Common brushtail possum

Brushtail possum in tree

The common brushtail possum is perhaps the most widespread marsupial of Australia. It is found throughout the eastern and northern parts of the continent, as well as some western regions,[9] Tasmania[10] and a number of offshore islands, such as Kangaroo Island[11] and Barrow Island.[12][13] It is also widespread in New Zealand since its introduction in 1850. The common brushtail possum can be found in a variety of habitats, such as forests, semi-arid areas and even cultivated or urban areas.[6][7] It is mostly a forest inhabiting species, however it is also found in treeless areas.[7] In New Zealand, possums favour broadleaf-podocarp near farmland pastures.[14] In southern beech forests and pine plantations, possums are less common.[14] Overall, brushtail possums are more densely populated in New Zealand than in their native Australia.[15] This may be because Australia has more fragmented eucalypt forests and more predators. In Australia, brushtail possums are threatened by humans, tiger quolls, dogs, foxes, cats, goannas, carpet snakes, and powerful owls. In New Zealand, brushtail possums are threatened only by humans and cats.[15] The IUCN highlight the population trend in Australia as decreasing.

Dentition of a common brushtail possum

The common brushtail possum can adapt to numerous kinds of vegetation but it is largely omnivorous.[15] It prefers Eucalyptus leaves, but also eats flowers, shoots, fruits, and seeds.[15] It may also consume animal matter such as insects, birds' eggs and chicks, and other small vertebrates.[16] Brushtail possums may eat three or four different plant species during a foraging trip, unlike some other arboreal marsupials, such as the koala and the greater glider, which focus on single species. The brushtail possum's rounded molars cannot cut Eucalyptus leaves as finely as more specialised feeders. They are more adapted to crushing their food, which enables them to chew fruit or herbs more effectively. The brushtail possums' caecum lacks internal ridges and cannot separate coarse and fine particles as efficiently as some other arboreal marsupials.[15] The brushtail possum cannot rely on Eucalyptus alone to provide sufficient protein.[17] Its more generalised and mixed diet, however, does provide adequate nitrogen.[18]

Brushtail possums feeding

Brushtail possum (brown form)

The common brushtail possum is largely arboreal and nocturnal. It has a mostly solitary lifestyle, and individuals keep their distance with scent markings (urinating) and vocalisations. They usually make their dens in natural places such as tree hollows and caves, but also use spaces in the roofs of houses. While they sometimes share dens, brushtails normally sleep in separate dens. Individuals from New Zealand use many more den sites than those from Australia.[19] Brushtail possums compete with each other and other animals for den spaces, and this contributes to their mortality. This is likely another reason why brushtail possum population densities are smaller in Australia than in New Zealand.[15] Brushtail possums are usually not aggressive towards each other and usually just stare with erect ears.[15] They vocalise with clicks, grunts, hisses, alarm chatters, guttural coughs, and screeching.[6][7]

Brushtail possum with young

The common brushtail possum can breed at any time of the year, but breeding tends to peak in spring, from September to November, and in autumn, from March to May, in some areas. Mating is promiscuous and random; some males can sire several young in a season, while over half sire none.[15] In one Queensland population, males apparently need a month of consorting with females before they can mate with them.[20] Females have a gestation period of 16–18 days, after which they give birth to single young.[6][7] A newborn brushtail possum is only 1.5 cm long and weighs only 2 g. As usual for marsupials, the newborn may climb, unaided, through the female's fur and into the pouch and attach to a teat. The young develops and remains inside the mother's pouch for another 4–5 months. When older, the young is left in the den or rides on its mother's back until it is 7–9 months old.[6][7] Females reach sexual maturity when they are a year old, and males do so at the end of their second year.[6][7] Brushtail possums can live up to 13 years in the wild.[6][7]

Female young have a higher survival rate than their male counterparts due to establishing their home ranges closer to their mothers, while males travel farther in search of new nesting sites, encountering established territories from which they may be forcibly ejected. In New Zealand's Ōrongorongo population, female young have been found to continue to associate with their mothers after weaning, and some inherit the prime den sites.[21] A possible competition exists between mothers and daughters for dens, and daughters may be excluded from a den occupied by the mother.[22] In forests with shortages of den sites, females apparently produce more sons, which do not compete directly for den sites, while in forests with plentiful den sites, female young are greater in number.[22]

Brushtail possum exhibiting exudative dermatitis, a condition that often results from stress associated with overcrowding, particularly in young males attempting to assert territory

Its fur has been considered valuable and has been harvested. Although once hunted extensively for its fur in Australia, the common brushtail possum is now protected in mainland states, but it has only been partially protected in Tasmania, where an annual hunting season is used. In addition, Tasmania gives crop-protection permits to landowners whose property has been damaged.[8]

While its populations are declining in some regions due to habitat loss, urban populations indicate an adaptation to the presence of humans.[23] In some mainland states, possum trapping is legal to catch when attempting to evict possums from human residences (e.g. roofs), but possums must be released after dusk within 24 hours of capture, no more than 50 m from the trapping site. In some states, e.g. Victoria, trapped possums may be taken to registered veterinarians for euthanasia.[24] In South Australia, they are fully protected and permits are required for trapping possums in human residences[25] or for keeping or rescuing sick or injured wild possums and other native animals.[26] In Queensland, they can only be trapped by licensed commercial relocators who must release possums within 25 metres of the point of capture to ensure that an animal is not released into another possum's home range (possums are less likely to survive if they are released into a new area where they do not have access to a den or must compete with a neighbouring possum for den space).[27]

Since its introduction from Australia by European settlers in the 1850s, the common brushtail possum has become a major threat to New Zealand native forests and birds. It is also a host for the highly contagious bovine tuberculosis.[8] (This is not an issue in Australia, where the disease has been eradicated.[28])

By the 1980s, the peak population had reached an estimated 60–70 million, but is now down to an estimated 30 million due to control measures. The New Zealand Department of Conservation controls possum numbers in many areas via the aerial dropping of 1080-laced bait.[29] Hunting is not restricted, but the population seems to be stable despite the annual killing of thousands of the animals.[8]