Ontogenetic growth and size-at-age shape metabolic scaling

Metabolic scaling describes the relationship between metabolic rate and body mass and has traditionally been investigated across individuals or species. Scaling of metabolic rate within individuals as they grow through ontogeny has received far less attention. This is presumably because it is temporally and logistically cumbersome to do the longitudinally repeated measurements on the same individuals that are required to quantify direct (within-individual) ontogenetic scaling. Here, we argue for the importance of studying within-individual ontogenetic metabolic scaling to understand energy-allocation trade-offs and variation in metabolic scaling at all biological levels. Using our own data on within-individual ontogenetic metabolic scaling of fish, we explore debated links between metabolic scaling and growth. We outline how covariation between ontogenetic metabolic scaling and growth rate can generate predictable variation in higher-level scaling across individuals. We further demonstrate how timing of sampling during an individual's growth trajectory can introduce variation in metabolic scaling relationships, because growth rate changes through ontogeny and varies among individuals. Thus, variation in metabolic scaling can be driven by often unknown variation in the age composition and growth trajectories among individuals within the species from which they are sampled. We believe that an increased focus on within-individual ontogenetic metabolic scaling and its relationships with other physiological and life-history traits can help clarify the century-long debate about what causes variation in metabolic scaling relationships and the many theories associated with it.