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.
Male conceptuses are less viable than female and the ratio of male:female drops to 105:100 at birth.
Researchers have not been able to find major differences in size and shape between X and Y sperm.
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:
There are equal numbers of X and Y chromosomes in mammalian sperm
X and Y stand equal chance of achieving conception
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.