Gender determination in humans: Male birth over female due to physical constraints

[Genecks]

About two years ago, I read an article about males having a slightly (fractional) chance of being born over females due to the physical processes of fertilization and the energy used to make the processes happen. In the end, males are born more than females.

 

Does anyone have a link to the article or remember where to find it? I can't remember.

[Kyrisch]

I think it's due to the fact that the mass of a sperm containing a Y chromosome as compared to an X chromosome is slightly less, allowing them to move slightly faster.

[Genecks]

I think it's due to the fact that the mass of a sperm containing a Y chromosome as compared to an X chromosome is slightly less, allowing them to move slightly faster.

 

Yeah, but I'm looking for the article.

[GDG]

I don't know if this is the article you are looking for, but here is a 1999 paper about gender selection using a "swim up" procedure. The procedure is based on the theory that Y sperm (because they are less massive) swim faster than X sperm. In this study, 86.7% of those seeking a girl were successful, and 89.2% of those seeking a boy were successful.

 

M.A. Khatamee et al., "A controlled study for gender selection using swim-up separation" Gynecol Obstet Invest (1999) 48(1):7-13.

[Mr Skeptic]

On the other hand, males also have a higher mortality rate.

[dougalbod]

While the average speed of 'male' vs 'female' sperm may be a factor it's far too simplistic to think that this by itself is the cause of an imbalance in the gender of offspring. If there are more male offspring than female then their is a selective advantage to producing offspring of the less common gender.

As Mr Skeptic said, males (at least for humans and I guess for many other mammal species) have a higher mortality rate before reaching sexual maturity. This does seem like a better reason for the gender imbalance at birth.

 

Does anybody know of a species where the imbalance is more significant? - I know social insects produce far more males than females but they have weird genetics -

My guess is that for most species the a 50:50 balance (more or less ) is maintained.

 

Dougal

[GDG]

While the average speed of 'male' vs 'female' sperm may be a factor it's far too simplistic to think that this by itself is the cause of an imbalance in the gender of offspring.

 

Except that the actual OP was directed toward that physical difference.

 

Just saying that there must be some evolutionary reason for the difference does not explain the mechanism that implements the difference.

[Mr Skeptic]

I've also heard that in some species, more males are born if the mother is well-nourished, whereas more females when she is malnourished. A weak female will have less trouble finding a mate than a weak male, whereas a strong male might get several mates to reproduce with but the female's reproductive rate can't increase by much. I don't know whether the mechanism for this is pre or post fertilization.