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Anatomy & physiology

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

Cleva, GM, Stone, GM & Dickens, RK 1994, Reproduction, Fertility and Development, vol. 6, pp. 713-719.

The concentration of plasma androgens was measured in six male koalas of sexually mature age (3-10 years) over a period of 23 months. Four of these koalas were situated away from females, while two were housed with females. The concentration of androgens appeared to change seasonally, with the lowest levels observed mid-year, and increased steadily coming into the beginning of breeding season. Males living in female presence had higher androgen concentrations, even without females being in oestrus. These androgen concentration changes did not correlate with any changes in volume of the testicles.

  Blood was collected through cephalic vein venipuncture on restrained koalas with testis volume predicted by measurements of the right testis using Vernier calipers. Concentration of androgens was then measured via radioimmunoassay cortisol measurement. Further examination of the plasma samples using high pressure liquid chromatography revealed the major androgen present to be testosterone with minimal amounts of 5a-dihydrotestosterone. Due to the fluctuating nature of the plasma androgen levels during the day, a diurnal rhythm of secretion was suggested. An additional two animals then had vascular access ports attached for more regular blood sampling which was collected every alternate day over 2 months. The concentration of androgens was found to range from 0 to 20 nmol L-1, which indicates episodic secretion.  Cortisol concentrations (a measure of stress) remained low with the exceptions of instances where animals underwent surgery or were relocated to alternate enclosures. No association was found between androgen and cortisol concentrations in the plasma.

  The lowest testosterone concentrations were found at 0 to 2.5 nmol L-1 in the non-breeding season and peaks at 23 nmol L-1 in August to September. This mirrors previous findings for marsupials.  It has been postulated that the early increase in these androgen concentrations may have an important role in preparing the reproductive system for mating. Marsupials often produce of sperm throughout the year with testicular regression being rare. Other authors have suggested that the higher androgen concentration may be necessary in the stimulation of the accessory glands, yet decreased concentrations promote the support of libido and production of sperm. Having female koalas present appeared to increase the concentrations of the plasma androgens but the overall pattern of change remained the same regardless of whether females were present. The large sample variation demonstrates the inherent ability for short-term changes in reproductive parameters, suggesting that androgen secretion acts episodically. This episodic nature has been detected in many other species and may be attributed to stress from handling.

  The present study demonstrates that male koalas, like many other marsupial species, have increased androgen concentrations during and prior to the breeding season. Future studies should measure the concentration of androgens as well as luteinising hormone in the plasma across a 24-hour period to further investigate the possible episodic and diurnal mechanism.

 

Summarised by Alexander Murdoch

 

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