Fish may be the most diverse vertebrate group, with incredibly athletic species like the tunas as well as more sedentary species such as sculpins and flatfish. Why does this variation exist? Why don’t all species simply evolve to be ever bigger, stronger, and faster? Are there advantages to being more sluggish?

Shaun Killen and colleagues examined this issue across 131 fish species and found that maximum metabolic rate—a trait which is related to swimming ability and athleticism—shows a 40-fold range across species. Species with more active lifestyles or that have higher positions within food-webs—pelagic predators, for example—tend to have higher maximum metabolic rates.

Importantly, maximum metabolic rate is closely related to the minimum metabolic rate that a species needs to sustain life. In other words, species that are built for a higher peak performance also spend more energy when they are at rest. An analogy is a high-performance car that requires more fuel than a slower car, even while both are idling.

This is likely a key limitation preventing the evolution of ever higher capacities for aerobic metabolism and swimming ability. A higher minimum metabolic rate will cause a species to require more food and oxygen—resources that are often in short supply in aquatic environments.

Read more in the ahead of print link here:

Citation: Killen, S.S., Glazier, D.G., Rezende, E.L., Clark, T.D., Atkinson, D., Willener, A., Halsey, L.G. 2016. Ecological influences and morphological correlates of resting and maximal metabolic rates across teleost fish species. American Naturalist. DOI: 10.1086/685893