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MedicalDaily writes in Male Fertility 101: 7 Little-Known Facts About Sperm For Men's Health:

Not all sperm are chromosomally male. Several sperm do carry the X chromosome, while others carry the Y chromosome. However, female sperm is actually stronger than male sperm, according to Harvard Health Publications. This means the likelihood of getting pregnant by a female swimmer is higher than a male one.

Is this accurate?

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    I do not find the claim in question in the primary source, Harvard Health Publications. Could you cite it directly, or is MedicalDaily misreading HarvardHP? – DJohnM Oct 21 '15 at 23:55
  • Gah. I have most of an answer, but as I wrote it, I realised I am missing one important statistic, that must have been studied but is elusive. – Oddthinking Oct 22 '15 at 0:36
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    Interesting given that male births exceed female births with an expected ratio of 0.515 ncbi.nlm.nih.gov/pmc/articles/PMC102777 i.e. if both claims are correct then there must be significantly more terminations of female pregnancies. – Dale M Oct 22 '15 at 3:51
  • I remember reading a book sometime ago that stated the reason more males are born is because the Y chromosome is light. It is missing an entire arm if you will that the X chromosome has. Therefore sperm with Y chromosomes move faster and can cover more ground. It's not a huge difference in weight, but it is significant. – Nathan Oct 22 '15 at 4:09
  • @Nathan: We already addressed that in this question: skeptics.stackexchange.com/questions/8001/… – Oddthinking Oct 22 '15 at 10:27
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  1. In humans, the X-bearing sperm has 2.8% more total DNA than Y-bearing sperm. Thus, when stained with a DNA-specific fluorochrome, the fluorescent signal emitted by an X-bearing sperm is greater than that emitted by a Y-bearing sperm which is utilized in the Microsort method for gender selection.

  2. Male conceptuses are less viable than female and the ratio of male:female drops to 105:100 at birth.

  3. Researchers have not been able to find major differences in size and shape between X and Y sperm.

  4. Research suggests that Y sperm may actually survive longer than X sperm in vitro.

Referring to William H James in 2008, conventional assumptions have been:

  1. There are equal numbers of X and Y chromosomes in mammalian sperm

  2. X and Y stand equal chance of achieving conception

  3. Therefore equal number of male and female zygotes are formed, and that therefore any variation of sex ratio at birth is due to sex selection between conception and birth.

The ideal ESS to maintain an optimal balance of different characteristics in the population is a 1:1 ratio of males to females. This is supported by the fact that there is no sex ratio bias in X to Y sperm.

Various factors are discussed in the William H James paper which affect the ratio of men’s X-bearing and Y-bearing sperm.

However, historical records and current studies confirm that more males than females are conceived (primary sex ratio) and born (secondary sex ratio). This suggests that under certain conditions, natural selection favors a deviation from the 50/50 ESS sex ratio at conception and birth per research by Curt A Sandman et.al. in 2013.

Males exposed to early adversity suffer a much higher risk than females of fetal and infant morbidity and mortality. Because they have been eliminated (high mortality) or weakened (morbidity), the surviving males constitute a relatively homogenous, less variable cohort. In contrast, females adjust to early adversity with a variety of less extreme individually determined strategies and have escaped the severe consequences of high risk for mortality and morbidity.

The consequences of male exposure to early adversity threaten their viability, effectively culling the weak and the frail and creating a surviving cohort of the fittest. Females adjust to early adversity with a variety of strategies, but their escape from the risk of early mortality and morbidity has a price of increased vulnerability expressed later in development.

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