Growth and mortality as causes of variation in metabolic scaling among taxa and taxonomic levels

Abstract

Metabolic rate (MR) usually changes (scales) out of proportion to body mass (BM) as MR = aBM^b, where a is a normalisation constant and b 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. Here, I show that individual variation in fish growth under naturally restricted food availability can explain variation in within-individual (ontogenetic) b for standard metabolic rate (SMR) of brown trout (Salmo trutta); the fastest growers had the steepest metabolic scaling, and within-individual b varied much more widely than previously assumed from work on different individuals or different species, from –1 to 1. The negative b for some individuals was caused by reductions in metabolic rate, likely to maintain positive growth, which resulted in a mean within-individual b that was significantly lower than the across-individual (‘static’) b, a difference that also existed for another species, cunner (Tautogolabrus adspersus). I also show that across-species (‘evolutionary’) b for SMR of 134 fishes is significantly steeper than the mean ontogenetic b for the trout and cunner. I hypothesise that the steeper evolutionary than ontogenetic scaling for fishes could arise as a by-product of natural selection for fast-growing individuals with steep metabolic scaling early in life, where size-selective mortality is high. I support this by showing that b for SMR tends to increase with natural mortality rates of fish larvae within taxa.