Abstract In social hierarchies, winning contests confers better access to food, shelter and reproductive opportunities. Clear dominance relationships are often apparent even when opponents are matched for experience, age, size and other obvious characteristics. One explanation for this could be among‐individual differences in mitochondrial function, as this could be associated with the cellular allocation of energy to dominance‐associated traits. Here we tested whether the dominance status of juvenile brown trout ( Salmo trutta ) was related to their mitochondrial function. We determined each fish's dominance status by staging a series of contests for a feeding territory in an artificial stream tank; observations of colouration, food acquisition and aggressive behaviour were then combined into a single dominance score. Mitochondrial function (both the capacity and efficiency of oxidative phosphorylation) was then measured in the muscles used for swimming, analysed at the level of both homogenised tissue and isolated mitochondria. We found that the dominance score of a fish was predicted by its capacity for oxidative phosphorylation (a proxy for ATP production capacity) per unit mass of muscle and volume of mitochondria (estimated from citrate synthase activity). Moreover, capacity for oxidative phosphorylation was also a significant predictor of whether fish were categorised as either subordinate or dominant in a binary classification. While it is now well established that organism‐level metabolism is a fundamental trait linking an individual's ecology, physiology and life history, it is only relatively recently that the causes of among‐individual variation in this trait have been investigated, with a shift in focus towards the ways in which energy is produced at a cellular level. By showing that mitochondrial function is related to competitive ability, this study exemplifies how the capacity for cellular energy production can influence contest outcomes and thus the architecture of social hierarchies. Read the free Plain Language Summary for this article on the Journal blog.
McLennan et al. (Fri,) studied this question.