Consequences of model specification for the determination of gut evacuation rates: redefining the linear model

The combination of gut contents and evacuation rate is an important tool to determine in situ feeding rates of many organisms. Traditionally used equations of gut evacuation models, however, have made a systematic comparison among models difficult. We changed the notation of the linear gut evacuation model to provide an algorithm compatible with the commonly used exponential model. Using examples from the literature, we demonstrate that prolonged retention of food after a true linear pattern of gut evacuation can be mistaken for an exponential pattern. In many of these examples, use of an exponential gut evacuation model causes overestimation of food consumption by approximately twofold. In situations where the entire gastrointestinal tract is examined instead of the stomach only, the time course of the gut content is best described by a plug-flow reactor for which the "input = output" rule applies. In contrast, the assumption of a "proportional release" must be met rigorously for the exponential model to be valid, which requires more complex feedback mechanisms than the simple plug-flow reactor. The new algorithm for the linear model is also consistent with the empirical observation that the gut evacuation rate is proportional to the initial gut content.