Diego R. Barneche, Miki Jahn, Frank Seebacher
Growth is a fundamental biological process in ecology and evolution. From the individual’s point of view, larger bodies may establish dominance or achieve better reproductive capacity. At the ecosystem level, growth rates also determine how much energy (as mass) can move from prey to predators, so it directly limits the amount of energy that can move from photosynthesis in plants all the way to the top of the food web. Surprisingly, we still understand little about how much it actually costs to grow in animals. We know that the energy that fuels growth comes mostly from cellular respiration. Most studies assume that the energy that is necessary to fuel growth is fixed for any given species, and that it does not vary with changes in the environment. In our study, we tested this assumption by directly measuring how much energy zebrafish need to grow as they age at different test temperatures (20˚C, 26˚C, 29˚C, 32˚C). We found that the cost of growth was significantly higher at earlier life stages, and that it increased twofold between 20˚C and 32˚C. Higher temperature also decreased the size of adults between 26˚C and 32˚C. To the best of our knowledge, these findings are the first to characterise how the cost of growth varies with time and across temperatures in a model vertebrate species. Our study indicates that warming waters will change the distribution of biomass in ecosystems because they drive a decline in the average size of individuals, and because higher costs of growth imply inefficient energy transfer between trophic levels.