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 equa tions of gut evacuation models, however, have made a systematic comparison among models difficult. We changed the notation of the linear gut evacuatio n model to provide an algorithm compatible with the commonly used exponenti al model. Using examples from the literature, we demonstrate that prolonged retention of food after a true linear pattern of gut evacuation can be mis taken for an exponential pattern. In many of these examples, use of an expo nential gut evacuation model causes overestimation of food consumption by a pproximately twofold. In situations where the entire gastrointestinal tract is examined instead of the stomach only, the time course of the gut conten t is best described by a plug-flow reactor for which the "input = output" r ule applies. In contrast, the assumption of a "proportional release" must b e met rigorously for the exponential model to be valid, which requires more complex feedback mechanisms than the simple plug-flow reactor. The new alg orithm for the linear model is also consistent with the empirical observati on that the gut evacuation rate is proportional to the initial gut content.