|Ph.D Student||Tsahar Ella|
|Subject||Protein Balance in Birds: Physiological Mechanisms and|
|Department||Department of Biology||Supervisors||Professor Emeritus Arad Zeev|
|Professor Ido Izhaki|
|Dr. Carlos Martinez Del Ri|
|Full Thesis text|
This study compared the nitrogen requirements of nectarivorous and frugivorous birds with those of omnivorous birds, and explored physiological mechanisms that enable these birds to subsist on a low protein diet. An allometric comparison of the minimal nitrogen requirements (MNR) and total endogenous nitrogen loss (TENL) was performed between frugivorous/nectarivorous birds and omnivorous birds. Both parameters were found to be around four times higher in the omnivorous birds. The hypothesis that the low MNR and TENL of nectar-feeding birds are the consequence of their sugary-watery diet was tested by feeding an omnivorous species, the European starling, a nectar-like diet. Contrary to the hypothesis, the starling's nitrogen requirements on this diet were not different from what is allometrically expected for an omnivorous species. It suggests that the diet itself cannot fully explain the low nitrogen requirements of nectarivorous birds. The effect of protein and water intake on the nitrogenous waste composition of two frugivorous species, the yellow-vented bulbul and Tristram's grackle, was tested. Their MNR and TENL were similar to other fruit-eating birds. The grackles were uricotelic and the chemical composition of their nitrogenous waste was relatively independent of water and protein intake. The bulbuls were "apparently ammonotelic". Their ammonotely was related to the low protein intake and high water flux, and resulted from post-renal urine modification. Two mechanisms are suggested for the post-renal urine modification: bacterial catabolism of uric acid (and indeed, bacteria species with uricase activity were identified in the bulbul's lower gut) and reabsorption of uric acid in the hindgut. As uric acid functions as an antioxidant, birds might benefit from its reabsorption.
Birds do not possess the enzyme uricase that oxidizes uric acid to allantoin. Hence, the presence of allantoin in their plasma should have resulted from non-enzymatic oxidation. The concentrations of allantoin and uric acid in the plasma and urine of white-crowned sparrows were measured at rest, after 30 min of exercise, and after 1 h of recovery. The plasma allantoin concentration and the allantoin-to-uric acid ratio did not increase during exercise. However, a positive relationship was found between the concentrations of allantoin and uric acid in the plasma. The slope of this correlation was significantly higher after exercising compared with the other phases. I suggest that the slope of the correlation reflects the rate of uric acid oxidation, and that during activity this rate increased as a result of higher production of free radicals.