Department of Animal Ecology and Physiology, Institute for Water and Wetland Research, Radboud University, Heyendaalseweg 135, 6525 AJ, Nijmegen, The Netherlands.
Department of Evolution, Ecology and Behaviour, University of Liverpool, Liverpool, L69 7ZB, U.K.
Biol Rev Camb Philos Soc. 2021 Feb;96(1):247-268. doi: 10.1111/brv.12653. Epub 2020 Sep 22.
Body size is central to ecology at levels ranging from organismal fecundity to the functioning of communities and ecosystems. Understanding temperature-induced variations in body size is therefore of fundamental and applied interest, yet thermal responses of body size remain poorly understood. Temperature-size (T-S) responses tend to be negative (e.g. smaller body size at maturity when reared under warmer conditions), which has been termed the temperature-size rule (TSR). Explanations emphasize either physiological mechanisms (e.g. limitation of oxygen or other resources and temperature-dependent resource allocation) or the adaptive value of either a large body size (e.g. to increase fecundity) or a short development time (e.g. in response to increased mortality in warm conditions). Oxygen limitation could act as a proximate factor, but we suggest it more likely constitutes a selective pressure to reduce body size in the warm: risks of oxygen limitation will be reduced as a consequence of evolution eliminating genotypes more prone to oxygen limitation. Thus, T-S responses can be explained by the 'Ghost of Oxygen-limitation Past', whereby the resulting (evolved) T-S responses safeguard sufficient oxygen provisioning under warmer conditions, reflecting the balance between oxygen supply and demands experienced by ancestors. T-S responses vary considerably across species, but some of this variation is predictable. Body-size reductions with warming are stronger in aquatic taxa than in terrestrial taxa. We discuss whether larger aquatic taxa may especially face greater risks of oxygen limitation as they grow, which may be manifested at the cellular level, the level of the gills and the whole-organism level. In contrast to aquatic species, terrestrial ectotherms may be less prone to oxygen limitation and prioritize early maturity over large size, likely because overwintering is more challenging, with concomitant stronger end-of season time constraints. Mechanisms related to time constraints and oxygen limitation are not mutually exclusive explanations for the TSR. Rather, these and other mechanisms may operate in tandem. But their relative importance may vary depending on the ecology and physiology of the species in question, explaining not only the general tendency of negative T-S responses but also variation in T-S responses among animals differing in mode of respiration (e.g. water breathers versus air breathers), genome size, voltinism and thermally associated behaviour (e.g. heliotherms).
体型大小是生态学的核心，从个体生殖力到群落和生态系统的功能都有涉及。因此，理解温度诱导的体型变化具有基础和应用两方面的重要性，但人们对体型的热响应仍知之甚少。温度-体型（T-S）响应往往是负向的（例如，在温暖条件下生长时，成熟时的体型较小），这被称为温度-体型法则（TSR）。解释强调了生理机制（例如，氧气或其他资源的限制以及与温度相关的资源分配）或大体型（例如，增加生殖力）或短发育时间（例如，对温暖条件下死亡率增加的响应）的适应性价值。氧气限制可能是一个近因因素，但我们认为它更可能是在温暖条件下减少体型的选择压力：由于进化消除了更容易受到氧气限制的基因型，氧气限制的风险将会降低。因此，T-S 响应可以用“过去氧气限制的幽灵”来解释，由此产生的（进化的）T-S 响应在温暖条件下保障了足够的氧气供应，反映了祖先经历的氧气供应与需求之间的平衡。T-S 响应在物种间差异很大，但其中一些变化是可以预测的。与陆地生物相比，水生生物的变暖体型缩小幅度更大。我们讨论了大型水生生物是否特别面临更大的氧气限制风险，因为它们在生长过程中会出现这种情况，这可能在细胞水平、鳃水平和整个生物体水平上表现出来。与水生物种相反，陆生变温动物可能不太容易受到氧气限制的影响，并且可能会优先考虑早期成熟而不是体型较大，这可能是因为越冬更具挑战性，因此在季节结束时会受到更强的时间限制。与时间限制和氧气限制相关的机制并不是 TSR 的相互排斥的解释。相反，这些和其他机制可能会协同运作。但是，它们的相对重要性可能因物种的生态学和生理学而异，这不仅解释了 T-S 响应的一般负向趋势，还解释了呼吸方式（例如，水生呼吸者与空气呼吸者）、基因组大小、多化性和与温度相关的行为（例如，喜热动物）不同的动物之间 T-S 响应的变化。
