Mitochondrial function predicts dominance status in brown trout

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, since 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 allowed the assignment of dominance. Mitochondrial function (the capacity for ATP production, and the efficiency with which ATP is produced) was then measured in the muscles used for swimming, analysed at the level of both homogenised tissue and isolated mitochondria.

We found that a fish’s net phosphorylation efficiency (i.e., the efficiency with which ATP is produced) was a significant predictor of its dominance-associated colouration and the level of aggression it exhibited towards an opponent. However, a fish’s overall dominance status was instead predicted by its capacity for oxidative phosphorylation (i.e. ATP production capacity). Moreover, oxidative phosphorylation was also a significant predictor of whether a fish would be categorised as being 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 cellular energy production can influence contest outcomes and thus the architecture of social hierarchies.