Macroevolution of Steep Interspecific Metabolic Allometry in an Old Insect Order

Endotherms exhibit metabolic allometric slopes of 0.75, which were long viewed as evolutionary constraints but which are increasingly questioned. In insects, allometric relationships between body size and metabolism are understudied. We used comparative phylogenetic methods to investigate macroevolution of metabolic allometry in Odonata (dragonflies and damselflies), an insect group with a unique ecology and dramatic macroevolution of body size. We found an interspecific allometric slope close to one (isometry). Metabolic rates evolved gradually with a weak pullback force and multiple shifts in metabolic optima, body size and flight behaviour. Cell size, nucleus size and body surface area did not explain this steep allometric slope. Instead, metabolic isometry likely reflects the high aerobic demands of flight and evolutionary increases in body size via cell numbers rather than cell size. Our results challenge traditional allometric theories and suggest that metabolic allometries can and do evolve, with far-reaching implications for insect size evolution.