Co-variation and trade-offs in ontogenetic scaling of growth and metabolic rates in teleost fish

The influential metabolic theory of ecology proposes that metabolic rate determines growth and ecological processes in a universal, size-dependent manner, scaling with body mass^0.75. Conversely, newer life-history-optimisation theory suggests that metabolic scaling varies due to evolutionary optimisation of energy allocation, predicting negative correlation between metabolic rate and growth. However, metabolic scaling has almost exclusively been investigated across individuals or species, not within individuals through ontogeny. By measuring body mass and metabolic rate longitudinally an average 6.6 times within the same 389 individuals from seven fish species, we find that within-individual ontogenetic scaling of standard (maintenance) metabolic rate correlates positively with scaling of growth, while scaling of aerobic scope correlates negatively. Accelerating ontogenetic growth thus appears to come at a cost of reduced metabolic scope to support functions beyond maintenance. Our results suggest that underappreciated variation in growth can explain why metabolic scaling varies, challenging dogmatic ¾-power scaling and life-history-optimisation theory.