Dj. Levey et Cm. Del Rio, Test, rejection, and reformulation of a chemical reactor-based model of gut function in a fruit-eating bird, PHYSIOL B Z, 72(3), 1999, pp. 369-383
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.