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

Abstract

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 of³H-PEG, sodium, and potassium were determined. Retrograde movement of urine into each gut segment was estimated from the quantity of³H-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.