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?
The researchers found a clever way by analyzing zoo animals, which do have records. It still took years of work though.
Of course they didn't "prove" anything. But you are setting a ridiculously high bar for scientific progress, which is generally very incremental. They took an open question that was undecided for decades and found a way to collect a large amount of data to support the hypothesis. Now that that is - finally! - done, work can proceed to focus on the mechanism.
Mothers with multiple sons (as opposed to mixed gender or primarily female) will have a higher likelihood of having a dominant son, thus more grandchildren, due to one or more of the combination of: 1) competition among sons generates sons able to compete against the larger herd/pack, 2) multiple raffle tickets are better than one, 3) cooperative dominance among siblings (they all work to dominate the rest), 4) in some species, only dominant males are allowed to breed (or are allowed to breed more often), which means that a male will father offspring from multiple mothers, of which sons will tend to be dominant based on their father's genetics, which feeds back on #1, #2, and/or #3.
I'm not saying that any of these are the cause, but they are reasonably plausible, and have an obvious behavioral mechanism that could easily result in the numbers that the research shows.
They took a multi-generational history of a collection of mammals. Then they look at how many 2nd generation children are produced when 1st generation tended to be mixed in gender, vs. predominantly males. From this data, they determined that on average, the when the 1st generation of children tended to be male, they had 2.7 times (on average) as many 2nd generation children than if they had been in a mixed-gender 1st generation.
From my perspective, it seems as though the study looked at all of the outliers in the population, then used an unsubstantiated hypothesis from 30 years ago to fit the outlier data, despite the fact that if you look at the general population as a whole, those outliers are expected in the sample, and might be explainable when you look at herd/group behaviors. There are potentially many factors that can lead to more children in the 2nd generation for male-dominated 1st generation, of which I listed 4 possible factors that doesn't rely on the mother determining the sex of their offspring.
Color me a skeptic, but I'll believe social behavior before uterine gender manipulation any day of the week - especially in animals that are known to be social.
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.
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.
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.
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.
In particular, the statement "men who have higher testosterone levels are more likely to produce male offspring" is pretty much equivalent to the statement "women having sex with men who have higher testosterone levels are more likely to produce male offspring", unless it was measured by impregnating women in a double-blind setting. If it was based on conceptions from a sperm bank, it might be better evidence, though my understanding is they still generally report some information to the women about potential fathers and give her some choice in the matter (but I could easily be mistaken, there).
Anyways, gender is a fairly modern term and it certainly doesn't refer to the sex of an animal. Although, for most animals I would guess that their gender and sex are the same, unless sexual camouflage is used in that species as a reproductive strategy. Humans are unique in divorcing sex (and gender) from reproduction.
That has changed. You can now find many studies in biology talking about the gender of animals (frogs, insects, anything) when they literally mean what you refer to as sex here.
> Humans are unique in divorcing sex (and gender) from reproduction.
That is highly speculative. We do not understand animals well enough to even know if that is a meaningful question.
Selecting X or Y sperm cells may be easy, if there is some sort of external effect to the genes in those. For example, perhaps X causes sperm cells to be more robust to acidity, and the female can select for X by increasing acidity levels.
IVF in humans can be very telling if number crunched properly - the eggs are extracted and the mother body is out of the equation for a moment.
Now if the sex selection/discrimination is population based - it is simpler and easier to make it in the males. If it is per embryo - than you have no choice but make it in the females no matter how complex.