One of the primary determinants of organismal demands for energy and matter is body size. Two main theoretical frameworks have been proposed to explain and predict the role of body size on the use, storage, and transfer of energy and matter in ecological systems: Metabolic Scaling Theory (MST) and Ecological Stoichiometry Theory (EST). Theoretical and empirical work derived from MST suggests that the pools of matter, such as nitrogen (N) and phosphorus (P), exhibit predictable quarter-power law size scaling. The growth rate hypothesis, central to EST, predicts that P and N content in invertebrates should scale inversely with body size. In contrast, as vertebrates get larger they are required to invest more P into skeletons. However, universal elemental-body size relationships have been difficult to infer and remain contentious. Here we take a macroecological approach and ask if global-scale variation in organismal stoichiometry (C, N, P) of invertebrates and vertebrates is consistent with scaling relationships predicted from MST and EST principles. Using a global database on animal elemental stoichiometry we ask: (i) Does nutrient content scale with body size as predicted by MST and EST principles? (ii) What is the role of phylogenetic relatedness on body size scaling relationships?
Results/Conclusions
Our database takes a global scope, encompassing animal stoichiometry across a body size range of 10 orders of magnitude (4x10-8 to 190 grams) using data for 1941 species from 8 phyla from terrestrial, freshwater and marine habitats. As predicted from MST and EST the slope of phosphorus content scaled positively with body size in vertebrates, but contrary to our predictions phosphorus content was size-invariant in invertebrates. The slopes did not exhibit quarter-power scaling predicted by MST. Overall, body size explained a small fraction of the variance in nutrient content across individuals regardless their taxonomic affiliation. However, when we accounted for phylogenetic relatedness we found that body size explained a much greater variance in nutrient content (~80%). These results suggest that evolutionary history plays an important role in determining body size scaling- nutrient content relationships.