Mechanical determinants of nectar-feeding energetics in butterflies: muscle mechanics, feeding geometry, and functional equivalence.

We develop a mechanistic model for nectar feeding in butterflies that integrates the two basic components of the feeding process: the fluid dynamics of nectar flow through the food canal and the contractile mechanics of the muscular, cibarial pump. We use the model to predict the relation between rate of energy intake during feeding and nectar concentration. We then identify nectar concentations that maximize energy intake rates (the optimal concentrations). We illustrate the model using measurements of the food canal and cibarium of Pieris butterflies. The model predicts an overall optimal range of nectar concentration of 31-39% sucrose for butterflies, which is in agreement with previously reported laboratory values. The model also predicts an interaction among the geometries of the food canal, the cibarial cavity, and the cibarial muscles, that allows us to identify the combinations of food canal, cibarium, and muscle dimensions that yield the highest rates of energy intake. Nectar-feeding is "functionally equivalent" in butterflies and hummingbirds: two physically different feeding mechanisms can yield identical energy intake rates. This equivalence results from a mathematical and physical similarity between quasi-steady-state fluid flow in hummingbrid tongues and the force-velocity characteristics of contracting cibarial muscle in butterflies.