Revista Chilena de Historia Natural 80 (4): 447-454, 2007
RESEARCH ARTICLE
Osmoregulatory responses to dietary protein and water intake in the granivorous
Zonotrichia capensis (Passerine, Emberizidae)
We studied the osmoregulatory responses of the granivore Zonotrichia
capensis, acclimated to high- and lowprotein diets as well as two levels of water intake. We tested whether Z. capensis
has the ability to modify the proportion of nitrogenous waste in the excreta when protein intake and water intake varies. After 21
days of acclimation, plasma osmolality was not significantly affected by dietary treatment; however, excreta osmolality was higher in
the high-protein group compared to the low-protein group. Nitrogenous wastes in Z. capensis are excreted
mainly in the form of uric acid. The proportion of kidney devoted to medullary tissue was 40 % higher in dehydrated birds than in
hydrated birds. Excreta osmolality was higher in dehydrated birds, and in all cases higher than plasma concentration by more than 300
mOsm kg<sup>-1</sup>. Our data do not support the hypothesis that Z. capensis can switch nitrogen excretion
pathways. We hypothesize that the low water content of the seed-base diet, the comparatively low water intake, and the large
difference between urine and plasma concentrations may minimize the retrograde flux of urine to the lower intestinal tract, thereby
reducing the potential for post-renal urine modification.
Key words:
granivory, kidney,
nitrogenous waste, osmoregulation, passerines
