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Abstract

Foregut fermentation in the Hoatzin (Opisthocomus hoazin) appears to be a unique adaptation among birds. Passage rates of liquid and solid digesta were measured in five Hoatzins. Four markers were given to captive birds: Cr-EDTA (a liquid marker), ytterbium (Yb) oxide mordanted on plant fiber particles of 1 mm2 (solid marker), and 1 mm2 and 4 mm2 plastic particles (solid markers). The markers were given orally as a single pulse dose. Hoatzins were fed ad libitum and housed in metabolic cages with removable floor trays. Excreta were sequentially collected 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 mm2 (7.5 hr ± 1.0) or 4 mm2 (10.7 hr ±4.6). Mean retention times were significantly shorter for the liquid marker (18.3 hr ± 3.3) than for Yb (25.1 hr ± 2.8), 1 mm2 (33.6 hr ± 11.3) or 4 mm2 (45.7 hr ± 7.8). Mean retention times of the Yb particles were significantly shorter than for the 4 mm2 plastic marker. Hoatzin passage rates are among 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 utilization of both plant cell contents and cell walls. This digestive strategy contrasts with that of other herbivorous birds, in which fast passage rates maximize the rate of assimilation from cell content at the expense of little cell wall digestion.

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© The Cooper Ornithological Society 1995
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