General Discussion

cthulu2016

(10,960 posts)

21. For most species, the sex ratio is approximately 1:1

Sat Feb 18, 2012, 03:30 PM

Feb 2012

There is a lot of variation and a lot of ways that parents can skew the ratio depending on the environment -- some birds can change the sex of embryos by heating them differently, humans can abort, etc. My impression is that for mammals, species wide, anything more extreme than 100 0 is unusual and unstable, but within groups it can be quite different. Small groups of Rhesus monkeys produce almost twice as many daughters, larger groups slightly more sons. It's amazing that they "know" how much room they have to gamble, since daughters are safer bets.

But though an environment can favor one over the other, in the big picture all sexual species seem to oscillate around 1:1 as the evolutionarily stable "sweet spot"

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Why are males and females 50%-50%? [View all] cthulu2016 Feb 2012 OP

Silly me, I learned the natural ratio at birth was 107 boys to 100 girls Brother Buzz Feb 2012 #1

That's birth, not conception. cthulu2016 Feb 2012 #2

Primary sex ratio up to 170 males to 100 females muriel_volestrangler Feb 2012 #8

Fair enough cthulu2016 Feb 2012 #12

I've read that there are more males than females are born in the U.S. Honeycombe8 Feb 2012 #19

I ammended the OP to reflect cthulu2016 Feb 2012 #23

Also, boys traditionally had a higher mortality rate. Arugula Latte Feb 2012 #7

Speak for yourself karynnj Feb 2012 #3

The ratio of boys to girls at birth ranges from 1.03 to 1.07 generally, depending on country FarCenter Feb 2012 #4

Sex ratio is a feature of a species natural history traits HereSince1628 Feb 2012 #5

Maybe it maximizes the experiment potential. gulliver Feb 2012 #6

Assume there is such a thing as truly random, then random coin flips are 50% heads 50% tails. retread Feb 2012 #9

By population, women in the US make up about 52%, partially by attrition. HopeHoops Feb 2012 #10

I think the ratio is about bottlenecks napoleon_in_rags Feb 2012 #11

That's attractive but cannot be right cthulu2016 Feb 2012 #13

Hmm, interesting. napoleon_in_rags Feb 2012 #15

No here is the answer, Fisher's principle. napoleon_in_rags Feb 2012 #17

Exactly (I was trying to say that in the OP but may have failed) cthulu2016 Feb 2012 #18

It's probably more complex than that, particularly in species which are polygynous and bear litters FarCenter Feb 2012 #20

Very true cthulu2016 Feb 2012 #24

Yeah, you're right... Here's a good article: napoleon_in_rags Feb 2012 #30

Again, note the "afford to gamble" factor cthulu2016 Feb 2012 #34

Yes indeed. Do you ever see this stuff holistically cthulu? napoleon_in_rags Feb 2012 #37

Me to. My approach to posting is atypical cthulu2016 Feb 2012 #39

You bring tough questions to the surface. napoleon_in_rags Feb 2012 #40

well said. n/t lumberjack_jeff Feb 2012 #28

It is not a gene quaker bill Feb 2012 #31

That process is not as random as it would seem though. napoleon_in_rags Feb 2012 #38

Unless you are a mouse, no you can't. quaker bill Feb 2012 #42

It seems unlikely to me that a bottle-neck would lead to a change in species mating-systems HereSince1628 Feb 2012 #22

Nicely written. cthulu2016 Feb 2012 #25

My theory is that it only became important to have sons when we became Cleita Feb 2012 #14

Here's a question. Igel Feb 2012 #16

For most species, the sex ratio is approximately 1:1 cthulu2016 Feb 2012 #21

This from an earlier post of mine TexasProgresive Feb 2012 #26

How a sperm cell is formed has to do with it. 2ndAmForComputers Feb 2012 #27

True, but that is not the mechanism cthulu2016 Feb 2012 #33

Mitochondrial DNA Another Bill C. Feb 2012 #29

Wikipedia has a sortable chart on gender ratio: more guys at birth, more gals after 65 yurbud Feb 2012 #32

And with the 1:1 crossover ocurring cthulu2016 Feb 2012 #35

I just realized this balances out a biological injustice: yurbud Feb 2012 #36

I have a question.... unkachuck Feb 2012 #41

Except now in some places... like India and China JCMach1 Feb 2012 #43