Data from: Variation in offspring size with birth order in placental fish: a role for asymmetric sibling competition?

Asymmetric sibling competition arises when siblings with different competitive abilities share a limited resource. Such competition occurs in species with postnatal parental care and may also occur when mothers provision embryos between fertilization and birth (matrotrophy). We hypothesized that the combination of matrotrophy and the simultaneous provisioning of embryos in different stages of development (superfetation) leads to asymmetric competition between sibling embryos. Moreover, we expect the intensity of this competition to increase with the level of superfetation as high levels of superfetation result in greater temporal overlap between broods. This hypothesis predicts that offspring from early broods, which predominantly compete with less developed siblings, will be larger at birth than offspring from later broods, which experience competition from more and less developed siblings. Data on offspring size at birth from two populations of the highly matrotrophic fish, Heterandria formosa, and similar studies of poeciliid fish spanning a range of life histories are consistent with our hypothesis. Together these results suggest that sibling competition is a direct consequence of the evolution of matrotrophy and superfetation in poeciliid fish.

当具有不同竞争能力的同胞个体共享有限资源时，便会产生不对称同胞竞争（asymmetric sibling competition）。这类竞争常见于具有产后抚育行为的物种，同时也可能出现在母体在受精至分娩期间为胚胎提供营养的情形中，即母体营养（matrotrophy）。

我们提出假说：母体营养与同时抚育不同发育阶段胚胎的超胎发育（superfetation）相结合，会引发同胞胚胎间的不对称竞争。此外，该竞争的强度会随超胎发育程度的提升而增强，因为高水平的超胎发育会使不同批次胚胎间的时间重叠程度进一步升高。

该假说预测：早期胎次的后代主要与发育程度更低的同胞竞争，因此出生时的体型会大于晚期胎次的后代——晚期胎次的后代需要与更多且发育程度各异的同胞展开竞争。

针对高度依赖母体营养的美丽异齿鳉（Heterandria formosa）的两个种群的出生后代体型数据，以及涵盖多种生活史类型的花鳉科鱼类（poeciliid fish）的相关研究结果，均与我们的假说相符。综合以上结果可见，同胞竞争是花鳉科鱼类演化出母体营养与超胎发育这两种特征的直接结果。