristics of human semen and facts affecting semen specimen quality. Journal of Reproduction and Fertility, 4, 143-159.

Gallup, G. G., & Burch, R. L. (2004). Semen displacement as a sperm competition strategy in humans. Evolutionary Psychology, 2, 12-23.

Harcourt, A. H., Harvey, P. H., Larson, S. G., & Short, R. V. (1981a). Testis weight, body weight and breeding system in primates. Nature, 293, 55-57.

Harcourt, A. H., Purvis A., & Liles L. (1995). Sperm competition: mating system, not breeding season, affects testes size of primates. Function Ecology, 9, 469-476.

Masters, W. H., & Johnson, V. E. (1966). Human Sexual Response. Little, Brown and Company: Boston.

Mautz, B. S., Wong, B. B. M., Peters, R. A., & Jennions, M. D. (2013). Penis size interacts with body shape and height to influence male attractiveness. Proceedings of the National Academy of Sciences of the United States of America, 110, 6925-6930.

Moller, A. P. (1988). Ejaculate quality, testes size and sperm competition in primates. Journal of Human Evolution, 17, 479-488.

Simmons, L. W., Firman, R. C., Rhodes G., & Peters, M. (2003). Human sperm competition: testis size, sperm production and rates of extra pair copulations. Animal Behaviour, 68, 297-302.

Weijmar Schultz, W., van Andel, P., Sabelis, I, & Mooyartm E. (1999). Magnetic resonance imaging of male and female genitals during coitus and female sexual arousal. British Medical Journal, 319, 18-25. JS.Chester (talk) 16:38, 2 March 2016 (UTC)

Human Sexuality