Monday, April 09, 2007

Honey Possum, Tarsipes rostratus

It is cute, but not a possum, nor does it eat honey.

The Honey Possum split from the glider and ringtail possum linage around 2 million years ago where upon it underwent rapid evolutionary change to become the sole member from the Family Tarsipedidae. Honey possums live almost exclusively on a diet of pollen and nectar (not honey, only bees make honey), and hence only survive in regions where nectar producing blossom are available year round. This only occurs in the SW of Western Australia (comprising the region known as the Wheatbelt) extending from Israelite Bay on the south coast, to the Murchison River on the west coast. Due to extensive clearing of vegetation in much of the inland mallee region for wheat production, its stronghold is on the coastal sand belt, much of which remains reasonably intact.

A very thoughtful little honey possum in an Adenanthos cuneatus shrub.

Honey Possum, Tarsipes rostratus

The honey possum is tiny, being smaller than the mouse, Mus musculus and weighing half as much. Their eyes are unusual, inasmuch they do not reflect torch light like true possums, so are very difficult to find amongst the dense foliage of the predominantly Proteaceae plants on which they feed. However partly because of their small size, they hold a couple of world records; apparently the males have the largest mammal testes in relation to their size (around 4.2% of their body weight), but surprisingly they also produce the largest sperm (to 0.36 mm in length). Not to be outdone, the female then produces the smallest young of any mammal, weighing only 0.0005 gm. So a very remarkable little animal and of great interest to the scientific community.

As an indication of size, that Banksia speciosa flower is 150 mm in height

Contemplating its future in a Banksia nutans shrub.

The long slender snout, houses a long slender tongue with a brush-like tip for reaching and lapping up the nectar from the generally long slender tubular Proteaceae flowers. Due to this adaptation it is thought to be an important pollinator of many Banksia and other Proteaceae species. The honey possum has teeth, but they are very specialised, the lower fangs are designed (along with groves in the palate) to scape off the nectar/pollen from the tongue, whilst the other teeth are tiny, peg-like, widely spaced and totally ineffectual for eating solids. However in captivity honey possums are known to eat soft-bodied insect larvae.

Females are about a third larger than males and occupy the dominant position in any relationship. However they generally lead a solitary life, but with overlapping territories, remain quite sociable with others except when the females have young. Their life span is between 1-2 years, but they breed year round providing there is sufficient food. I encountered a mum with two young sitting along side of her, they were only 600 mm away, stationary and in an exposed position ideal for photographing, but they looked so terrified that I returned my camera in its box and left them undisturbed.

Female honey possum about to disappear into an Adenanthos cuneatus shrub.

Honey possums are great climbers, but unlike most other mammals, they do not use their claws, but grasp branches like a monkey with opposable toes. Its genus name of Tarsipes, is derived from the Tarsier (a primitive primate, with a similar foot structure). The honey possum also has a prehensile tail that can support its weight, so is very agile and well adapted to its arboreal home. When caught by torchlight, it usually freezes and remains so unless you come too close, when it can move very quickly. I even had a fleeting glimpse during the day of one (probably disturbed by a bird or reptile) moving over the ground from one bush to another, it was very fast and with a mixture of galloping and leaping, it soon disappeared.

Typical rusty brown stripes and long prehensile tail of the honey possum.

Although this tiny animal is known to climb high into Banksia trees to reach the blossom, I have only seen them within 2 metres of the ground and usually only between a ½ and 1½ metres. Being above this height on the terminal Banksia speciosa flowers, places them in considerable danger of predation from night birds like owls and frogmouths, and when at lower elevations from cats and foxes. I have encountered dead honey possums (and pygmy possums) that had been caught and mauled by cats, and have watched a fox so intent in tracking one down in a clump of low Banksia pulchella, that I almost caught it in my hands.

The reason I suspect the honey possum does not feed from lofty exposed Banksia flowers, is the Tawny Frogmouth, Podargus strigoides, who will happily dine on frogs, reptiles, moths and honey possum.

