Gender determined by food scarcity?

[MarkE]

In this video it is mentioned that food scarcity influences the gender of a newborn child (skip to 1:27).

How can we possibly know that this is the cause?

 

Edited March 5, 2018 by MarkE

[Silvestru]

I have a dark joke in mind but I will restrain. It has to do with the direct proportion of sandwich ingredients to women.

[Strange]

5 minutes ago, MarkE said:

How can we possibly know that this is the cause?

If it is true, then presumably it would be based on birth statistics in periods of starvation or malnutrition.

[MarkE]

1 minute ago, Strange said:

If it is true, then presumably it would be based on birth statistics in periods of starvation or malnutrition.

Yes, but on what scientific grounds is this relation considered a correlation?

[Strange]

1 minute ago, MarkE said:

Yes, but on what scientific grounds is this relation considered a correlation?

If you find the same relation in multiple cases, then there is a correlation. Now, obviously, correlation is not causation. But we can eliminate the future birth ratios as a cause of starvation. And, by looking at other factors, presumably it is possible to eliminate those as a common cause. So you would be left with the correlation as the most likely cause. (This isn't very unusual; it is how the health effects of smoking or diet are investigated.)

More here: https://www.smithsonianmag.com/smart-news/times-of-famine-linked-to-disproportionate-number-of-female-births-1110208/

(Luckily, it seems to support my guesswork! )

[MarkE]

26 minutes ago, Strange said:

If you find the same relation in multiple cases, then there is a correlation. Now, obviously, correlation is not causation. But we can eliminate the future birth ratios as a cause of starvation. And, by looking at other factors, presumably it is possible to eliminate those as a common cause. So you would be left with the correlation as the most likely cause. (This isn't very unusual; it is how the health effects of smoking or diet are investigated.)

More here: https://www.smithsonianmag.com/smart-news/times-of-famine-linked-to-disproportionate-number-of-female-births-1110208/

(Luckily, it seems to support my guesswork! )

I guess it's really true then, thanks! The question that still remains is how the fertilisation process is influenced by external conditions. How does the body actually know that there is food scarcity, and how is this translated to the gametes?

Edited March 5, 2018 by MarkE

[StringJunky]

12 minutes ago, MarkE said:

 How does the body actually know that there is food scarcity, and how is this translated to the gametes?

Probably availability, or lack thereof,  of certain nutrients to the relevant cells in the body.

[MarkE]

On 05/03/2018 at 3:37 PM, Strange said:

More here: https://www.smithsonianmag.com/smart-news/times-of-famine-linked-to-disproportionate-number-of-female-births-1110208/

In the article I've read two things that could explain the how and why of gender determination:

"The Harvard-based pair theorized that as the physical condition of a female declines — if she’s nutritionally deprived, for example — she’ll tend to produce a lower ratio of male to female offspring. Evidence of the theory came from red deer and humans; in both species, adverse conditions in the mother’s environment during pregnancy are correlated with a shift toward female births".

and:

"The mechanism behind this finding, and what it takes to trigger this decline, however, are harder questions to answer. One study, Discover reports, found that males during pre-embryonic development have lower rates of survival than females when a mother’s blood sugar levels are down, so it could be that the selection pressure happens after conception".

If this is indeed true, then there's no direct correlation between the womb/gametes and the external environment. It would als mean that @StringJunky was right after all. But I'm not yet convinced by this explanation, because how then would you explain the average 50-50 ratio birth rate of boys and girls in famine threatened African countries?

Take the common reed frog, who can change from a female into a male, which likely occurs when the population does not have enough males to allow procreation. That's an example of the external environment that accounts for determining gender. The same thing has been observed in clownfish, when a female clownfish dies, a male will transform into a female (females are highest in dominance). So this is again an example of an external environmental cause, not the mother's metabolism. Yes, the mother's metabolism and hormone levels change, but this happens as a reaction to something else in the external environment.

Of course, gender shift after birth is different from gender determination before birth, but a mother's internal milieu doesn't have to be the only explanation for human gender determination.

Edited March 6, 2018 by MarkE

[Strange]

18 minutes ago, MarkE said:

But I'm not yet convinced by this explanation, because how then would you explain the average 50-50 ratio birth rate of boys and girls in famine threatened African countries?

