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Additionally, fluctuating asymmetry has been shown to predict atypical asymmetry of the brain

We would like to expand the section on penis adaptations. We will do this by splitting penis adaptations into 3 types:

1. Testis and penis size (references below) Gallup, G. G., & Burch, R. L. (2004). Semen displacement as a sperm competition strategy in humans. Evolutionary Psychology, 2, 12-23.

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.

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.

2. Ejaculate adjustment

Baker, R. R., & Bellis, M. A. (1989). Number of sperm in human ejaculaes varies in accordance with sperm competition theory. Animal Behaviour, 37, 867-869.

Shackelford, T. K., LeBlanc, G. J., Weekes-Shackelford, V. A., Bleske-Rechek, A. L., Euler, H. A., & Hoier, S. (2002). Psychological adaptation to human sperm competition. Evolution and Human Behavior, 23, 123-138.

Shackelford, T. K., Pound, N., & Goetz, A. T. (2005). Psychological and physiological adaptations to sperm competition in humans. Review of General Psychology, 9, 228-248.

3. Semen displacement

Burch, R. L., Gallup, G. G., Pervez, R. A., Stockwell, M. L., & Zappieri, M. L. (2003). The human penis as a semen displacement device. Evolution and Human Behaviour, 24, 277-289. (Although already mentioned, more detail can be added).

Burch, R. L., Gallup, G. G., & Mitchell, T. J. (2006). Semen displacement as a sperm competition strategy: Multiple mating, self-semen displacement, and timing of extra-pair copulations. Human Nature: An interdisciplinary Biosocial Perspective, 17, 253-264.

Let us know of any suggestions or queries!

Penis Adaptations Section: Semen Displacement

Semen displacement is an adaptive mechanism of the penis to deal with the event of infidelity by a woman. It can be categorised as a counter-insemination strategy, and it is an adaptation of a human penis, to draw foreign semen away from the cervix.

Research supports this idea, having shown that "when combined with thrusting, the coronal ridge of the penis is able to remove the seminal fluid previously placed by a rival male within the female reproductive tract. It does this by forcing the semen under the frenulum of the coronal ridge, causing it to collect behind the coronal ridge shaft. By removing the rival's sperm, the male increases his own chances of fertilising the female." Males have to ensure they do not displace their own sperm, however. It is thought that the relatively quick loss of erection after ejaculation, and the shallower, slower thrusting of the male after ejaculation, prevents this from occurring.

The use of semen-displacement strategies has been found to increase when the likelihood of sperm competition increases. Additionally, semen-displacement is thought to be used alongside mate retention behaviours to reduce the likelihood of cuckoldry.

The shape of the human penis is likewise thought to have evolved as a result of sperm competition.[41 ] Semen displacement is an adaptation of the shape of the penis to draw foreign semen away from the cervix. This means that in the event of a rival male's sperm residing within the reproductive tract of a female, the human penis is able to displace the rival sperm, replacing it with his own.[42 ]

Semen displacement has two main benefits for a male. Firstly, by displacing a rival male's sperm, the risk of the rival male's sperm fertilizing the egg is reduced, thus minimizing the risk of sperm competition.[43 ] Secondly, the male replaces the rival's sperm with his own, therefore increasing his own chance of fertilizing the egg and successfully reproducing with the female. However, males have to ensure they do not displace their own sperm. There are three primary ways to prevent one's own semen being displaced. These include the relatively quick loss of erection after ejaculation, penile hypersensitivity following ejaculation, and the shallower, slower thrusting of the male after ejaculation.[44 ]

The coronal ridge is the part of the human penis thought to have evolved to allow for semen displacement. Research has been carried out looking at how much semen is displaced by different shaped, artificial genitals.[45 ] This research showed that, when combined with thrusting, the coronal ridge of the penis is able to remove the seminal fluid of a rival male from within the female reproductive tract. It does this by forcing the semen under the frenulum of the coronal ridge, causing it to collect behind the coronal ridge shaft.[45 ] When model penises without a coronal ridge were used, less than half the artificial sperm was displaced, compared to penises with a coronal ridge.[45 ]

The presence of a coronal ridge alone, however, is not sufficient for effective semen displacement. It must be combined with adequate thrusting to be successful. It has been shown that the deeper the thrusting, the larger the semen displacement. No semen displacement occurs with shallow thrusting.[45 ] Some have therefore termed thrusting as a semen displacement behaviour.[46 ]

The behaviors associated with semen displacement, namely thrusting (number of thrusts and depth of thrusts), and duration of sexual intercourse[46 ] have been shown to vary according to whether a male perceives the risk of partner infidelity to be likely or not. Males and females report greater semen displacement behaviors following allegations of infidelity. In particular, following allegation of infidelity, males and females report deeper and quicker thrusting during sexual intercourse.[45 ]

Circumcision has been suggested to affect semen displacement. Circumcision causes the coronal ridge to be more pronounced, and it has been hypothesized that this could enhance semen displacement.[28 ] This is supported by females' reports of sexual intercourse with circumcised males. Females report that their vaginal secretions diminish as intercourse with a circumcised male progresses, and that circumcised males thrust more deeply.[47 ] It has therefore been suggested that the more pronounced coronal ridge, combined with the deeper thrusting, causes the vaginal secretions of the female to be displaced in the same way as rival sperm can be.[28 ]