Measurements of energy use, and its scaling with size, are critical to understanding how organisms accomplish myriad tasks. For example, energy budgets are central to game theory models of assessment during contests and underlie patterns of feeding behavior. Clear tests connecting energy to behavioral theory require measurements of the energy use of single individuals for particular behaviors. Many species of mantis shrimp (Stomatopoda: Crustacea) use elastic energy storage to power high-speed strikes that they deliver to opponents during territorial contests and to hard-shelled prey while feeding. We compared the scaling of strike kinematics and energetics between feeding and contests in the mantis shrimp Neogonodactylus bredini. We filmed strikes with high-speed video, measured strike velocity, and used a mathematical model to calculate strike energy. During contests, strike velocity did not scale with body size but strike energy scaled positively with size. Conversely, while feeding, strike velocity decreased with increasing size and strike energy did not vary according to body size. Individuals most likely achieved this strike variation through differential compression of their exoskeletal spring prior to the strike. Post-hoc analyses found that N. bredini used greater velocity and energy when striking larger opponents, yet variation in prey size was not accompanied by varying strike velocity or energetics. Our estimates of energetics inform prior tests of contest and feeding behavior in this species. More broadly, our findings elucidate the role behavioral context plays in measurements of animal performance.