Fraternal twins could be the byproduct of an evolutionary ‘insurance policy’ for women as they age.
A woman’s ovaries usually release one egg in a single ovulatory cycle, but occasionally release two — potentially leading to the birth of fraternal twins. A woman’s chance of having fraternal twins rises with age, peaks in her 30s, and then falls.
To explain this pattern, Wade Hazel at DePauw University in Greencastle, Indiana, Joseph Tomkins at the University of Western Australia in Perth and their colleagues hypothesized that as women age, they are more likely to release two eggs in one cycle. This strategy safeguards against the increased risk of prenatal embryo loss, which is more common in older women.
The researchers built a model of fraternal-twin frequency and compared its predictions with the actual ‘twinning rate’ in nine human populations. The twinning rate for women of varying ages was best explained if the model included a double-ovulation rate that rises with age and a foetal-survival rate that falls with age.
Although double ovulation is a reproductive insurance policy, sometimes both embryos survive — and twins are born.