Effects of water restriction on nitrogen metabolism and urea recycling in the macropodid marsupials Macropus eugenii (tammar wallaby) and Thylogale thetis (red-necked pademelon).

The effects of water restriction on nitrogen metabolism were compared in the semi-arid adapted tammar wallaby (Macropus eugenii) and a wallaby from a moist forest environment, the red-necked pademelon (Thylogale thetis). On a medium (9 to 13%) protein diet water restriction depressed dry matter and nitrogen intakes in both species. Nitrogen balance remained positive, but fell further in the pademelons. Urea excretion decreased in the tammars, but increased in the pademelons. Urea recycling as a proportion of urea entry rate tended to increase in the tammars but not in the pademelons. These findings suggest that T. thetis is not as well adapted as M. eugenii to coping with water shortages. In a second experiment water restriction depressed nitrogen balance in tammars on both high (15%) and low (5%) protein diets, but only on the latter diet did nitrogen balance become negative. Urea recycling was greater on the low than on the high protein diet, but was unaffected by water restriction. Although better able to withstand water stress, M. eugenii was unable to cope with both water stress and a low protein diet together. The reported ability of M. eugenii to maintain water intake by drinking sea water during the dry season when fresh water is unavailable and vegetation is of low protein content is thus of great ecological significance.