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

Abstract

Many idle insects exhibit discontinuous gas exchange cycles (DCGs). During DCGs, CO2 is released in discrete bursts, followed by periods of negligible gas exchange. The standard metabolic rate (SMR) is thus determined to first order by the product between cycle frequency (fc) and burst volume (Vb, SMR {approx} fc{middle dot} Vb). The evolutionary allometry of these parameters is well studied, but it remains unclear if their static allometry, measured in individuals of the same species, sharing the same ontogenetic stage, follows the same patterns. To address this question, we investigate the static allometry of DCGs in Atta cephalotes leaf-cutter ants workers varying by two orders of magnitude in body mass. The SMR allometry significantly exceeded the standard prediction from the nutrient supply network model, and differed from the SMR allometry observed across insects. This disproportional increase was exclusively achieved by an increase in Vb, perhaps because fc is stabilised by neural and mechanical constraints. It may be necessitated by the positive allometry of the largest muscle in Atta--the mandible closer muscle--which increases with a virtually identical allometric coefficient, providing further evidence that the principles of symmorphosis may be upheld in insects.