The allometry of discontinuous gas exchange cycles in Atta cephalotes leaf-cutter ants

Abstract The variation of metabolic rate with body size is a fundamental aspect of animal biology, yet the extent to which metabolic scaling arises from interactions between functional demand and physiological constraints remains debated. Many idle insects exhibit discontinuous gas exchange cycles (DGCs), in which CO2 is released in periodic bursts separated by intervals of minimal gas exchange. Standard metabolic rate (SMR) is then determined to first order by the product of cycle frequency (fc) and burst volume (Vb; SMR ≈ fc·Vb). Variation in these respiratory parameters may provide insight into the mechanistic basis of metabolic scaling. Here we investigate DGCs in adult workers of Atta cephalotes leaf-cutter ants spanning nearly two orders of magnitude in body mass and exhibiting strong positive allometry of the mandible closer muscles, potentially imposing increasing metabolic demand. The SMR scaling exponent significantly exceeded the prediction of the nutrient supply network model, and differed from exponents observed across insects. This disproportionate increase was driven entirely by an increase in Vb, perhaps because fc is stabilised by neural and mechanical constraints. Notably, Vb scaled with an exponent indistinguishable from that of the mandible closer muscle volume, providing further evidence that the principles of symmorphosis may be upheld in insects.