Test, rejection, and reformulation of a chemical reactor-based model of gut function in a fruit-eating bird

We explored modulation of retention time in cedar waxwings (Bombycilla cedr orum) by feeding them diets varying in hexose concentration. Our goals were to (1) test three predictions of a chemical reactor-based model of how gut s might respond optimally to diet shifts; (2) determine whether modulation of retention time can occur quickly, thereby facilitating rapid changes in diet; (3) tease apart the relative influence of ingestion rate and nutrient concentration on retention time; and (4) examine the degree of axial mixin g in the intestine and its relationship with retention time. The model's pr edictions were rejected: mean retention time did not decrease, ingestion ra te did not increase, and glucose assimilation efficiency did not decrease w ith increased hexose concentration of the diet. Instead, birds displayed ma ximal intake rate at intermediate sugar concentration, and mouth to cloaca mean retention times increased with hexose concentration. Significant modul ation of retention time occurred quickly, within 3 h of exposure to a diffe rent diet. Birds did equally well in terms of total energy assimilated on d iets differing 3.3-fold in hexose concentration (from 500 mmol/L to 1660 mm ol/L) but showed reduced intake when fed food with low hexose concentration (110 mmol/L). Far more variation in retention time was explained by direct effects of ingestion rate than by direct effects of hexose concentration. Finally, a gut dispersion index that measured degree of axial mixing was po sitively correlated with mean retention time, indicating that higher retent ion times are accompanied by increased axial mixing. We propose a modificat ion of the assumptions of the original model. The resulting "osmotic constr aint" model better captures the interaction between feeding rate and digest ive function in fruit-eating birds.