Flexibility in the timing of parturition: the key to viviparity in lizards?

分娩时间的灵活性：蜥蜴胎生的关键？胎生蜥蜴的胚胎通常从蛋黄中获得足够的营养。那么，为什么一些物种进化出了胎盘营养，它又带来了什么新的机会呢？答案可能在于出生时间的灵活性所带来的好处，这种优势是通过在怀孕期间将储存脂肪补充转移到胚胎中实现的。我们认为，这为进化分化提供了一个关键的机会：一些高山物种利用灵活的分娩；其他物种用强制性转移取代了可选的胎盘营养。我们独特的实验方法，使用了两个经过充分研究的胎生石龙子分支，将导致关于胎生进化的新的可检验的假说。

Flexibility in the timing of parturition: the key to viviparity in lizards? Embryos of viviparous lizards usually gain adequate nutrients from yolk. Why then has placentotrophy evolved in some species, and what new opportunities has it opened up? The answer may lie in the advantages gained from flexibility in the timing of birth, made possible by supplementary transfer of storage lipids to embryos during gestation. We argue that this provides a pivotal opportunity for evolutionary divergence: some alpine species exploit flexible parturition; other species replace optional placentotrophy with obligate transfer. Our unique experimental approach, using two well-researched clades of viviparous skinks, will result in new testable hypotheses about the evolution of viviparity.