PASSAGE RATES OF DIGESTA MARKERS IN THE GUT OF THE HOATZIN, A FOLIVOROUS BIRD WITH FOREGUT FERMENTATION

Foregut fermentation in the Hoatzin (Opisthocomus hoazin) appears to b e a unique adaptation among birds. Passage rates of liquid and solid d igesta were measured in five Hoatzins. Four markers were given to capt ive birds: Cr-EDTA (a liquid marker), ytterbium (Yb) oxide mordanted o n plant fiber particles of 1 mm(2) (solid marker), and 1 mm(2) and 4 m m(2) plastic particles (solid markers). The markers were given orally as a single pulse dose. Hoatzins were fed ad libitum and housed in met abolic cages with removable floor trays. Excreta were sequentially col lected for 4-5 days. Plastic markers were visually counted and Cr and Yb were measured by atomic absorption spectrometry. Transit times were significantly shorter for the liquid marker (2.6 hr +/- 0.5) than for solid particles. Transit lines of solid particles were significantly longer for the largest particles: Yb (4.5 hr +/- 3.0), 1 mm(2) (7.5 hr +/- 1.0) or 4 mm(2) (10.7 hr +/- 4.6). Mean retention times were sign ificantly shorter for the liquid marker (18.3 hr +/- 3.3) than for Yb (25.1 hr +/- 2.8), 1 mm(2) (33.6 hr +/- 11.3) or 4 mm(2) (45.7 hr +/- 7.8). Mean retention times of the Yb particles were significantly shor ter than for the 4 mm(2) plastic marker. Hoatzin passage rates are amo ng the longest recorded for a bird; they are similar to mean retention times found in some large foregut fermenting mammals. Long retention times, efficient separation of solid and liquid digesta and selective particle size retention probably maximize energy and nutrient utilizat ion of both plant cell contents and cell walls. This digestive strateg y contrasts with that of other herbivorous birds, in which fast passag e rates maximize the rate of assimilation from cell content at the exp ense of little cell wall digestion.