Simple Summary
Why do a tiny insect and a large fish have such different energy needs? Scientists use a framework called the Metabolic Theory of Ecology to predict how an animal’s energy use relates to its body size and the temperature of its environment. A key question is whether these predictions work the same way for individuals within a species as they do for comparisons between different species. We analyzed metabolic rate data from 174 species, using statistical approaches that account for species differences and evolutionary relationships. We found that the effect of body size is highly variable within species but converges toward a consistent scaling pattern across species. In contrast, the influence of temperature is strong within a species, but appears weaker when comparing different species. This shows that the “rules” for energy use depend on the scale of observation. Our findings will help build better models to predict how species, from the smallest insects to the largest fish, will respond to environmental changes like global warming.
Abstract
The Metabolic Theory of Ecology (MTE) proposes universal constants for the scaling of metabolic rate (BMR) with body mass and temperature. However, the validity of these constants across different biological hierarchies, specifically within versus between species, remains debated. Using a comprehensive dataset of 3767 metabolic measurements across 174 species, we tested for systematic differences in scaling relationships. We employed a unified linear mixed-effects model to estimate intraspecific parameters and phylogenetic generalized least squares (PGLS) regression for interspecific comparisons. Our results reveal a decoupling of scaling parameters across hierarchical levels. The overall intraspecific mass-scaling exponent (b = 0.760 ± 0.012, fixed effect ± SE) was not significantly different from the phylogenetically corrected interspecific exponent (0.768 ± 0.023). In contrast, the overall intraspecific activation energy (E = 0.601 ± 0.016 eV) was significantly higher than the attenuated interspecific value (0.403 ± 0.073 eV). Taxonomic variation was prominent for mass-scaling, with fish exhibiting a significantly higher exponent than most other groups, whereas activation energy did not differ significantly among groups. We conclude that while the mass-scaling relationship converges through interspecific averaging, the sensitivity of metabolism to temperature is robust within species but becomes diluted in broad-scale comparisons. This demonstrates that metabolic scaling is inherently hierarchical, necessitating scale-explicit models rather than the pursuit of universal constants.