Variation in reproductive parameters in the captive male koala (Phascolarctos cinereus).

Plasma androgen concentration was measured in six mature male koalas over 23 months. During this time, four animals were housed isolated from females. Androgen concentration showed a seasonal change, with a nadir about the middle of the year, and began to increase sometime before the start of the breeding season. The concentration was greater when males were housed with females, even when the females were not yet cycling themselves. Changes in androgen concentration were not associated with changes in testicular volume. When plasma extracts were examined following separation by high pressure liquid chromatography, the major circulating androgen was testosterone with only trace amounts of 5 alpha-dihydrotestosterone present. Plasma androgen fluctuated during the day suggesting some form of diurnal rhythm of secretion. Two further animals were fitted with vascular access ports to allow more frequent blood collection and samples were taken every second day for two months, a period that included a major portion of the breeding season. Androgen concentration fluctuated widely from 0 nmol L-1 to 20 nmol L-1 over successive sampling and indicating an episodic pattern of secretion. Plasma cortisol concentration was measured in all animals in these studies to monitor stress and remained low except when animals were undergoing surgery, were being moved to different enclosures or were exhibiting reproductive behaviours. Overall, there was no significant correlation between the concentration of androgen and cortisol in plasma (n = 215). This contrasted with a group of wild-bled animals (n = 4) where there was a highly significant negative correlation.