Honey possums

(Tarsipedidae)

Class Mammalia

Order Diprotodontia

Family Tarsipedidae

Thumbnail description
Very small mouselike possum with long, tapering snout and very long, partially prehensile tail; eyes large and round, ears large, rounded, and sparsely haired; fingers and toes are long with rounded tips and small nails; fur is grayish brown with three dark dorsal stripes

Size
Head and body length 1.6–3.7 in (4–9.5 cm); tail length 1.8–4.3 in (4.5–11 cm); weight 0.2–0.6 oz (7–16 g)

Number of genera, species
1 genus, 1 species

Habitat
Arboreal in flowering trees and shrubs, mainly in heathland

Conservation status
Not listed as threatened, but may be at risk from habitat loss

Distribution
Southwestern Australia

Evolution and systematics

The Australian honey possum, Tarsipes rostratus, was formerly classified along with other Australian possums in the family Phalangeridae. It was deemed unusual enough to deserve its own subfamily (Tarsipedinae), but it was not until the mid-1970s that the true extent of its uniqueness became apparent. Today, this tiny possum is known to be one of the most divergent marsupials, easily worthy of its own family, Tarsipedidae. It is the sole survivor of an otherwise extinct marsupial group that diverged from the diprotodont lineage during the Pleistocene, about two million years ago. In this relatively short geological time span, the lineage has evolved rapidly and developed one of the most specialized lifestyles of any mammal group. Its closest relatives appear to be the feather-tailed possums and gliders of the family Acrobatidae.

The taxonomy for this species is Tarsipes rostratus Gervais and Verreaux, 1842, Western Australia, Australia.

Physical characteristics

The honey possum is a dainty animal, adapted for climbing. Its hands and feet bear long, grasping, monkeylike fingers

and toes. As in all diprotodont marsupials, the second and third toes of the hind feet are fused, or syndactylous. Only the fused toes of the hind foot have claws, which are used for grooming. The other digits are equipped instead with a small, round nail. The first toes are opposable, like those of a climbing primate. The honey possum's agility is greatly enhanced by its long tail, the tip of which is prehensile. The tail is used as a counter-balance, a safety line, and a fifth limb. It is very sparsely haired, and naked on the underside toward the tip. The bare skin provides excellent grip when the tail is curled around branches and stems.

The honey possum's head is distinctive, with a long, shrew-like snout bearing long whiskers. The tongue is longer still and can be extended the length of the head beyond the tip of the snout. The teeth are reduced. Only the lower front incisors are at all developed, while the others are little more than weak pegs. The fur is coarse and grayish brown, with very long guard hairs. The honey possum's eyes are large, as befits a nocturnal animal, and situated so that they face both forward and upward. This arrangement combines the advantages of binocular vision, which they need to move confidently around a three-dimensional environment, as well as be alert for airborne predators such as owls.

Distribution

Honey possums have a very limited distribution. They live in the extreme southwestern corner of Australia, hemmed in to the south and west by oceans, to the east by the vast desert of the Nullabor and to the north by high ground.

Habitat

Within its limited range, the honey possum lives only on the sandy and coastal heaths, where conditions suit nectar-rich myrtles, proteas, banksias, and other flowering plants that, between them, bloom throughout the year.

Behavior

Honey possums are nocturnal. Their foraging behavior is a cross between that of a monkey and a honeybee—they scurry nimbly about the branches of flowering trees and shrubs, pausing often to thrust their snout into every floret. Being small, the honey possum uses a lot of energy keeping warm and, when resting in cool conditions, it curls up to reduce its surface area. For most of the year, the animals live alone and rest in tree holes or abandoned bird nests. In periods of food scarcity, the honey possum runs low on energy and is sometimes forced into periods of inactivity during which it enters a deep torpid sleep; its heart rate is reduced and its body temperature drops. Under these circumstances, several animals

may cluster together because, by sharing small amounts of body heat, they can save more energy still. They wake periodically and when things have improved, they resume diligent solitary feeding to replenish their depleted reserves.

Feeding ecology and diet

The teeth of honey possums are few and underdeveloped—little more than short pegs incapable of biting or chewing. Despite their common name, honey possums rarely eat honey. In fact, they feed more or less exclusively on pollen and nectar, a diet that makes them highly unusual among mammals other than bats. In a dramatic example of convergent evolution, these tiny marsupials are equipped with a long tongue, like that of nectar-feeding insects or hummingbirds, with which they can reach into the nectaries of deep flowers. Unlike an insect proboscis, however, the tongue is not tubular. Instead, it bears at its tip a highly specialized arrangement of bristles, like a tiny brush. This collects pollen grains and sticky nectar and delivers them into the possum's mouth where they are sucked off or scraped onto ridges on the roof of the mouth. Even in very small doses, this diet provides ample sugar and protein to fuel and maintain a small body, so long as there is a more or less continuous supply.

Reproductive biology

Female honey possums are larger than males and socially dominant. They can come into breeding condition at any time of the year, providing food is plentiful, though there is often a lull during mid-summer when nectar and pollen are less abundant. Males gather around an estrous female and compete

for the right to mate, before moving quickly on. Amazingly for such small animals, the sperm produced by males are very large—at 0.01 in (0.3 mm), they are the longest of any mammal. As if to redress the balance, female honey possums give birth to the smallest mammalian babies, each one weighing no more than 0.002 oz (5 mg). Most females will mate again almost immediately after giving birth. This second litter is an insurance policy—the fertilized eggs do not develop beyond a very early stage until the previous litter has left the pouch or died. Hence, while honey possum gestation requires only 3 weeks, pregnancies can last anywhere up to 13 weeks. Litters never contain more than four young; this is the maximum number a female can suckle on the four teats in her pouch. The litter spends about eight weeks in the pouch, during which time each baby grows to about 0.1 oz (2.5 g), which is a remarkable 500 times its birth weight. After that, the young are too big to fit in the pouch and the female deposits them in a spherical woven nest until they are weaned at about 10 weeks of age. A healthy female will normally rear two litters in a year, but will rarely live long enough to breed a third time.

Conservation status

This species remains common in areas of suitable habitat, despite predation by introduced carnivores such as foxes and cats. The biggest threat comes from habitat loss due to urbanization and land development, but conservation authorities are monitoring the situation closely to ensure that this unique marsupial lineage continues.

Significance to humans

Honey possums are popular animals, though their secretive, nocturnal lifestyles make them difficult to see. They do no harm to human interests and perform a valuable service in helping to pollinate many species of heathland flower.

Resources

Books

Aplin, K. P., and M. Archer. "Recent Advances in Marsupial Systematics with a New Syncretic Classification." In Possums and Opossums: Studies in Evolution. Sydney: Surrey Beatty and Sons and Royal Zoological Society of New South Wales, 1987.

Morris, P., and A. J. Beer. "Honey Possum." In World of Animals: Mammals. Vol. 10, Marsupials. Danbury, CT: Grolier, 2002.

Nowak, R. "Honey Possum (Diprotodontia; Family Tarsipedidae)." In Walker's Mammals of the World, 6th ed., Vol I. Baltimore and London: Johns Hopkins University Press, 1999.

Strahan, R. The Mammals of Australia, Revised Edition. Sydney: New Holland, 1998.

Organizations

CALM (Department of Conservation and Land Management) South Coast Office. 120 Albany Highway, Albany, Western Australia 6330 Australia. Phone: (08) 9841 7133. Fax: (08) 9842 4500. E-mail: [email protected] Web site: <http://www.calm.wa.gov.au>

Amy-Jane Beer, PhD