Differential fitness operates through survival and reproduction. In energetically constrained animals that survive on small amounts of nectar, adaptations to enhance caloric intake have been naturally selected. On the other hand, animals compete to reproduce, and when this turns into physical combat, intrasexually selected weapons evolve. In this talk, I present theoretical and empirical findings on each of these fronts, along with a case study of their intersection, namely, hummingbird bill weapons. These discoveries revive questions dating back to Darwin about how hummingbirds budget energy gain and expenditure to enable hovering, the most expensive form of locomotion, establishing coevolutionary relationships with flowers. These novel perspectives of a data-rich mutualistic system, open the door to quantitative and comparative assessments of trade-offs between energy optimality and fighting proficiency. My research is question-, rather than technique-driven; thus, I present cross-disciplinary approaches aimed to achieve a more complete integration across biological levels of organization.