Life-history theory and metabolic theory offer powerful tools for understanding processes driving phenotypic variance, yet traditionally these fields have done so independently of the other. This thesis provides some of the first steps towards integrating two fundamental fields in ecology: life-history theory and metabolic theory. Through integrating two mostly separate fields, I can address how variation in key fitness traits arise, how they might be maintained, and our expectations of how these traits will evolve across environments. I combine empirical work, both within and among species, in the laboratory and field, with meta-analyses to address questions central to life history and metabolic theory.