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.当竞争能力存在差异的同胞个体共享有限资源时，便会产生不对称同胞竞争。这类竞争常见于存在产后双亲抚育的物种中，同样也可能出现在母体在受精至分娩期间为胚胎提供营养的胚营养（matrotrophy）场景中。我们提出假说：胚营养与为不同发育阶段胚胎同时提供营养的超孕（superfetation）相结合，会导致同胞胚胎间出现不对称竞争。此外，我们推测该竞争的强度会随超孕程度提升而增强——高水平的超孕会使同窝胚胎间的时间重叠度进一步升高。该假说预测：相较于后期产仔的后代，前期产仔的后代出生时体型更大；前者主要与发育程度更低的同胞竞争资源，而后者则会面临来自更多且发育程度各异同胞的竞争。针对高度依赖胚营养的鱼类翼鳍胎鳉（Heterandria formosa）的两个种群的出生后代体型数据，以及覆盖多种生活史类型的花鳉科（poeciliid fish）鱼类的相关研究结果，均与我们的假说相符。综合上述结果可知，同胞竞争是花鳉科鱼类演化出胚营养与超孕特性的直接结果。