Pettersen Amanda K, Schuster Lukas, Metcalfe Neil B
School of Life and Environmental Sciences, University of Sydney, Sydney, NSW, Australia.
Institute of Biodiversity, Animal Health and Comparative Medicine, University of Glasgow, Glasgow, UK.
Integr Comp Biol. 2022 Jun 3;62(5):1492-502. doi: 10.1093/icb/icac076.
Size at the start of life reflects the initial per offspring parental investment - including both the embryo and the nutrients supplied to it. Initial offspring size can vary substantially both within and among species. Within species, increasing offspring size can enhance growth, reproduction, competitive ability, and reduce susceptibility to predation and starvation later in life, that can ultimately increase fitness. Previous work has suggested that the fitness benefits of larger offspring size may be driven by energy expenditure during development - or how offspring metabolic rate scales with offspring size. Despite the importance of early life energy expenditure in shaping later life fitness trajectories, consideration of among-species scaling of metabolic rate at the time of birth as a potential source of general metabolic scaling patterns has been overlooked by theory. Here we review the patterns and processes of energy expenditure at the start of life when mortality is often greatest. We compile existing data on metabolic rate and offspring size for 191 ectotherm species spanning eight phyla and use phylogenetically-controlled methods to quantify among-species scaling patterns. Across a 109-fold mass range, we find that offspring metabolic rate scales hypometrically with size, with an overall scaling exponent of 0.66. This exponent varies across ontogenetic stage and feeding activity, but is consistently hypometric, including across environmental temperatures. Despite differences in parental investment, life history and habitat, large-offspring species use relatively less energy as a proportion of size, compared with small-offspring species. Greater residual energy can be used to fuel the next stages of life, particularly in low resource environments. Based on available evidence, we conclude that, while large knowledge gaps remain, the evolution of offspring size is likely shaped by context-dependent selection acting on correlated traits, including metabolic rates maintaining hypometric scaling, that operates within broader physical constraints.
生命起始时的体型反映了亲代对每个后代的初始投资——包括胚胎及其所获得的营养物质。初始后代体型在物种内部和物种之间都可能有很大差异。在物种内部，后代体型的增加可以促进生长、繁殖和竞争能力，并降低其在生命后期遭受捕食和饥饿的易感性，这最终可以提高适合度。先前的研究表明，较大后代体型带来的适合度优势可能是由发育过程中的能量消耗驱动的——或者说是后代代谢率如何随后代体型变化。尽管生命早期的能量消耗对于塑造后期生命适合度轨迹很重要，但理论上却忽略了将出生时代谢率的种间缩放作为一般代谢缩放模式潜在来源的考量。在这里，我们回顾了生命起始时死亡率通常最高的阶段的能量消耗模式和过程。我们汇总了涵盖八个门的191个外温动物物种的代谢率和后代体型的现有数据，并使用系统发育控制方法来量化种间缩放模式。在跨越109倍的质量范围内，我们发现后代代谢率随体型呈亚比例缩放，整体缩放指数为0.66。这个指数在个体发育阶段和摄食活动中有所变化，但始终是亚比例的，包括在不同环境温度下。尽管亲代投资、生活史和栖息地存在差异，但与小后代物种相比，大后代物种在体型比例上消耗的能量相对较少。更多的剩余能量可用于为生命的下一阶段提供能量，特别是在低资源环境中。基于现有证据，我们得出结论，虽然仍存在很大的知识空白，但后代体型的进化可能是由作用于相关性状的上下文依赖选择塑造的，这些性状包括维持亚比例缩放的代谢率，并且在更广泛的物理限制内起作用。
