

Mammals can 'choose' sex of offspring, study finds - wikiburner
http://med.stanford.edu/ism/2013/july/ratios.html

======
driverdan
Original, non-blogspam article:
[http://med.stanford.edu/ism/2013/july/ratios.html](http://med.stanford.edu/ism/2013/july/ratios.html)

This PR is ridiculously unscientific. This doesn't "prove" their hypothesis.
They've found no mechanism of action meaning they found a correlation.
Correlation is not causation. Maybe the actual study found something more than
the PR says but it doesn't seem like it. Also, they called it the "holy
grail". Seriously?

~~~
venomsnake
Actually I thought of very fun experiment - because the gender of the baby is
determined by the father - make two groups live in extreme condition bio-dome
style for a month or two - 900 males and 100 females and in the other 100
males and 900 females in the second. Then at the end of the period just
examine the ratio of sperm cells with X and Y chromosome in the males' semen.
If there is sizable difference means there are mechanisms for regulation and
selection.

Anecdotal evidence is that even after very bloody wars where the males die off
disproportionately the population does not stray off a lot in the gender
balance after a few years.

Bonus points if you make it with humans as reality TV on Bravo.

~~~
dllthomas
The hypothesis here is that it is _not_ the father that determines the gender
of the baby.

~~~
venomsnake
The father determines the gender in a sense that he provides both X and Y. The
woman only provides X.

In the facts given in the article I found nothing convincing that the gender
selectors are females i.e. they conclude that they are.

Now I haven't studied seriously biology but to change the meiosis of sperm
cells to skew the odds seems more effective (and simple) than to have the egg
cell select the sperm with the X or Y chromosome during the fertilization that
is messy as it is. And if X and Y sperm cells were somehow different I think
we may have found out.

~~~
dllthomas
> The father determines the gender in a sense that he provides both X and Y.
> The woman only provides X.

That's not a meaningful sense. Obviously it's a question of whether a sperm
cell carrying an X or Y chromosome gets into the egg, but both parties have a
lot of biology involved between meiosis and fertilization that might impact
that.

> Now I haven't studied seriously biology but to change the meiosis of sperm
> cells to skew the odds seems more effective (and simple) than to have the
> egg cell select the sperm with the X or Y chromosome during the
> fertilization that is messy as it is. And if X and Y sperm cells were
> somehow different I think we may have found out.

X and Y sperm cells are different - one is carrying an X and the other is
carrying a Y and these will therefore have different masses and probably some
other characteristics will differ. As female mammals tend to invest hugely
more resources in the individual child, it's more likely to be worthwhile to
them to sacrifice some chance of conception to favor a particular gender.
Furthermore, if the child is more likely to stay with the female, the female
is a better predictor of the environment the child will be raised in. Since
meiosis produces an X and Y cell inherently (it splits a cell with an X+Y in
half, with one chromosome going each way) and furthermore happens well in
advance of mating when the partner may not even be determined, it seems less
likely that the cause for any pattern in SR would be manipulation of the ratio
of X vs Y cells during meiosis. Of course, biology surprises on occasion, I
just think you're making entirely unwarranted assumptions.

~~~
craigyk
Still the mechanisms by which the male body might control the ratio of sperms
with X vs. Y is astoundingly simpler than mechanisms by which an egg can
somehow "sense" and reject sperm. On the other hand, a mechanism by which the
female body can selectively abort an embryo is way more plausible. Either way,
this leads to some pretty testable predictions. You might not even need to do
much direct experimentation, an outcome of the second hypothesis is that if
gender selection is highly activated than more eggs/embryos are wasted, which
might lead to smaller litters/ longer time between pregnancies.

~~~
dllthomas
> mechanisms by which an egg can somehow "sense" and reject sperm.

Evolution doesn't work like an engineer. If you or I were going to sit down
and say, "Hmm, how do we control the sex ratio from within the female's body?"
we'd come up with things like a mechanism to differentiate the sperm and then
try to apply that to a mechanism to block or help one group or the other,
because modularity is good and such.

Evolution doesn't pay attention to the mechanism, just the result. Maybe it
happens that one type of sperm is disproportionately affected by acidity
levels or viscosity of cervical mucous or any of potentially hundreds of
attributes of the environment the sperm will be encountering between meiosis
and conception, and that's assuming any differentiation happens pre-
conception. If there is such an effect, there's a good chance evolution will
"spot it" and exploit it where the gain outweighs the cost; the female has
(evolutionarily speaking) far more motivation to make use of it, if it
involves any tradeoff with chance of conception.

