An age-dependent ovulatory strategy explains the evolution of dizygotic twinning in humans

Dizygotic twinning, the simultaneous birth of siblings when multiple ova are released, is an evolutionary paradox. Twin bearing mothers often have elevated fitness1-5; but despite twinning being heritable6, twin births only occur at low frequencies in human populations7. We resolve this paradox by showing that twinning and non-twinning are not competing strategies, instead dizygotic twinning is the outcome of an adaptive conditional ovulatory strategy of switching from single to double ovulation with increasing age. This conditional strategy when coupled with the well-known decline in fertility as women age, maximizes reproductive success and explains the increase and subsequent decrease in twinning rate with maternal age that is observed across human populations8-10. We show that the most successful ovulatory strategy would be to always double ovulate as an insurance against early fetal loss, but to never bear twins. This finding supports the hypothesis that twinning is a byproduct of ...

异卵双胎妊娠（Dizygotic twinning）指当多个卵子排出时，同胞同时降生的现象，这是一个进化悖论。生育双胎的母亲通常具有更高的达尔文适合度（fitness）1-5；但尽管双生子性状具有可遗传性6，人类群体中双胎分娩的发生率始终处于较低水平7。我们通过研究破解了这一进化悖论：双胎妊娠与单胎妊娠并非相互竞争的生殖策略，异卵双胎妊娠实则是一种适应性条件排卵策略的结果——该策略会随着女性年龄增长，从单次排卵切换为双次排卵。这一条件性排卵策略结合广为人知的女性生育力随年龄增长而衰退的规律，可使生殖收益达到最大化，同时也解释了全球人类群体中观测到的双胎分娩率随母亲年龄先升后降的现象8-10。我们的研究表明，最优的排卵策略应为：始终以双次排卵作为早期胎儿丢失的保险手段，但却不会诞下双胎。这一发现支持了“双胎妊娠是……的副产物”这一假说。