No abstract

SUMMARYThree processes central to osmoregulation of marine birds were compared in three species of ducks that differ in habitat affinity, diet and saline tolerance. These processes are filtration of Na+ and water from the plasma by the kidneys, their reabsorption along the renal tubules, and secretion by the salt glands. Barrow's goldeneyes Bucephala islandica, the most marine species, have the highest rates for all three processes and only this species can secrete all the infused salt viathe salt glands. Rates of all three processes are lower in mallards Anas platyrhynchos, the most freshwater species. Following saline acclimation,mallards could excrete all the infused Na+ by a combined Na+ excretion of the kidneys and salt glands. Canvasbacks Aythya valisineria, despite being more saline tolerant than mallards,are unable to excrete all the infused Na+. They produce a large volume of urine (like mallards) that has a low [Na+] (like goldeneyes). Salt gland secretion Na+ concentration did not differ among the three species, but only goldeneyes secrete at a rate sufficient to eliminate all infused Na+via the salt glands. Differences in saline tolerance of these ducks species cannot be fully explained by differences in their filtration, reabsorption and secretion of Na+and water, suggesting that the intestinal tract plays an important role.

Ducks absorb imbibed Na+ and water in the anterior gut and reabsorb Na+ and water from urine refluxed into the hind gut. In Mallards (Anas platyrhynchos) this process is sexually disparate: males reflux and reabsorb more water, mainly in the ceca. We examined the effect of saline acclimation on the size of Mallard organs, especially the gut and other osmoregulatory organs (kidneys, salt glands) in both sexes. We tested and accept two hypotheses: (1) saline increases the mass of the Mallard hind gut and other osmoregulatory organs and (2) saline has a greater effect on the organs of males. Drinking saline did not affect the mass of body, kidney, heart, or liver, but increased the mass of the salt glands, ileum, and ceca. Increases in organ mass were greater in males than in females. Saline acclimation increased the length of the jejunum only in males and decreased the length of the esophagus and the length and mass of the proventriculus only in females. Our data suggest that the upper and lower gut segments may play somewhat different roles in ion and water transport in the two sexes.

The influence of sex and salt-water intake on retrograde movement of urine into the hindgut was examined in captive adult wild Mallards (Anas platyrhynchos). Ducks were given intravenous injections of tritiated polyethylene glycol (3H-PEG) and killed 3 h later. The fluid content of each gut segment and its concentrations of 3H-PEG, sodium, and potassium were determined. Retrograde movement of urine into each gut segment was estimated from the quantity of 3H-PEG recovered there. In both ducks that drank tap water and those that drank saline, about 20% of the urine was moved into the hindgut, two-thirds of this into the ceca. Males deposited more urine in the ceca and reabsorbed more water from this urine than did females (P < 0.01). We conclude that retrograde movement of urine into the hindgut was unaffected by the salinity of drinking water and was greater in males than in females. Reabsorption of urinary sodium and water in the hindgut, particularly the ceca, may play an important role in osmoregulation in birds with salt glands.

No abstract