Sexual maturity, factors affecting the breeding season and breeding in consecutive seasons in populations of overabundant Victorian koalas (Phascolarctos cinereus)
McLean, N & Handasyde, KA 2006, Australian Journal of Zoology, vol. 52, pp. 385-392.
At sexual maturity, female koalas in Victorian koala populations had an average age of 24.4 months, head length of 125 mm and body mass of 6.6 kg. The length and timing of the breeding season differed significantly between the island populations and inland populations, with those in island populations commencing earlier and lasting longer than those in inland populations. More male offspring were born earlier in the breeding season than females, and Chlamydia appeared to affect the frequency of breeding in consecutive seasons.
A majority of female koalas in the Victorian populations studied here reached sexual maturity at 2 years of age, with little variation observed around the age at sexual maturity. Similarly, most females had a mean mass of 6.6 kg at sexual maturity, with females producing young at a body mass of less than 6 kg experiencing reduced success in raising their young to independence. These findings agree with existing literature that a threshold body mass exists before female koalas can successfully reproduce. This limited ability to advance the timing of sexual maturation reflects a possible compromise between survival and reproductive success if mating at an earlier age. Breeding seasons at Snake and French Islands were significantly longer and commenced earlier than at Brisbane Ranges, Framlingham and Mt Eccles. This difference between island and inland populations may be attributed to differences in environmental factors. Island sites generally have milder climatic conditions than inland sites, which allow for better quality resources and more energy to be allocated to reproductive activities than to thermoregulation. This study observed a higher number of male offspring produced earlier in the breeding season than female offspring. This can be explained by the koala’s sexual dimorphism life-history strategy, as producing male offspring earlier in the season allows for a greater resource and maternal investment to support the development larger and more competitive male offspring. The authors of the present study also noticed that Chlamydia-infected female koalas failed to reproduce in consecutive years regardless of food abundance and quality, suggesting the disease has severe impacts on fecundity in koala populations.
Understanding the factors that may influence the population dynamics of koalas is essential for effective management, and it is important to acknowledge that these dynamics vary geographically. In order to understand these differences, there is a need to continue and extend current research by that addresses the significant challenge of collecting sufficient long-term koala tracking data.
Summarised by Cherie Chan
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