Use of Nest Trees by the Mountain Brushtail Possum (Trichosurus caninus) (Phalangeridae : Marsupialia). III. Spatial Configuration and Co-occupancy of Nest Trees

Radio-tracking was used to examine the spatial configuration and co-occupancy of large trees with hollows occupied by 16 mountain brushtail possums (Trichosurus caninus) at Cambarville in the central highlands of Victoria, south-eastern Australia. The distance that animals moved between trees on successive days was also examined. Our analyses showed that animals often remained in a given den tree on successive days. On nights when animals did shift between den sites, they typically moved to a new tree that was relatively nearby (< 200 m). Long-distance movements (e.g. > 300 m) between den trees were rare. Considerable variation was found between individuals in the size of areas encompassing trees used frequently (≥5 times) (‘core denning range’) and those encompassing all occupied trees [i.e. including those used infrequently (< 5 times)] (‘total denning range’). The mean area of the core denning range averaged about 1·1 ha for males and 0·7 ha for females. The mean value for the total denning range was approximately 2·6 ha for males and females respectively. Considerable overlap was found in the total denning ranges of pairs of adult males and pairs of adult females. For most animals, the total denning range was shared with the total denning ranges of several other animals. There was substantial variation in the extent of this overlap, ranging from complete enclosure to the sharing of a single tree. The extent of overlap was more limited for the core denning ranges, particularly among females. We observed differences in the extent of the overlap of the total denning ranges of pairs of males and pairs of females in the breeding season (January–March) and non-breeding season (the remaining months of the year). Fewer instances of overlapping total denning ranges among pairs of both males and females were found during the breeding season. The total denning ranges in the breeding season were generally smaller than those in the non- breeding season. Simultaneous co-occupancy of a given den tree by T. caninus was relatively common. Unexpectedly, there was a number of instances of groups of three or four adult animals sharing the same den site on the same night. We recorded several instances of sharing by pairs of animals of the same sex, especially adult males. However, most records of simultaneous tree use were by an adult male and an adult female T. caninus. The extent of overlap in the denning ranges of animals and the prevalence of simultaneous co-occupancy of den trees indicate that the social behaviour of T. caninus at Cambarville may be different from that observed among populations of the species elsewhere in Australia. Possible reasons for these differences are outlined.