Function of the lower intestine and osmoregulation in the ostrich: preliminary anatomical and physiological...

The gross anatomy of the lower intestine of the African ostrich (Struthio camelus) was investigated in four adult birds shot in the wild. The presence of 80 cm long paired caeca, and approximately 10 m of colon between ileum and the cloaca was confirmed. Urine, but not faeces, was found in the coprodeum. Retrograde flow into the colon was not observed. Samples of contents from the lower intestine were secured from these birds and the water content, osmolality and concentrations in the supernatant of short-chain fatty acids (SCFA), acetate, Na, K and Cl and pH were measured. In the caeca and the orad wide part (2-3 m) of the colon an avid production of SCFA takes place since the concentration of SCFA reached around 200 mM in these segments. As judged from a pronounced fall in the concentrations of Na and SCFA along the length of the colon, these ions are absorbed together with water. The water content falls from 92 to 67% (i.e. from 11.5 to 2.0 ml H2O/g dry matter). The mid-gut fermentation of carbohydrate is in agreement with the observation that the birds were feeding exclusively on Euphorbia heterochroma. This succulent plant (water content 87%) apparently makes the birds independent of surface water. Laboratory studies were performed on two captive male chicks. Hyperosmotic NaCl loading was unable to activate the nasal glands to secretion. Their ducts passed directly from the frontal/lacrimal bone to the nasal cavity. Dehydration confirmed a maximal osmolality of the cloacal urine of around 800 mosmol/l, the osmotic urine to plasma ratio being 2.5. Feeding of either a low- or a high-NaCl diet did not affect the transepithelial electrical potential difference of the coprodeal wall. It remained less than 5 mV, lumen negative.