Data from: Does the cost of development scale allometrically with offspring size?

1.Within many species, larger offspring have higher fitness. While the presence of an offspring size-fitness relationship is canonical in life-history theory, the mechanisms that determine why this relationship exists are unclear. 2.Linking metabolic theory to life-history theory could provide a general explanation for why larger offspring often perform better than smaller offspring. In many species, energy reserves at the completion of development drive differences in offspring fitness. Development is costly so any factor that decreases energy expenditure during development should result in higher energy reserves and thus subsequently offspring fitness. 3.Metabolic theory predicts that larger offspring should have relatively lower metabolic rates and thus emerge with a higher level of energy reserves (assuming developmental times are constant). The increased efficiency of development in larger offspring may therefore be an underlying driver of the relationship between offspring size and offspring fitness, but this has not been tested within species. 4.To determine how the costs of development scale with offspring size, we measured energy expenditure throughout development in the model organism Danio rerio across a range of natural offspring sizes. We also measured how offspring size affects the length of the developmental period. We then examined how hatchling size and condition scale with offspring size. 5.We find that larger offspring have lower mass-specific metabolic rates during development, but develop at the same rate as smaller offspring. Larger offspring also hatch relatively heavier and in better condition than smaller offspring. That the relative costs of development decrease with offspring size may provide a widely applicable explanation for why larger offspring often perform better than smaller offspring.1. 在众多物种中，体型更大的后代具有更高的适合度（fitness）。尽管后代体型与适合度之间的关联是生活史理论（life-history theory）中的经典结论，但决定这一关联为何存在的具体机制仍不明晰。 2. 将代谢理论（metabolic theory）与生活史理论相结合，或可为“体型更大的后代通常表现优于体型更小的后代”这一现象提供普适性解释。在众多物种中，发育完成时的能量储备差异是后代适合度分化的核心驱动因素。发育过程存在能量成本，因此任何能降低发育期间能量消耗的因素，均可使后代积累更多能量储备，进而提升后代适合度。 3. 代谢理论预测：若发育时长保持恒定，体型更大的后代相对代谢率更低，因此发育完成时的能量储备水平更高。由此可见，体型更大的后代在发育过程中更高的能量利用效率，可能是后代体型-适合度关联的潜在驱动因素，但该假说尚未在物种内部得到验证。 4. 为明确发育成本如何随后代体型变化而缩放，我们以模式生物斑马鱼（Danio rerio）为研究对象，在一系列自然存在的后代体型梯度下，测量了其整个发育过程中的能量消耗。此外，我们还测定了后代体型对发育时长的影响，并进一步考察了初孵幼体的体型与身体状况如何随后代体型变化而缩放。 5. 我们的研究结果显示：体型更大的后代在发育期间的单位体重代谢率（mass-specific metabolic rates）更低，但其发育速率与体型更小的后代并无差异。同时，体型更大的后代孵化时体重相对更重，身体状况也更佳。发育相对成本随后代体型增大而降低这一结论，或可为“体型更大的后代通常表现更优”这一现象提供普适性解释。