From the point of view of fertility of women is most advantageous to ovulate once per cycle in his youth, and twice in adulthood. To such conclusion scientists have come, having collected a computer model of the data across multiple populations. According to their calculations, the most successful are those who “switched” with a single ovulation to double in 25 years. In a later age of two embryos, often only one survives, and it increases the fertility of women. In rare cases, this strategy has failed — then there are fraternal twins. Work published in the journal Nature Ecology & Evolution.
It is estimated that every month in Mature women ovulidae one egg, and so if a woman gets pregnant, the light appears most often one child. However, sometimes this rule is violated, and are born fraternal twins — a product of a parallel to ovulation of two ova fertilized by different sperm. It is known that this property — to violate the rule of “one egg per cycle” — is inherited. Therefore, we can assume that it is some specific mechanism that probably gives the mother a distinct advantage.
For an explanation of this phenomenon came up with the hypothesis of insurance: a woman may be beneficial to ripen several eggs because the uterus take root, not all fertilized embryos. Therefore, the more eggs ovulidae, the higher a woman’s chance of getting pregnant, and with the higher the fecundity, the evolutionary benefits are obvious.
But this strategy has a side effect: sometimes double ovulation still leads to the birth of twins. And that in itself is unsafe: when in the uterus instead of a fetus be two, increasing the risk of premature birth, and children often are born weaker than loners. This means that they are more likely to die in infancy, and a woman loses an evolutionary benefit. However, according to the statistics, mothers of twins are more fecundthan women who have twins have never been born.
As for solving these contradictions, a group of researchers under the leadership of Joseph Tomkins (Tomkins Joseph) from the University of Western Australia put forward the assumption, according to which woman’s body is able to regulate the amount baliruhya at a time eggs. The researchers drew attention to the fact that the birth rate of fraternal twins varies with maternal age: it first increases and then decreases. Therefore, they suggested that the woman may be advantageous to change the number baliruhya eggs to adjust to age-related changes: the older you are, the less likely that the child will take root in the uterus, and the double ovulation is more profitable.
To test their hypothesis, the authors had to confirm that with age, the frequency of double ovulation increases due to lower birth rates. They collected demographic data on different populations from Europe, Asia, USA and Africa. But since you can’t measure directly the number of the eggs impossible, the researchers had to count the frequency of double ovulation on circumstantial evidence — the number of twins being born and getting accustomed to the number of embryos. Following this, the researchers found that the best way to explain the frequency of multiple births allows a model in which women are switching over time from a single to a double ovulation. At the same time, this transition can be different: in Japanese women, according to the calculations of the author of the work, it needs to be around 46 years, and Nigerian — 33.