Ratites differ distinctively in the anatomy of their digestive tracts. For example, Common Ostriches (Struthio camelus, hereafter Ostriches) have a particularly long, voluminous colon and long, paired caeca; Rheas (Rhea spp.) are characterized by a short colon with particularly prominent paired caeca; and Emus (Dromaius novaehollandiae) have neither prominent caeca nor a prominent colon. We tested whether digesta excretion patterns corresponded to these differences in anatomy, expecting Ostriches to have the longest and Emus the shortest digesta retention times, and Rheas possibly showing a selective retention of fluids observed in other birds and mammals with prominent caeca. We used 6 Ostriches (97–123 kg), 5 Greater Rheas (R. americana, 22–27 kg), and 2 Emus (32–34 kg) fed a common diet of alfalfa pellets ad libitum in captivity. Intake per unit of metabolic body mass did not differ between Ostriches and Greater Rheas but was significantly higher in Emus, which also displayed higher defecation frequencies and lower fiber digestibility. Mean digesta retention time for small fiber particles (2 mm) differed significantly among species (Ostrich: 30–36 h; Greater Rhea: 7–19 h; Emu: 1.3–1.8 h), but there were no differences between the retention of 2 mm or 8 mm particles or a solute marker within species. The shape of the marker excretion curves corresponded to digesta mixing in the digestive tract of Ostriches and Greater Rheas but not Emus. The calculated dry matter gut fill (% of body mass) was significantly higher in Ostriches (1.6–1.8) than Greater Rheas (0.3–1.0) and Emus (0.2). Ostriches had the highest and Emus the lowest fecal dry matter concentration. These physiological findings match the differences in digestive anatomy and support the concept that in ratites, herbivory—and hence flightlessness—evolved repeatedly in different ways.
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Vol. 132 • No. 1