For decades, it has been regarded in biology as a natural law that there is a fixed relationship between body size and metabolic rate across all animal groups. According to this classical theory, an animal that weighs twice as much as a smaller animal does not have twice the metabolic rate. Instead, its metabolic rate increases proportionally more slowly. As a result, small animals consume more energy relative to their size than large animals.
However, in a newly published study, researchers from, among other institutions, Lund University have investigated how well this established theory holds true for insects.
“Our new results challenge this classical theory and show that it does not apply to all organisms,” says Erik Svensson, Professor at Lund University.
Dragonflies and damselflies included in the study
Erik Svensson and his colleagues focused on the charismatic insect group comprising dragonflies and damselflies. The researchers examined how variation in metabolic rate is explained by variation in body size among these insects, and how the relationship has evolved over more than 200 million years of evolution.
The results show that metabolic rate increases almost entirely in proportion to body size. In dragonflies and damselflies, metabolism increases by nearly 100 per cent rather than the typical 67–75 per cent predicted by the classical theory.
“The study suggests that the relationship between body size and metabolism is not a fixed evolutionary constraint shared by all animal groups. Instead, this relationship has been modified over the course of evolution,” says Erik Svensson.
Aiming to improve scientific models
What is the value of knowing whether metabolic rate increases more slowly than body size, or at the same rate, and that the relationship varies between different animal groups?
Erik Svensson points out that because metabolism reflects how organisms use energy, the study increases our understanding of how species may respond to environmental change. Such changes may include increases or decreases in food availability, or shifts in temperature in an increasingly variable climate.
“This knowledge enhances our understanding of how energy use changes and co-varies with animal body size and life-history traits over the course of evolution. We hope that our new findings will help to develop and improve models in both ecology and climate science,” says Erik Svensson.