Viviparity does not affect the numbers and sizes of reptile offspring

Viviparity (live-bearing) has independently evolved from oviparity (egg-laying) in more than 100 lineages of squamates (lizards and snakes). We might expect consequent shifts in selective forces to affect per-brood reproductive investment (RI = total mass of offspring relative to maternal mass) and in the way in which that output is partitioned (number versus size of offspring per brood). Based on the assumption that newly-born offspring are heavier than eggs, we predicted that live-bearing must entail either increased reproductive investment or a reduction in offspring size and/or fecundity. However, our phylogenetically-controlled analysis of data on 1,259 squamate species revealed no significant differences in mean offspring size, clutch size or RI between oviparous and viviparous squamates. We attribute this paradoxical result to (1) strong selection on optimal offspring sizes, unaffected by parity mode, (2) the lack of a larval stage in amniotes, favouring large eggs even ...胎生（Viviparity，即产活幼体）已在有鳞目（Squamates，涵盖蜥蜴与蛇类）的100余个演化支系中，从卵生（Oviparity，即产卵）类群独立演化而来。 我们或可预期，选择压力随之发生的改变，会影响单窝繁殖投入（per-brood reproductive investment，RI = 后代总质量与母体质量的比值），以及该繁殖产出的分配模式——即每窝后代的数量与个体大小。基于新生幼体体重高于卵粒的前提假设，我们曾预测：胎生必然会伴随繁殖投入的提升，或是后代体型、繁殖力的降低。 然而，我们针对1259种有鳞目物种数据开展的系统发育校正分析（phylogenetically-controlled analysis）显示，卵生与胎生有鳞类在平均后代体型、窝产数量（clutch size）以及繁殖投入RI上均无显著差异。 我们将这一悖论性结果归因于两点：(1) 最优后代体型受到强烈定向选择，且不受生殖方式（parity mode）的影响；(2) 羊膜动物（Amniotes）不存在幼虫阶段，这使得大型卵粒更具演化优势……