Water and Electrolyte Metabolism of the Tammar Wallaby Macropus eugenii

Abstract

Seasonal changes in body condition and water and electrolyte metabolism in two populations of tammar wallabies (Macropus eugenii) are described in this paper. One population lives in a beach environment and has no access to freshwater, whereas the second population lives in the vicinity of a continuously flowing freshwater bore. During normal (mild) summers, the availability of free water has no influence on the condition of the tammars or on their water and electrolyte metabolism. Only during periods of extended summer drought is free water marginally advantageous to the bore population, as indicated by lower circulating antidiuretic hormone levels (ADH), a higher condition index, and lower total body water content. Both populations showed low plasma sodium concentrations after a prolonged drought, indicating that tammars are exceptionally resistant to the effects of hyponatraemia. After the onset of the winter rains, plasma sodium concentrations increase and the water turnover rises dramatically. The high water content of the vegetation at this time appears to prevent the tammars from ingesting enough food to satisfy their nutritional requirements, and their condition continues to decline, reaching its annual minimum in midwinter. It is evident that the breeding season of the tammars on Garden Island, which falls in early summer, is attuned to changes in condition and represents the only period of the year when the animals are neither water deprived nor suffering from nutritional deficiencies.