Note that by his hypothesis, smaller litters or longer time between
pregnancies probably is what you'd see if the decision was being made in the
female's body, whether pre- or post-conception.

~~~
dllthomas
s/by his hypothesis/by this hypothesis/

------
andrewcooke
_Hamilton [2], focused on scenarios specific to particular insect groups. In
Mammals and birds a more general principle applies: the number of offspring a
male produces is often limited by how many females he can mate with, while a
female is limited by how many offspring she can physiologically produce [5],
[6].This generates a tendency for males to vary more in first-generation
success than females. Thus male offspring are a high-risk-high-reward bet for
potential grandparents in the genetic lottery; while females are a safe,
hedged bet [5], [6]. However, just like in insects, if a grandparent ‘knows’
that a male offspring is a low-risk-high-reward bet, then they can beat the
house, and hit a jackpot (in terms of grandchildren produced) [5], [6].
Furthermore, grandparents can beat the house in other more subtle ways,
leading later authors to propose a variety of advantages to SR manipulation
that might apply to vertebrate species (e.g. local resource competition or
enhancement [3])._

some basic background from the paper. lets try and drag the level of debate up
a little here?

[http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjourna...](http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0067867)

------
jamestomasino
Some of the worst science reporting I've ever read. That said, after reading
the journal article itself, I found plenty of false assumptions there as well.

"... if functional consequences of SR manipulation were to be found in
mammals, then it would suggest that mammals (either in individual species, or
in general), possess unknown physiological mechanisms to control birth SR."

This sentence in the introduction assumes a level of choice on the part of the
mammals that is pervasive in the article but never substantiated or even
formalized to the point of a testable hypothesis. It is the type of sloppy
science that takes away from the truth, and it's becoming way too common. It's
as if scientists feel that if they just have some good hard numbers, they get
the freedom to toss conjecture around as Theory for a few paragraphs.

The study found that gender bias in birth rates can affect the number of
grandchildren. That is the takeaway here. No evidence whatsoever of what
caused that bias is present, nor do I accept their statement that the study
taking place in a zoo means that their findings are somehow more likely to be
present in the wild. That would require a different experiment on its own.

~~~
azakai
> This sentence in the introduction assumes a level of choice on the part of
> the mammals that is pervasive in the article but never substantiated or even
> formalized to the point of a testable hypothesis.

The field here is evolutionary biology, and specifically the area of parental
investment and applications of game theory. It is very common in that area to
talk about "choice" etc. while not meaning anything conscious. For example, it
is acceptable to say that genes "want" to propagate copies of themselves, but
of course this is just shorthand for something very different. In the field,
this is not misunderstood, it is the norm.

> The study found that gender bias in birth rates can affect the number of
> grandchildren.

No, the point was that they found that gender bias in birth rates appeared to
be not entirely random (50-50) and weighted towards what generates an optimal
number of grandchildren. That exactly supports a hypothesis that has been
around for decades that through female choice (again, choice is not meant in
the sense that I can choose what to eat for lunch), animals can affect gender
birth ratios in order to maximize the number of grandchildren. (Here you can
see the connection to game theory.)

It is very hard to study that empirically, which is why this study is
receiving a lot of attention - after decades, it is among the first to
actually give measurements.

> nor do I accept their statement that the study taking place in a zoo means
> that their findings are somehow more likely to be present in the wild. That
> would require a different experiment on its own.

This is indeed a weakness in their methodology, and surely being debated
heavily.

~~~
gcb0
So it's like comparative lit in biology?

Joking aside, the grand patent post is right, all the article does is get one
consequence (not 50/50 sex split) and immediately assumes parent control.

in reptiles it's know that egg temperature determine offspring sex. But it's
also believed that parents are ignorant of this fact.

~~~
azakai
It is not just a non-50/50 split. It is also that the bias, when present, is
tuned to optimize for the number of grandchildren. That is the long-standing
hypothesis which they have supported.

------
brass9
Much better coverage can be found here:
[http://www.sciencedaily.com/releases/2013/07/130710182941.ht...](http://www.sciencedaily.com/releases/2013/07/130710182941.htm)

~~~
1123581321
All that is being discovered is that sons mean more grandchildren, and a few
groups (billionaires, top-ranking wives) are cherry-picked because they have
more sons. Dung flies are mentioned but they are not even mammals, and they
aren't sex selecting their sperm.

That said, though the article claims a mechanism is unknown, diet can
influence whether a girl or boy is conceived:

[http://genderdreaming.com/2011/05/the-french-gender-
diet/](http://genderdreaming.com/2011/05/the-french-gender-diet/)

[http://www.rbmojournal.com/article/S1472-6483(10)00549-3/abs...](http://www.rbmojournal.com/article/S1472-6483\(10\)00549-3/abstract)

My own wild guess is that the observed diet factors, or at least certain
states of the body that diet can cause, account for any statistically
significant distributions of sex, not a mysterious sperm-guiding mechanism.

~~~
upquark
Thank you for the links, very interesting.

There is also this: [http://genderdreaming.com/family-balancing/natural-
gender-sw...](http://genderdreaming.com/family-balancing/natural-gender-
swaying/science-behind-gender-swaying/ph-balance/)

I have noticed that in some small immigrant population pockets in the US the
majority of offspring is boys, and I've been wondering about the biological
causes for this phenomenon. The explanation I heard is that the acidity can be
regulated by stress levels of the female, causing more boys to be born in a
more stressful environment. At the time I thought it's pure pseudoscience.

------
randartie
I wanted to see some numbers and some stats.

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clubhi
So everyone that majored in comp sci has daughters?

