Norin Tommy
DTU Aqua: National Institute of Aquatic Resources, Technical University of Denmark, Kemitorvet, Building 202, 2800 Kgs. Lyngby, Denmark.
Integr Comp Biol. 2022 May 17. doi: 10.1093/icb/icac038.
Metabolic rate (MR) usually changes (scales) out of proportion to body mass (BM) as MR = aBMb, where a is a normalisation constant and b is the scaling exponent that reflects how steep this change is. This scaling relationship is fundamental to biology, but over a century of research has provided little consensus on the value of b, and why it appears to vary among taxa and taxonomic levels. By analysing published data on fish and taking an individual-based approach to metabolic scaling, I show that variation in growth of fish under naturally restricted food availability can explain variation in within-individual (ontogenetic) b for standard (maintenance) metabolic rate (SMR) of brown trout (Salmo trutta), with the fastest growers having the steepest metabolic scaling (b ≈ 1). Moreover, I show that within-individual b can vary much more widely than previously assumed from work on different individuals or different species, from -1 to 1 for SMR among individual brown trout. The negative scaling of SMR for some individuals was caused by reductions in metabolic rate in a food limited environment, likely to maintain positive growth. This resulted in a mean within-individual b for SMR that was significantly lower than the across-individual ("static") b, a difference that also existed for another species, cunner (Tautogolabrus adspersus). Interestingly, the wide variation in ontogenetic b for SMR among individual brown trout did not exist for maximum (active) metabolic rate (MMR) of the same fish, showing that these two key metabolic traits (SMR and MMR) can scale independently of one another. I also show that across-species ("evolutionary") b for SMR of 134 fishes is significantly steeper (b approaching 1) than the mean ontogenetic b for the brown trout and cunner. Based on these interesting findings, I hypothesise that evolutionary and static metabolic scaling can be systematically different from ontogenetic scaling, and that the steeper evolutionary than ontogenetic scaling for fishes arises as a by-product of natural selection for fast-growing individuals with steep metabolic scaling (b ≈ 1) early in life, where size-selective mortality is high for fishes. I support this by showing that b for SMR tends to increase with natural mortality rates of fish larvae within taxa.
代谢率（MR）通常与体重（BM）不成比例地变化（缩放），因为MR = aBM^b，其中a是一个归一化常数，b是缩放指数，反映了这种变化的陡峭程度。这种缩放关系是生物学的基础，但一个多世纪的研究对b的值以及它为何在不同分类群和分类水平上似乎有所不同几乎没有达成共识。通过分析已发表的鱼类数据，并采用基于个体的方法来研究代谢缩放，我发现，在自然食物供应受限的情况下，鱼类生长的差异可以解释褐鳟（Salmo trutta）标准（维持）代谢率（SMR）的个体内（个体发育）b值的差异，生长最快的个体具有最陡峭的代谢缩放（b≈1）。此外，我还表明，个体内b的变化范围比之前基于不同个体或不同物种的研究假设的要大得多，个体褐鳟的SMR的b值从-1到1。一些个体的SMR负缩放是由于食物受限环境中代谢率的降低，这可能是为了维持正生长。这导致个体内SMR的b平均值显著低于个体间（“静态”）b，另一个物种康氏躄鱼（Tautogolabrus adspersus）也存在这种差异。有趣的是，同一批褐鳟的最大（活跃）代谢率（MMR）在个体发育过程中b值的广泛变化并不存在，这表明这两个关键的代谢特征（SMR和MMR）可以相互独立地缩放。我还表明，134种鱼类的SMR的跨物种（“进化”）b值比褐鳟和康氏躄鱼的个体发育平均b值要陡峭得多（b接近1）。基于这些有趣的发现，我推测进化和静态代谢缩放可能与个体发育缩放系统地不同，并且鱼类进化缩放比个体发育缩放更陡峭是自然选择的副产品，自然选择有利于生命早期具有陡峭代谢缩放（b≈1）的快速生长个体，而鱼类的大小选择性死亡率很高。我通过表明SMR的b值倾向于随着分类群内鱼类幼体的自然死亡率增加来支持这一观点。
