Abstract/Summary

Ostriches (Struthio camelus) achieve digesta retention times, digesta particle size reduction and digestibilities equal to similar-sized herbivorousmammals, in contrast to some other avian herbivores. The sequence of digestive processes in their gastrointestinal tract, however, is still unexplored. Using two groups of four ostriches (mean body mass 75.1±17.3 kg) kept on fresh alfalfa, we tested the effect of two intake levels (17 and 42 g dry matter kg−0.75 d−1) on the mean retention time (MRT) of a solute and three different-sized (2, 10, 20 mm) particle markers, mean faecal particle size (MPS), and digestibility. Intake level did not affect MRT, but MPS (0.74 vs. 1.52 mm) and dry matter digestibility (81 vs. 78%). The solute marker (MRT 22–26 h) was excreted faster than the particlemarkers; there was no difference in the MRT of 10 and 20 mmparticles (MRT 28–32 h), but 2 mmparticles were retained longer (MRT 39–40 h). Because the solutemarker was not selectively retained, and wet‐sieving of gut contents of slaughtered animals did not indicate smaller particles in the caeca, the long MRT of small particles is interpreted as intermittent excretion from the gizzard, potentially due to entrapment in small grit. Themarker excretion pattern also showed intermittent peaks for allmarkers in five of the animals, which indicates mnon-continuous outflow from the gizzard.When adding our data to literature data on avian herbivores, a dichotomy is evident, with ostrich and hoatzin (Opisthocomus hoazin) displaying longMRTs, high digestibilities, and gut capacities similar tomammalian herbivores, and other avian herbivores such as grouse, geese or emuswith shorter MRTs, lower fibre digestibilities and lower gut capacities. In the available data for all avian herbivores where food intake and MRTs were measured, this dichotomy and food intake level, but not body mass, was related to MRT, adding to the evidence that body mass itself may not be sole major determinant of digestive physiology. The most striking difference between mammalian and avian herbivores from the literature is the fundamentally lower methane production measured in the very few studies in birds including ostriches, which appears to be at the level of reptiles, in spite of general food intake levels of amagnitude as inmammals. Further studies in ostriches and other avian herbivores are required to understand the differences in digestivemechanisms between avian and mammalian herbivores.