Mountain Pygmy Possum: the Marsupial Known From Fossils Before Anyone Saw One Alive

Most "living fossil" stories involve a species that has limped along mostly unchanged for millions of years. The mountain pygmy possum's story runs the other way round: scientists met the fossil first, decided the animal itself was long gone, and only learned seventy years later that they were wrong. Here's what it actually is, where the only population left on Earth survives, and why it spends more of the year asleep than awake.

What is a mountain pygmy possum?

The mountain pygmy possum (Burramys parvus) is a mouse-sized marsupial, the only living member of its genus and one of just two species left in the possum family Burramyidae. It weighs around 45 g, with a compact 11 cm body and a thin, scaly tail that runs about 14 cm, longer than the possum itself, and prehensile enough to grip a branch or a rock edge like a fifth limb. A soft grey-brown coat and dark eye-rings give it an unremarkable, almost rodent-like look, which is part of why it went unnoticed for so long. Australia's marsupials tend to specialise in extremes: the numbat is the only diurnal, termite-only marsupial, and the mountain pygmy possum has gone the opposite direction, becoming the only one truly built for an alpine winter.

Known from fossils seventy years before anyone saw one alive

In 1894, cave diggers at Wombeyan Caves in New South Wales turned up a scatter of fossil teeth and jaw fragments roughly 20,000 years old. Palaeontologist Robert Broom described them as a new genus, Burramys, in 1896, and for decades that was the entire record: an Ice Age marsupial known only from bone. Then, in August 1966, a skier at a lodge on Mount Hotham in Victoria watched a small, unfamiliar animal scurry across the kitchen floor. It was alive, it matched Broom's fossil genus exactly, and it was the first confirmed sighting of a living mountain pygmy possum in the 70 years since the species had been named. The find was startling enough that the 1967 Guinness Book of Records briefly listed it as the rarest animal on Earth, a title that said more about how little anyone had looked than about the animal's actual numbers: later surveys found it living at a reasonable density, just confined to a tiny, specific kind of ground almost nobody ever sets foot on.

Where it lives: the only Australian mammal confined to true alpine ground

That ground is boulder field. The mountain pygmy possum survives in three isolated pockets of the Australian Alps: deep scree and rock piles above the treeline around Mount Kosciuszko in New South Wales, and on Mount Bogong, Mount Higginbotham and Mount Buller in Victoria, all between roughly 1,300 and 2,230 m elevation. It is, by a wide margin, the only Australian mammal restricted to true alpine habitat and nowhere else; everything else that lives this high also lives lower down somewhere. The gaps between the boulders form a maze of tunnels and chambers under the winter snowpack, insulated enough to stay well above the outside air temperature through an alpine winter, which is exactly the shelter the species has built its entire life cycle around.

How it survives winter: one of the only marsupials that truly hibernates

Most small mammals cope with a cold night using a few hours of torpor and carry on the next morning, a trick even monotremes like the echidna use during a cold spell. The mountain pygmy possum goes much further: it hibernates for up to seven months of the year, among the longest hibernation of any Australian mammal, with its body temperature falling to around 2°C. It isn't continuously unconscious for all seven months; it wakes periodically to feed on seeds and other food it cached under the rocks in autumn, then drops straight back into torpor. Few mammals commit this hard to shutting down for winter; even a wolverine, built for Arctic cold, stays active and hunting all season rather than sleeping through it. That long, deep hibernation is also the reason the species went unnoticed for so long after Broom's fossil description: for well over half the year, there is simply no active animal above ground to find.

The Bogong moth connection

Each spring, the possum's wake-up call doubles as a food delivery. Bogong moths complete a seasonal migration out of the lowlands and arrive in vast numbers in the high country at almost exactly the time mountain pygmy possums are emerging from hibernation, and the moths make up roughly a third of the possum's diet through spring and summer. The rest comes from mountain plum pine seeds, the fruit of snow beard-heaths, and berries, but the moth arrival is the trigger: females time breeding to the spring moth glut, when there's enough protein-rich food on the ground to fuel a litter. Few local food webs depend this precisely on one insect showing up on schedule.

Is the mountain pygmy possum dangerous?

Not even slightly. It's a shy, nocturnal animal a few centimetres long that spends most of its life either asleep under rocks or quietly foraging for seeds and insects in the dark. It has no venom, no bite worth worrying about, and no interest whatsoever in people. The only real threat it poses is to the moths and seeds on its own menu.

Two more things worth knowing

• Climbing gear built in. Its hind feet carry an opposable big toe, and combined with the long prehensile tail, it can grip boulders and branches with real precision, useful for an animal that spends its waking life clambering through loose rock.
• One litter, once a year. Females breed once annually, giving birth after a gestation of just 14-16 days, among the shortest pregnancies of any mammal, and raising the usual handful of marsupial young from a teat inside the pouch, the same basic strategy covered in our piece on why kangaroos have pouches. Three or four young typically stay in the pouch about three weeks and are weaned at nine to ten weeks, and daughters tend to stay near their mother's territory for life while males disperse alone.

How rare is the mountain pygmy possum in Kaught?

In the Kaught catalog the mountain pygmy possum sits at the Epic tier, three diamonds out of four. That's a measure of how often the species is actually confirmed in the wild, not a conservation label: with a range limited to a handful of boulder fields above the treeline, and an animal that's invisible underground for up to seven months a year, a confirmed sighting is genuinely hard to come by even for people who go looking in the right place at the right time of year.