Data from: Placentation and maternal investment in mammals

The mammalian placenta exhibits striking interspecific morphological variation, yet the implications of such diversity for reproductive strategies and fetal development remain obscure. More invasive hemochorial placentas, in which fetal tissues directly contact the maternal blood supply, are believed to facilitate nutrient transfer, resulting in higher fetal growth rates, and to be a state of relative fetal advantage in the evolution of maternal-offspring conflict. The extent of interdigitation between maternal and fetal tissues has received less attention than invasiveness but is also potentially important because it influences the surface area for exchange. We show that although increased placental invasiveness and interdigitation are both associated with shorter gestations, interdigitation is the key variable. Gestation times associated with highly interdigitated labyrinthine placentas are 44% of those associated with less interdigitated villous and trabecular placentas. There is, however, no relationship between placental traits and neonatal body and brain size. Hence, species with more interdigitated placentas produce neonates of similar body and brain size but in less than half the time. We suggest that the effects of placental interdigitation on growth rates and the way that these are traded off against gestation length may be promising avenues for understanding the evolutionary dynamics of parent-offspring conflict.

哺乳动物胎盘存在显著的种间形态变异，然而这类多样性对繁殖策略与胎儿发育的影响仍不明朗。侵袭性更强的血绒膜胎盘（hemochorial placenta），即胎儿组织直接接触母体血液供应的胎盘类型，被认为可促进营养物质转运，提升胎儿生长速率，且在母-胎冲突的演化进程中属于相对有利于胎儿的状态。相较于侵袭性，母胎组织间的交错程度受到的关注较少，但该因素同样可能至关重要，因其会影响物质交换的表面积。

本研究证实，尽管胎盘侵袭性增强与交错程度提升均与更短的妊娠期相关，但交错程度才是关键变量。高度交错的迷宫状胎盘对应的妊娠期，仅为交错程度较低的绒毛状胎盘与小梁状胎盘的44%。不过，胎盘特征与新生儿体重及脑容量并无关联。换言之，胎盘交错程度更高的物种，仅需不足一半的妊娠期即可产下体重与脑容量相当的新生儿。

我们认为，胎盘交错程度对生长速率的影响，以及生长速率与妊娠期之间的权衡关系，或是解析母-胎冲突演化动力学的极具前景的研究方向。