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Concentrations of corticosteroids and aldosterone have been measured in the plasma and related to the maximum cortisol binding capacity, to the concentration of sodium and potassium of the plasma and to the condition of the animals, in a natural population of the quokka (Setonix brachyurus, Quoy & Gaimard) on Rottnest Island, Western Australia. A marked population in one area has been repetitively sampled at approximately 6 week intervals over a period of 2 years. Animals were found to be in significantly better condition during the hot–dry season. Their plasma concentrations of corticosteroids, 1·09 ± 0·11 (S.E.M.) and 1·47 ± 0·14 µg/100 ml and their plasma sodium concentrations, 147 ± 0·87 and 142 ± 1·17 mequiv./l, for the hot–dry and cold–wet seasons respectively, were both significantly different (P < 0·05). Plasma concentrations of aldosterone were 8·40 ± 0·57 and 7·06 ± 0·53 ng/100 ml for the hot–dry and cold–wet seasons. The level of aldosterone fell to levels comparable to those found in salt-replete laboratory animals only during the month of July.

In the laboratory, salt deprivation resulted in a significant rise in the concentration of plasma aldosterone, 2·33 ± 0·57 (S.E.M.) ng/100 ml when salt-replete and 16·95 ± 2·74 ng/ 100 ml when salt-deprived (P< 0·001). Total plasma corticosteroid levels were unaffected by changes in the sodium content of the diet.

Injections of ACTH at a rate of 2 i.u./kg caused a significant rise in the levels of plasma corticosteroids which were essentially identical in both high- and low-condition animals. Body temperature fell to a significantly lower level in low-condition animals held in cages after injection of ACTH. These results indicate that the seasonal mortality of the quokka on Rottnest Island does not result from any breakdown in adrenal function, despite a substantial decline in the condition of the animals.