Demographic consequences of sex‐biased dispersal in a population of brushtail possums

Abstract

1. A population of brushtail possums (Trichosurus vulpecula) in the Orongorongo Valley, near Wellington, New Zealand was live‐trapped three times a year for 15 years. Almost all births occurred in a single autumn pulse, and many young were tagged while still with their mothers in spring before becoming independent in summer. Yearlings could be distinguished from adults until the following summer. An age‐structured mark–recapture analysis distinguished in situ recruitment from immigration. 2. The sex ratio of young in June, soon after birth, was male‐biased (56·1%); by September, late in the pouch phase, the observed ratio was closer to parity (52·5%). Disappearance immediately after independence was not sex‐biased and did not coincide with noticeable immigration, so it is believed to be due almost entirely to mortality. 3. Recurrent seasonal patterns of disappearance and immigration differed between sex and age classes. Adult disappearance was high in winter (June–September), particularly among males, and low through the rest of the year. The disappearance of yearlings, which seldom bred, showed the least sexual dimorphism in winter. Almost no immigration was detected for either age class over winter. Winter disappearance was therefore attributed to in situ mortality, made worse in adult males by their abrupt loss of condition during the autumn breeding season. 4. The largest seasonal influx of immigrants was of nominally adult males in the intervals September to February and February to June. Many of these were young males in their second summer that could not be distinguished from adults by body size. It is inferred that the majority of natal dispersal was delayed until after their first winter of independence, which may have the effect of reducing dispersal‐related mortality. 5. Immigration by males exceeded that by females in both adult and yearling age classes, and at all times of year. In total, 74% of immigrants were males. About three in four breeding males and one in five breeding females were immigrants. 6. The inverse seasonal relationship between mortality and dispersal suggests that the proximate motivation for dispersal in brushtail possums is not related to weather or food shortage. Immigration probably exceeded emigration, indicating that dispersal was not a strategy to escape high densities. Dispersal appears to be an integral part of male development. The rules governing the settlement of dispersers remain unknown.