Organismal size, metabolism and the evolution of complexity in metazoans

Questions: What is the macroevolutionary relationship between body size, number of cell types and metabolism? Furthermore, why does the relationship between body size and the number of cell types hold between major metazoan clades but not within closely related taxa? Mathematical methods: Expand the allometric relationship between size and metabolism to include (1) the energetic costs of supporting an increased number of cell types and (2) the phylogenetic constraints governing the number of cell types. Key assumptions: An increase in organismal size selects for additional cell types. This is due to biophysical constraints and transport demands. The increase in cell types allows the organism to perform new functions. The extra cell types also require more intercellular networks. Therefore, the amount of energy required per unit of body mass should increase with the number of cell types. Phylogeny may also constrain the number of cell types within taxa. This constraint will limit the number of cell types to be approximately constant within a bauplan (a unique organismal form comprised of an anatomical and physiological design). Predictions: Organismal size should be positively correlated to the number of cell types across metazoan taxa. However, this relationship will not hold within clades due to energetic and phylogenetic constraints. The energetic constraint leads to a positive correlation between the number of cell types and metabolic intensity (the mass-specific rate of energy processing standardized to a given body size) across metazoan bauplans. Available data support these predictions. Metabolic intensity is positively related to the number of cell types in metazoan clades.