In response, ecologists and evolutionary biologists often record and include body mass as a co-variant in their studies and assume that animals with a higher body mass are in better condition, despite the knowledge that obesity can have numerous negative impacts on fitness. In 2011, Hill defined condition as the relative capacity to maintain optimal functionality of vital systems (Ecology Letters 14:625) and further suggested that cellular respiration efficiency would be among the best indicators of condition (Integrative and Comparative Biology, 2014,54:645). With this presentation, I will describe the relationship between body mass and mitochondrial performance in several species and contexts. I will highlight work on ICR mice, showing that the interaction between mitochondrial performance and body mass is highly dependent on reproductive experience. My lab group observed that non-reproductive mice displayed a negative relationship between body mass and mitochondrial respiratory capacity; in contrast, females that experienced four reproductive events displayed a positive relationship between body mass and mitochondrial respiratory capacity. I will discuss the significance of these findings to our understanding of the interactions between body mass, condition, and the fitness of animals.