Osmotic adaptation of the emu (Dromaius novaehollandiae)

Summary

• 1 The emu (Dromaius novaehollandiae) is well adapted to the Australian semi-desert, although it has a limited renal concentrating ability. One problem is the balance between the ability to resorb ions and water from ureteral urine during its reflux into the coprodeum and rectum (colon), and the rate of ureteral inflow. To elucidate this problem the osmotic and nutritional state of wild birds needs to be studied in order to integrate laboratory and field investigations.

• 2. This paper reports plasma values and the composition of gut contents from birds shot in the arid interior during the hot, dry summer. The wild birds showed little sign of osmotic stress with only marginally elevated plasma osmolality, and near-isotonic contents of the coprodeal/rectal segment (332±6 mOsm). Intestinal contents showed that the birds were feeding on berries of bushes with either a high (Rhagodia) or a low (Santalum) content of NaCl. The coprodeal/rectal concentration of Na⁺ and Cl^- were high enough to permit solute-linked water flow. There was little sign of fermentation.

• 3. In vitro investigation in the Ussing chamber of isolated coprodeum and rectum epithelium revealed an inwardly directed current which was amiloride inhibitable and amino acid independent, suggesting electrogenic Na absorption of around 4 μmol·h^-1·cm^-2 mucosal area regardless of the NaCl intake.

• 4. Based on previously determined transport parameters of the coprodeum/rectum epithelium (Dawson et al. 1985) and ureteral excretion rates of water and electrolytes (Dawson et al. 1991), it is concluded that the lower gut can resorb the major part of the water coming from the kidneys (and ileum) on both low and high NaCl intakes. The lower gut is most important in the xeric adaptation. The reason for the apparently high transport capacity, larger than in other birds, of the lower gut (serosal area) was a high degree of folding of the mucosal surface (increasing surface area by factor of five).