Leadbeater's Possum (Gymnobelideus leadbeateri) is an endangered possum restricted to small pockets of remaining old growth Mountain Ash forests in the cool, misty highlands of Victoria, Australia. Leadbeater's Possums can be moderately common within the very small areas they inhabit: their requirement for year-round food supplies and tree-holes to take refuge in during the day restricts them to mixed-age wet sclerophyll forest with a dense mid-story of Acacia.

The possum was not discovered until 1867, and was known only through five specimens; the last one was collected in 1909, and from that time on the fear that it might be extinct gradually grew into near-certainty. In 1961, a colony was discovered near Marysville, in the Upper Yarra Valley. Extensive searches since then have found a substantial population in the highlands. However, the availability of suitable habitat is critical: forest must be neither too old nor too young.

The current fairly healthy population is ascribed to the terrible Black Friday fires that swept through Australia in 1939: The combination of 40-year-old regrowth (for food) and large dead trees left still standing after the fires (for shelter and nesting) allowed the Leadbeater's Possum population to expand to an estimated peak of about 7500 in the early 1980s. However, the old trees are gradually decaying and the regrowth is maturing. Prior to European settlement, a similar situation would have forced migration to other areas—something which is not a realistic option now because of extensive land clearing over the last hundred years or so.

From its peak in the 1980s, the Leadbeater's Possum population is expected to decline rapidly, by as much as 90%. Failing human intervention, and assuming that a population of about 1000 can survive that long, natural tree hollows will begin to develop in the Black Friday regrowth as the trees reach about 150 years of age in the second half of the 21st century, and numbers begin to climb again.

Leadbeater's Possums are rarely seen: they are nocturnal, small (about 16 cm long and about 130 grams, or the size of a small rat), fast-moving, and occupy the upper story of some of the tallest forest in the world. They live in small family colonies of up to 8 individuals, usually a breeding pair, their offspring, and sometimes an unrelated extra male or two. All members sleep together in a nest made out of shredded bark in a tree hollow, anywhere from 6 to 30 metres above ground level and roughly in the centre of a territory of one or two hectares, which they defend actively. The senior female is the main defender: she is more active in expelling outsiders, and attacks her daughters when they reach sexual maturity at about 10 months of age, forcing them to disperse earlier than male children. In consequence, mortality among young female Leadbeater's Possums is high—average female lifespan is little more than 18 months, as opposed to about 7 years in captivity.

Solitary Leadbeater's Possums have difficulty surviving: when young males disperse at about 15 months of age, they tend to either join another colony as a supernumerary member, or gather together into bachelor groups while they await an opportunity to find a mate.

At dusk, Leadbeater's Possums emerge from the nest and spread out to forage in the canopy, often making spectacular leaps from tree to tree. Their diet is omnivorous: they take a range of saps and exudates, lerps, and a high proportion of arthropods which they find under the loose bark of eucalypts: spiders, crickets, beetles, and the like. Pland exudates make up 80% of their energy intake, but the protein provided by the arthropods is essential for successful breeding.

Births are usually timed for the beginning of winter (May and June) or late spring (October and November). Most litters are of one or two young, which stay in the pouch for 80 or 90 days, and first emerge from the nest about three weeks after that. Young, newly indedpendent Leadbeater's Possums are very vulnerable to owls.

In 1971, the State of Victoria made Leadbeater's Possum its faunal emblem.

See also:

• Possum
• Close relatives of Leadbeater's Possum: Sugar Glider, Squirrel Glider, Yellow-bellied Glider , Mahogany Glider , Striped Possum
• Favoured habitat trees: Mountain Ash, Alpine Ash, Shining Gum