Diurnal variation in plasma concentrations of testosterone, 5α-dihydrotestosterone,and corticosteroids in the Australian brush-tailed possum, Trichosurus vulpecula (Kerr)

Plasma concentrations of testosterone, 5α-dihydrotestosterone, and total adrenal corticosteroids were measured in intact and castrated males and female possums over a 24-h period. In males, testosterone levels varied diurnally, being significantly higher in the morning than in the evening, with an overall mean concentration of 3.30 ± 0.43 (SEM) ng/ml. Levels of 5α-dihydrotestosterone showed a similar variation but of lower magnitude and the overall mean plasma concentration was 0.63 ± 0.10 ng/ml. Total adrenal corticosteroid concentrations in males also appeared to cycle, but inversely to the androgens with a peak in the early evening, and the overall mean concentration was 0.61 ± 0.08 μg/100 ml. Testosterone concentrations averaged 0.46 ± 0.02 and 0.35 ± 0.01 ng/ml in castrated males and females, respectively, and the corresponding values for 5α-dihydrotestosterone were 0.20 ± 0.03 and 0.21 ± 0.03 ng/ml. These measurements indicate that plasma testosterone levels of the marsupial possum fall within the range reported for eutherian species.