A SIMPLE MATHEMATICAL-ANALYSIS OF THE LIMITATIONS TO INFERRING FEEDING-BEHAVIOR OF ZOOPLANKTON FROM GUT CONTENT

The instantaneous amount of food in an animal's gut, or gut content (G), is a function of its ingestion rate (I) minus its gut evacuation rate (E). We present simple models describing the short-term (minutes to hours) dynamics of G, incorporating I and E simultaneously. The models apply to situations in which animals start feeding after a period of starvation. Two possibilities are considered: (1) Both I and K (the coefficient of gut evacuation) remain constant during the feeding period. (2) K remains constant, but I decreases, after satiation, from the initial value (I(o)) to an equilibrium value (I(e)). Results from the models show that although gut content is usually asymptotic with time, the shapes of the curves vary considerably depending on the relationship between I(o) and K in the first instance, and I(o), I(e) and K in the second instance. The relationship between the time of first defecation (t1) and the gut passage time (K-1) is critical in these models. Results from the models are compared to data from published experiments in which gut content of copepods was monitored at intervals of minutes to hours. We conclude that short-term variability in feeding behavior may not always be inferred solely from changes in gut content with time.