Mammals
Honey Possum
Tarsipes rostratus · also called honey possum, noolbenger
The honey possum is a marsupial that lives like a hummingbird. Barely bigger than a mouse, it feeds only on nectar and pollen, lapping them up from banksia flowers with a brush-tipped tongue and a long pointed snout. It has almost no teeth left, because it never chews anything — and, at the other end of its biology, the male produces the longest sperm of any mammal.
Overview
The honey possum lives only in the far south-western corner of Western Australia, in coastal heath and shrubland rich in flowering banksias and related plants. It has a head and body of just 4–9 cm, a prehensile tail about as long, and weighs 7–12 g (female) or as little as 4 g (male). It is grey-brown with three dark back stripes.
A flower-only diet
It eats nectar and pollen only — one of the very few mammals, and the only marsupial, to live entirely on flowers without flying. Because it never chews, its teeth are reduced to tiny pegs, the fewest of any marsupial.
Record-holder at both ends
Males produce sperm about 0.36 mm long, longer than any other mammal’s, in testes that make up a large share of body weight — linked to sperm competition. And a newborn honey possum, at about 5 milligrams, is the smallest young of any mammal.
Living on a knife-edge
On cold nights or when flowers are scarce it drops into torpor, letting its body temperature fall to save energy. In carrying pollen as it feeds, it pollinates many south-western heath plants, including winter-flowering banksias.
Status
The honey possum is Least Concern but declining. It needs old, unburnt heath with plants in flower year-round, and it is vulnerable to frequent fire, land clearing, and a drying climate in an already tiny range.
Honey Possum: common questions
Is the honey possum related to other possums?
No, despite the name. It is the only species in its family, the Tarsipedidae, and its closest relatives among living marsupials are not clear, though it is placed loosely near the pygmy possums and gliders. It got the name possum because it is small, nocturnal, and has a prehensile tail, but its highly specialised nectar diet, its reduced teeth, its brush tongue, and its whole way of life are unlike those of any other marsupial. It is effectively a marsupial that has evolved to fill the niche of a nectar-feeding bat or a hummingbird.
How can an animal survive on nothing but nectar and pollen?
With difficulty, and only by being very small and very economical. Nectar provides sugar for energy, and pollen provides the protein and other nutrients that nectar lacks, so the two together make a complete, if marginal, diet. The honey possum minimises its needs by being tiny, by moving efficiently through the shrubs on gripping hands and a prehensile tail rather than flying, and by entering torpor whenever conditions allow, so that it is not burning energy staying active and warm when there is nothing to eat. It also needs a habitat where something is always in flower, which is why it is tied to the particular heathlands of south-western Australia.
Why does such a small animal have record-breaking sperm and newborns at opposite extremes?
The two records reflect different pressures. The extremely large sperm, and the very large testes that produce them, are thought to result from sperm competition. Female honey possums mate with several males, so a male's reproductive success depends partly on outcompeting other males' sperm inside the female, which appears to have driven the evolution of unusually long, and unusually numerous, sperm. The tiny newborn, on the other hand, is simply the marsupial pattern taken to its logical end in the smallest marsupial — the young is born at a very early stage and does almost all of its growing while attached to a teat.
Is the honey possum endangered?
It is currently listed as Least Concern, so not formally threatened, but its situation is watched closely. It lives only in the south-western tip of Western Australia, one of the world's biodiversity hotspots, and it depends on old, unburnt heath where a succession of plants flowers through the whole year. That kind of habitat is reduced by land clearing for agriculture and housing, and it is degraded by fires that come too often or burn too hot, removing the mature nectar plants the possum needs. Drying of the region's climate adds further pressure, so the trend is thought to be downward.