A Breeding Season Difference in Two Populations of the Australian Macropod Marsupial (Setonix brachyurus)

Abstract

The population of Setonix at Rottnest Island (Western Australia) has a season of birth restricted to 6 months, but the population on the adjacent mainland was found during this study to breed continuously throughout the year. As both populations are in the same latitude and less than 50 miles apart, it is unlikely that climatic conditions are responsible for the different breeding patterns. Previous work showed that the Rottnest Island population annually suffered semistarvation in the late summer. This recurrent population debility is thought to be the main factor maintaining the 6-month anestrous period, as females from Rottnest Island maintained in captivity on liberal rations are known to decrease, and after a period of 2 or more years, finally delete the anestrous period. The period of gestation and pouch life of Setonix is a little longer than 6 months. Mainland females, breeding continuously, will produce 2 joeys in a little over a year, but Rottnest Island females which have only 1 young each year are also forced to delay their first joey 6 months. The decreased fertility of the insular population as a result of the 6-month anestrous period and later breeding may then be regarded as a population-limiting factor, only affected by long-term environmental changes but acting in concert with the short-term Malthusian factors of disease and starvation. A possible adult age structure difference resulting from the anestrous period is also discussed.