Retrograde movement of urine into the gut of salt water acclimated Mallards (Anas platyrhynchos)

The influence of sex and salt-water intake on retrograde movement of urine into the hindgut was examined in captive adult wild Mallards (Anas platyrhy nchos). Ducks were given intravenous injections of tritiated polyethylene g lycol (H-3-PEG) and killed 3 h later. The fluid content of each gut segment and its concentrations of H-3-PEG, sodium, and potassium were determined. Retrograde movement of urine into each gut segment was estimated from the q uantity of H-3-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 a nd reabsorbed more water from this urine than did females (P < 0.01). We co nclude that retrograde movement of urine into the hindgut was unaffected by the salinity of drinking water and was greater in males than in females. R eabsorption of urinary sodium and water in the hindgut, particularly the ce ca, may play an important role in osmoregulation in birds with salt: glands .