How is this ratio achieved? since sex is determined by the sperm, does this mean that male and female sperm are present in the same 106:100 ratio? Or are more male fetuses spontaneously miscarried than female fetuses?
Does this 106:100 ratio get anywhere close to 1:1 by the time human being reach their reproductive age?
Do other animals exhibit a similarly biased male/female ratio at birth?
It is my, possibly incorrect, understanding that male sperm are just a little quicker. Maybe because the Y chromosome is bare? However, looking it up, I see “there’s no explanation for this fact.”
ETA: Here’s an article that mentions my “theory” and that’s it’s probably not the reason:
Concerning point 1: you probably mis-typoed and meant “more female fetuses spontaneously miscarried than female fetuses”, but there seems to be no evidence of that that I can find.
Concerning point 2, yes, definitively. It changes from place to place and from time to time, but male teenagers engage more often in dangerous activities, are killed more often, go to war… After WWII there were many countries in Europe with a surplus of women. Today less so, but still yes. This point, of course, is highly variable.
Other animals have all kind of sex ratios and factors that determine them. Some reptiles like turtles and crocodiles have a sex ratio that is highly dependant on the temperature their eggs are incubated. Some fish are influenced by the metabolites of people’s urine in the sewers, depending on how many take what medicines, for instance the pill or anabolica. Mammals that tend to gather harems “need” less males in proportion to females, but still deer, sea-lions and the like have a 50:50 ratio at birth, more or less (BTW: 106 to 100 is in my view remarcably equal, particularly when considering that it seems to hold true across many cultures and times). That is bad for many males, who live frustrated lifes. Humans, who more or less try to form stable pairs, are better served and more peaceful when the ratio is even, or slightly more women than men.
Insects are very interesting too, specially social insects, but that only shows that nature is complex and interesting.
While my wife was pregnant with my daughter I asked our gynecologist about this. She told me that “male” sperm swim faster but don’t last as long. “Female” sperm are slower but have more staying power.
I then, two years later, used an app designed to tell women about their periods, to deduce the correct date to increase the possibility of getting a son.
Note that in a “harem” mating system a male’s expected number of offspring at his birth (prior to discovering if he will become a dominant breeding male) is exactly the same as any other mating system, because all progeny in the subsequent generation still have one father and one mother.
So the mating system does not usually push the sex ratio away from 50:50, because evolution is driven by the reproductive prospects of the individual, not by the “need” of the population as a whole. If the sex ratio were to differ significantly from 50:50, there would be an advantage to producing offspring of the rarer sex.
Yes, indeed, that is why we both wrote “need” in “”. One big rule, this Fisher’s principle, and easy to understand. The interesting thing to explain would IMO be how nature manages to achieve this equilibrium of the sexes. Because it mostly does, through all kind of circumstances: young and old parents, parents that mate often and regularly and parents that only mate once in their life, monogamous and polygamous parents… And it works! Leaving apart some millions of exceptions that only make the whole more interesting to study
Way back in college, I learned that at conception, the M:F ratio is something like 120:100, but that male fetuses spontaneously abort at a higher rate so that the ratio at birth is 105 or 106 to 100.
OK, I will assume that your college was better than my Google-Fu, so I’ll suppose this to be true. Isn’t it incredibly complex how nature/natural selection takes so many things into account (i.e.: speed of male sperms vs female sperms, spontaneous abortions, juvenile violence…) to arrive at the desired outcome?
Of course “desired” is a metafore, there is no agent who expresses his wish, it is what happens when life goes on and stuff happens. Still I think it is fascinating: it mostly works! Not for every individual, see the spontaneously aborted fetuses, but for the species and for the ecosystem.
And then we humans screw it up, but that is another story and we haven’t reached the conclusion yet. Should be interesting too, and whatever happens afterwards, but I may not witness it. Or, who knows? Catastrophic can also mean very fast.
When you say “the natural sex ratio at birth,” I’m not sure if you mean actual real-world stats, or if you mean the ratio in a situation unaffected by outside factors. By “outside factors,” I mean Chinese gov’t regs, in-vitro fertilization and abortions intended for sex selection. These factors allow parents-to-be to choose the baby’ sex.
As you know, the Chinese family size policy resulted in a generation of skewed birth ratios.
Machine_Elf , I failed to click on the link in your OP. I see now that the linked page’s Summary lists all the factors I mentioned, and some more as well. These days, the places in the world unaffected by IVF, abortion, or outright sex-selective infanticide are few. I don’t know what the ratio would be in those places, where the ratio would really be “natural.”
Maybe I’m naive, but my impression is that outside of a few better-off members of certain immigrant communities, most of the population of western countries don’t put any effort into sex selection. After all, by the time a couple resorts to IVF they are probably happy to have any child at all. How often does the average western family have an abortion because they want one of the other gender?
The article links to another - why are there 123 boys for every 100 girls in Sweden? Apparently, anyone under 18 who becomes a refugee there can request family reunification, so 15 to 21 year old male refugees seem to predominate, especially from Afghanistan. Some are aware of and use this policy. Plus, it’s far easier for a young man to travel across two continents safely than for a 16-year-old unacompanied female.
