Chemical Reactor Models of Digestion Modulation

In this paper we give an overview of the application of chemical reactor theory to models of digestion processes and indicate how those models extend to eco-physiological questions of modulation of digestion and feeding. In particular, we discuss processes that organisms may use to manage their nutrient intake, including optimality, ecological stoichiometry, and regulatory physiology. In the context of regulatory physiology we present a new, detailed model that forges an eco-physiological connection between animal foraging and digestion modulation. The discussion focuses on insect herbivores, in particular common grasshoppers and locusts. In presenting the highlights of reactor models we set up a mathematical model of preand post-ingestive changes that an insect may employ when faced with suboptimal foods. To achieve this post-ingestive change we employ a simple control mechanism that is dependent on the ratio of the concentration of two nutrients in the hemolymph. The control serves to limit the absorption of the nutrient that is occurring in excess, thus allowing the insect to avoid “jamming” the regulatory mechanisms that affect feeding. At the same time, pre-ingestive controls influence the inter-meal time period to allow insects to eat more frequently when feeding on suboptimal food. We examine the effect that the control mechanisms have on nutrient uptake by simulating feeding on two foods of different quality. Numerical simulations give results that are qualitatively congruent with empirical data.