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Wikipedia Assignment V

Content added to Human penis article:

Ejaculate adjustment:
One of the primary ways in which a male's ejaculate has evolved to overcome sperm competition is through the speed at which it travels. Ejaculates can travel up to 30-60 centimetres at a time which, when combined with its placement at the highest point of the vaginal tract, acts to increase a male's chances that an egg will be fertilised by his sperm (as opposed to a potential rival male's sperm), thus maximising his paternal certainty.

In addition, males can - and do - adjust their ejaculates in response to sperm competition and according to the likely cost-benefits of mating with a particular female. Research has focused on two fundamental ways in which male's go about achieving this: adjusting ejaculate size and adjusting ejaculate quality.

Ejaculate size
The number of sperm in any given ejaculate varies from one ejaculate to another. This variation is hypothesised to be a male's attempt to eliminate, if not reduce, his sperm competition. A male will alter the number of sperm he inseminates into a female according to his perceived level of sperm competition, inseminating a higher number of sperm if he suspects a greater level of competition from other males.

In support of ejaculate adjustment, research has shown that a male typically increases the amount he inseminates sperm into his partner after they have been separated for a period of time. This is largely due to the fact that the less time a couple is able to spend together, the chances the female will be inseminated by another male increases, hence greater sperm competition. Increasing the number of sperm a male inseminates into a female acts to get rid of any rival male's sperm that may be stored within the female, as a result of her potential extra-pair copulations (EPCs) during this separation. Through increasing the amount he inseminates his partner following separation, a male increases his chances of paternal certainty. This increase in the number of sperm a male produces in response to sperm competition is not observed for masturbatory ejaculates.

Ejaculate quality
Males also adjust their ejaculates in response to sperm competition in terms of quality. Research has demonstrated, for example, that simply viewing a sexually explicit image of a female and two males (i.e. high sperm competition) can cause males to produce a greater amount of motile sperm than when viewing a sexually explicit image depicting exclusively three females (i.e. low sperm competition). Much like increasing the number, increasing the quality of sperm that a male inseminates into a female enhances his paternal certainty when the threat of sperm competition is high.

Ejaculate adjustment and female quality
A female's phenotypic quality is a key determinant of a male's ejaculate investment. Research has shown that males produce larger ejaculates containing better, more motile sperm when mating with a high quality female. This is largely to reduce a male's sperm competition, since more attractive females are likely to be approached and subsequently inseminated by more males than are less attractive females. Increasing investment in females with high quality phenotypic traits therefore acts to offset the ejaculate investment of others. In addition, female attractiveness has been shown to be an indicator of reproductive quality, with greater value in higher quality females. It is therefore beneficial for males to increase their ejaculate size and quality when mating with more attractive females, since this is likely to maximise their reproductive success also. Through assessing a female's phenotypic quality, males can judge whether or not to invest (or invest more) in a particular female, which will influence their subsequent ejaculate adjustment.