The placenta is a complex life-history trait that is ubiquitous across the tree of life. Theory proposes that the placenta evolves in response to high performance-demanding conditions by shifting maternal investment from pre- to post-fertilisation, thereby reducing a female's reproductive burden during pregnancy. We test this hypothesis by studying populations of the fish species Poeciliopsis retropinna in Costa Rica. We found substantial variation in the degree of placentation among natural populations associated with predation risk: females from high predation populations had significantly higher degrees of placentation compared to low predation females, while number, size and quality of offspring at birth remained unaffected. Moreover, a higher degree of placentation correlated with a lower reproductive burden and hence likely an improved swimming performance during pregnancy. Our study advances an adaptive explanation for why the placenta evolves by arguing that an increased degree of placentation offers a selective advantage in high predation environments.

中文翻译：

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捕食风险影响着活鱼自然种群中的胎盘繁殖程度。

胎盘是一种复杂的生命历史特征，在生命之树中无处不在。理论认为，胎盘可通过将产妇的投资从受精前转移到受精后，来响应对高性能要求的条件而进化，从而减轻女性在怀孕期间的生殖负担。我们通过研究哥斯达黎加鱼类品种Poeciliopsis retropinna的种群来检验这一假设。我们发现与捕食风险相关的自然种群中的胎盘繁殖程度存在很大差异：高捕食种群中的雌性与低捕食雌性相比具有更高的胎盘化程度，而出生后代的数量，大小和质量仍然不受影响。此外，较高的胎盘素与较低的生殖负担相关，因此可能在怀孕期间改善游泳性能。我们的研究提出了一种解释性的解释，说明胎盘为何会进化，理由是胎盘高度增加可在高捕食环境中提供选择性优势。