mikerbyrne73 t1_jdpxj5b wrote
I was taught it is because the Y chromosome sperm are slightly lighter, therefore swim faster and have a higher chance of fertilizing the egg.
Thin-Rip-3686 t1_jdqup8u wrote
There’s only about a 3% difference in payload mass between the two.
Although it’s also been theorized that the lion’s share of the difference is because female embryos spontaneously die at a higher rate, there’s also the theory that eggs and sperm communicate with chemical messengers and eggs may prefer one sperm to another.
This set of preferences as well as the distribution of X vs. Y sperm also correlates to both parents’ stress and methylation levels- an easy life so far means creating males is better because if successful they’ll make more babies of their own, whereas a hard life so far means creating females is better because their success is a safer bet, even if they can’t make as many babies.
That logic may be backwards, or even reversed between sperm and egg.
Tyrrox t1_jdqyov4 wrote
I’m no science dude, but I would think a difference of a 2% spread could easily be accounted for by a 3% shift. 51 is only 4% larger than 49, and 2% larger than 50.
Thin-Rip-3686 t1_jdr6ejt wrote
You forget the importance of percentage of overall mass. It’s probably more like the race performance difference of two identical cars, one with 3% more fuel in the tank, versus one with 3% more overall weight.
Little effects like this can produce outsized results or no change in results, but the “sized” result is way less than 2%.
kelldricked t1_jdt4sfu wrote
That second thing seems unlikely because it wouldnt explained the “returning soldier” effect. A weird “event” were more males (relative to woman) are born after things like wars.
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