Data from: The evolutionary economics of embryonic-sac fluids in squamate reptiles

The parchment-shelled eggs of squamate reptiles take up substantial water from the nest environment, enabling the conversion of yolk into neonatal tissue and buffering the embryo against the possibility of subsequent dry weather. During development, increasing amounts of water are stored in the embryonic sacs (i.e., membranes around the embryo: amnion, allantois, and chorion). The evolution of viviparity (prolonged uterine retention of developing embryos) means that embryonic-sac fluid storage now imposes a cost (increased maternal burdening), confers less benefit (because the mother buffers fetal water balance), and introduces a potential conflict among uterine siblings (for access to finite water supplies). Our data on nine species of squamate reptiles and published information on three species show that the embryonic-sac fluids comprise around 33% of neonatal mass in viviparous species versus 94% in full-term eggs of oviparous squamates. Data on parturition in 149 vipers (Vipera aspis, a viviparous species) show that larger offspring store more fluids in their fetal sacs and that an increase in litter size is associated with a decrease in fluid-sac mass per offspring. Overall, the evolutionary transition from oviparity to viviparity may have substantially altered selective forces on offspring packaging and created competition among offspring for access to water reserves during embryonic development.

有鳞目爬行动物（squamate reptiles）的革质壳卵会从巢穴环境中摄取大量水分，以支持卵黄向新生幼体组织的转化，并为胚胎抵御后续干旱天气的风险。在发育过程中，胚胎囊（即包裹胚胎的膜结构：羊膜（amnion）、尿囊（allantois）与绒毛膜（chorion））内储存的水量会逐渐增加。胎生（viviparity，即发育中的胚胎在子宫内长期滞留）的演化，使得胚胎囊储水行为如今会产生两项代价：一是增加母体负担，二是益处大幅降低（因为母体可直接缓冲胎儿的水平衡）；同时还会引发子宫内幼崽间的潜在竞争——即为获取有限的水资源而展开争夺。我们针对9种有鳞目爬行动物的实测数据，结合3个物种的已发表研究结果显示：胎生物种的胚胎囊液约占新生幼体体重的33%，而卵生（oviparity）有鳞目爬行动物的足月卵中，该比例可达94%。对149条极北蝰（Vipera aspis，一种胎生物种）的分娩数据分析表明，体型更大的幼崽在胎儿囊中储存的液体更多，而窝产仔数的增加则与单只幼崽的囊液质量下降存在关联。总体而言，从卵生到胎生的演化转变，可能极大地改变了对幼崽储水结构的选择压力，并在胚胎发育阶段催生了幼崽间对水资源储备的竞争。