According to my Jack Russell Terrier (who is under very strict orders not to hunt them), the honey possums do give off an odor, but it is disguised by the odor of the Banksia flowers, and fortunately they take a while to track down, nevertheless the dog is aware if any are around, so presumably cats and foxes would too. I dare say birds and reptiles also take a few, therefore not only are they endangered from predators, but equally so from land clearing and devastating wildfires.

Honey Possum on a Banksia speciosa flower.

Wildfires are caused when the native vegetation is permitted to grow for many years without periodic burn-offs (partly to reduce fuel loads); such fires are highly destructive particularly to the fauna. However if the preferable indigenous mosaic fire management were employed, then it would by its more open flora composition, advantage predators like the cat and fox, by leaving the honey possum more exposed and consequently more easily caught. (see item 14. Fire Management: European versus Indigenous).

From studies conducted in the Fitzgerald River National Park (200-300 km west of Esperance), it was concluded the honey possum did best when fire frequency was at least 20 years apart, in order to permit the floral community, particularly the Banksia species to mature. The situation in the Esperance region I believe is quite different. There are four plant species I have seen the honey possum using and they are also the most dominant in Esperance sandy heath. They comprise the largest local Banksia, B. speciosa that grows to around 6 metres and flowers May to Jan. Banksia nutans to around 1½ metres (flowering Nov to April), and Banksia pulchella usually less than 1 metre and commonly only ½ metre in height (flowering Jan to Oct). The fourth plant and another Proteaceae also very popular with the possum, is Adenanthos cuneatus, which has a very long flowering period, doing so off and on throughout the year. This can grow to 2 metres in height.

With some exceptions, after 20 years, the three Banksia species mentioned would be pretty much at the end of their useful life, with B. pulchella being smothered by larger vegetation, B. speciosa beginning to die off and B. nutans just hanging on. The Adenanthos would be incredibly leggy by this stage (if not smothered) and like the rest, not producing much blossom. However, all of these species would be flowering within three years of a bushfire, and with more blossom than an army of honey possums could consume by 5 years. Most of the largest plants Banksia speciosa and B. nutans would be at their peak (blossom wise within 15 years and beginning to deteriorate thereafter), the others deteriorating within a much shorter period.

Having said the above regarding the Esperance (honey possum) flora situation, I return to the concluding statement of the Fitzgerald River study, where it was found that honey possum numbers were at their highest 20-30 years without a bushfire. This may well be the same for Esperance, but not for the number of nectar producing blossoms available, but from the dense (overgrown) vegetation that is providing protection from cat and fox predators. As a cautionary note, the entire possum population can also be wiped out by a wildfire derived from this overgrown environment with its very high fuel load.

What I believe to be a sensible solution as long as the fox and feral cat populations remain high, is to provide pockets of aged vegetation, alongside areas with a mosaic of different aged flora. However the situation I regret will not improve to where the honey possum is secure in its native environment, unless predator numbers are substantially reduced.

An easily hidden, yet highly vulnerable honey possum.


huntervalley said...

What a thrill (and privilege) it must be to observe this fascinating creature go about its secret life.

I am surprised by its short life span. I presume with such tiny young, that the female must have a pouch.

Do you know how long it is until the young are capable of feeding themselves?

And does the female build a nest of grasses etc in a tree similar to a bird? I don't imagine hollows would be available in banksias.

An extremely interesting blog entry.


Esperance Blog said...

Hi Huntervalley,
Yes the honey possum is a very interesting little animal to observe, but even for me living in their environment, they are so difficult to find that sightings are very infrequent.

>>I presume with such tiny young, that the female must have a pouch<<

Yes like most marsupials the females have a well-developed pouch and initially carry the kids around with them (usually two, but up to four). Once they become too large for her, they are deposited in a safe place while see gos off looking for food.

>>Do you know how long it is until the young are capable of feeding themselves?<<

Around 10 weeks. The female usually breeds twice a year, but seldom produces a third litter before dying.

>>And does the female build a nest of grasses etc in a tree similar to a bird? <<

She sometimes build a spherical nest of woven grass for her young, but honey possums generally seek out small hollows amongst vegetation, with abandoned birds nests apparently being very popular.