Well, if it's true that it doesn't apply in African famines (is there any data for that conclusion?) then it may be because they haven't lasted long enough.

[MarkE]

1 minute ago, Strange said:

Well, if it's true that it doesn't apply in African famines (is there any data for that conclusion?) then it may be because they haven't lasted long enough.

That's true. Does anybody know if this has ever been researched?

[Strange]

https://www.ncbi.nlm.nih.gov/pubmed/9825599

Other stresses appear to have a similar effect: https://www.ncbi.nlm.nih.gov/pubmed/18250336

[CharonY]

1 hour ago, MarkE said:

Take the common reed frog, who can change from a female into a male, which likely occurs when the population does not have enough males to allow procreation. That's an example of the external environment that accounts for determining gender. The same thing has been observed in clownfish, when a female clownfish dies, a male will transform into a female (females are highest in dominance). So this is again an example of an external environmental cause, not the mother's metabolism. Yes, the mother's metabolism and hormone levels change, but this happens as a reaction to something else in the external environment.

I think this conflates different effects. Most data on humans suggest that malnutrition  of the mother has a stronger effect on the development of male compared to female fetuses (i.e. females are more resilient). The resulting sex ratio shift is driven by incomplete pregnancies and death. However, the data is confounded by the observation that in various populations where these studies have been conducted, mothers tend to give fewer births after their first male child, for example. Other issues include the proper measurement of nutritional status. For example, in an Ethiopian study low BMI was associated with a change in sex ration. However, in Urban areas, it is not.

Independently there have been some suggestion that maternal nutritional status (not necessary starvation, but overall nutrition) may have a weak effect before or around conception on the gender of the child. But these connections are even weaker and some metastudies failed to reproduce these findings.

Overall, there may be an effect, but it it does not seem to be clear or very strong one. Also, most point to survival-driven sex ratio deviation rather than a sex determining effect in humans.

Edited March 6, 2018 by CharonY

[MarkE]

@CharonY I guess we don't yet know the full story of gender determination, and why in most species both genders are on average in balance with each other, even though we're aware of the fact that food scarcity can be, in some cases, an indication of gender determination. In conclusion, I think it's interesting that it's surely not just the fastest swimming sperm/spore that determines gender.

[StringJunky]

I haven't read them thoroughly but you might find these interesting:

https://en.wikipedia.org/wiki/Sex_ratio

https://en.wikipedia.org/wiki/Human_sex_ratio

[MarkE]

On 07/03/2018 at 4:53 PM, StringJunky said:

I haven't read them thoroughly but you might find these interesting:

https://en.wikipedia.org/wiki/Sex_ratio

https://en.wikipedia.org/wiki/Human_sex_ratio

Thanks for that!

I've read one interesting thing (in your first URL), called 'Fisher’s principle'. It basically says that if there are more women than men, a newborn male would have better mating prospects than a newborn female. I think he has a point there, but it still doesn’t explain how this kind of information about the environment could have influenced the gametes. I guess the underlying chemical mechanism is still a mystery to science.

[StringJunky]

3 minutes ago, MarkE said:

Thanks for that!

I've read one interesting thing (in your first URL), called 'Fisher’s principle'. It basically says that if there are more women than men, a newborn male would have better mating prospects than a newborn female. I think he has a point there, but it still doesn’t explain how this kind of information about the environment could have influenced the gametes. I guess the underlying chemical mechanism is still a mystery to science.

I think you'd have to look into epigenetics.

[MarkE]

17 minutes ago, StringJunky said:

I think you'd have to look into epigenetics.

So the question is whether the genes in male sperm are actually turned on or off by methylation or not.

"Some areas of the genome are methylated more heavily than others, and highly methylated areas tend to be less transcriptionally active, through a mechanism not fully understood. Methylation of cytosines can also persist from the germ line of one of the parents into the zygote, marking the chromosome as being inherited from one parent or the other (genomic imprinting)."

Interesting, but that's not quite it. The direct connection between the external environment and the internal world is missing. The brain may observe the male-female ratio, but how is this translated to the germ line cells? It's known that bacteria in our gut are connected to the brain through the nervus vagus (and thereby are able to influence what we eat), but male sperm isn't connected to the brain the same way in order to send or receive direct messages (other than testosterone).

Edited March 13, 2018 by MarkE