Metabolic rate, or the rate at which organisms uptake, transform, and expend energy varies substantially across individuals of the same species, even after accounting for differences in body size and temperature. What drives this variation? Metabolic rates set the pace-of-life – higher metabolic rates are linked to faster growth, earlier onset of reproduction, and shorter lifespan, while low metabolic rates are associated with a slow pace-of-life (slow growth, late onset of reproduction and long lifespan).
Variation in metabolic rates are likely to have fitness consequences, and be under strong selection. Evolutionary theory predicts that over time, selection should deplete variation in traits, yet variation in metabolic rates is ubiquitous. Variable selection regimes may maintain trait variation, by selecting for different metabolic rates across different environments, where a high or low pace-of-life is advantageous. While this theory is well established, field estimates of selection on metabolism across environments are historically rare.
To investigate the role of environmental variation in maintaining trait variation, we measured the metabolic rates of individual marine bryozoans, experimentally manipulated their competitive environment, and monitored their survival, reproduction, and pace-of-life in the subtidal.
We found that selection on metabolic rate varies among competition environments separated by only a few centimetres – competition selects for a faster pace-of-life, relative to competition-free environments. High-metabolism individuals are better able to withstand intense competition, however low-metabolism individuals live longer, and are likely to have higher fitness under competition-free conditions. Hence, the environment-dependent nature of selection on metabolism and the pace-of-life is likely to maintain variation in metabolic rates.