The local-resource-competition hypothesis predicts that where philopatric offspring compete for resources with their mothers, offspring sex ratios will be biased in favour of the dispersing sex. This should produce variation in sex ratios between populations in relation to differences in the availability of resources for philopatric offspring. However, previous tests of local resource competition in mammals have used indirect measures of resource availability and have focused on sex-ratio variation between species or individuals rather than between local populations. Here, we show that the availability of den sites predicts the offspring sex ratio in populations of the common brushtail possum. Female possums defend access to dens, and daughters, but not sons, occupy dens within their mother's range. However, the abundances of possums in our study areas were determined principally by food availability. Consequently, in food-rich areas with a high population density, the per-capita availability of dens was low, and the cost of having a daughter should have been high. This cost was positively correlated with male bias in the sex ratio at birth. Low per capita availability of dens was correlated with male bias in the sex ratio at birth.

STUDIES of dispersal and philopatry in Macropodoid marsupials have revealed strong sexdifferential patterns, consistent with those found in other mammals (Johnson 1989;Greenwood 1980). In the macropodids (kangaroos and wallabies), males disperse at sexual maturity, over distances several times greater than the diameter of their mother's home range. Females typically remain close to their birth place, often settling within the maternal home range, resulting in long-term association of female kin. Dispersal in potoroids (rat-kangaroos) is far less well understood. Although movements of adults have been studied in several species, there are only two indications of the pattern of natal dispersal and philopatry. Christensen and Maisey (1987) suggested that in the woylie Bettongia penicillata sub-adults probably dispersed over short distances, and that such dispersal was male-biased. Pope (2001) used microsatellite assignment tests and pairwise analysis of relatedness to infer that dispersal was also malebiased in B. tropica.Here we provide data on natal dispersal and philopatry from a large capture-mark-recapture study of the rufous bettong Aepyprymnus rufescens. We define natal philopatry as continued residence by an adult offspring in a home range overlapping that of its mother, and natal dispersal as a shift in the home range of the offspring large enough to result in no overlap with the mother.Aepyprymnus rufescens was studied at four sites in the Mt Fox area (145 o 46′ E; 18 o 50′ S), about 50 km west of Ingham in north Queensland. All sites were flat or gently undulating and covered by open eucalypt woodland dominated by Eucalyptus crebra, E. intermedia, E. tesselaris and E. peltata. Two of the sites had basalt-derived and two had granite-derived soils; population densities were estimated at 2.64 per km 2 and 11.45 per km 2 on the granite sites and 20.8 per km 2 and 36.7 per km 2 on the basalt sites. A 600 m x 600 m trapping grid was laid at each site, and trapped using 36 wire cage traps at 100 m spacings. Traps were baited with a mixture of rolled oats, peanut butter, vanilla essence and honey. Traps were set in the late afternoon, and were checked during the night. Bettongs (including furred pouch young) were marked with subcutaneous transponder ('PIT') tags, and pouch young were aged using the growth curve provided by Johnson (1978). All four sites were trapped at intervals of one or two months from early 1996 until the end of 1998. A typical trap session lasted three nights on each grid. This program resulted in a total of 1,344 captures of 163 individuals. The number of new animals caught on each grid plateaued after the first five or six nights of trapping, suggesting that all residents had been identified.A total of 39 individuals (21 males, 18 females) were marked as pouch young, and of these three males and seven females were subsequently recaptured as independent subadults or adults on their home grids. Of the three males marked as pouch young and later recaptured on their home grids, two were re-cap...

We compared demography of populations along gradients of population density in two medium-sized herbivorous marsupials, the common brushtail possum Trichosurus vulpecula and the rufous bettong Aepyprymnus rufescens, to test for net dispersal from high density populations (acting as sources) to low density populations (sinks). In both species, population density was positively related to soil fertility, and variation in soil fertility produced large differences in population density of contiguous populations. We predicted that if source-sink dynamics were operating over this density gradient, we should find higher immigration rates in low-density populations, and positive relationships of measures of individual fitness--body condition, reproductive output, juvenile growth rates and survivorship--to population density. This was predicted because under source-sink dynamics, immigration from high-density sites would hold population density above carrying capacity in low-density sites. The study included 13 populations of these two species, representing a more than 50-fold range of density for each species, but we found that individual fitness, immigration rates and population turnover were similar in all populations. We conclude that net dispersal from high to low density populations had little influence on population dynamics in these species; rather, all populations appeared to be independently regulated at carrying capacity, with a balanced exchange of dispersers among populations. These two species have suffered recent reductions in range, and they are ecologically similar to other species that have declined to extinction in inland Australia. It has been argued that part of the cause of the vulnerability of species like these is that they exhibit source-sink dynamics, and disturbance to source habitats can therefore cause large-scale population collapses. The results of our study argue against this interpretation.