Figure 7: The complexity of vertebrates, expressed in terms of Φm, is shown here rising in order to highlight some of the increasingly intricate structures and functions for a variety of animals at various stages of biological evolution. Note how endotherms (including mammals and birds) have higher energy rate densities than ectotherms (including invertebrates as well as lower vertebrates such as fish, amphibians, and reptiles) among all taxonomic groups found on Earth.