Why does metabolic rate scale with body size?

Metabolic rates of all organisms change (scale) with body size, but usually out of proportion; for a given increase in size, metabolic rate increases less. Why this is so, is one of the big unanswered questions in biology. I have a new hypothesis that I believe can answer this: I propose that growth in early life stages shapes metabolic scaling, and that variation in selection pressures on early-life growth causes systematic variation in metabolic scaling among taxa, from ontogenetic to evolutionary levels. I will explore this empirically here using fish, but my hypothesis applies broadly. Confirmation of my hypothesis would be a first step towards a unifying answer to why metabolic rate scales with body size, and why there appears to be variation in how steep this scaling is among species and taxonomic levels. In addition to being of high fundamental interest, understanding metabolic scaling is very important now, as climate warming is changing the metabolic rates and body sizes of life on earth.