The Handbook of New Zealand Mammals

2Family Phalangeridae

The Family Phalangeridae is the second and smaller of the two families of marsupials represented in New Zealand. Phalangerids are classified into two tribes, the Trichosurini (one species of Wyulda, the scaly-tailed possum, plus three species of brushtail possums, Trichosurus, native to forests in Australia); and the Phalangerini (>10 species of cuscuses native to Papua New Guinea, and adjacent islands between Sulawesi and the Solomons). All weigh 1.5–5 kg and are mainly arboreal.²⁰⁸,³⁵⁰

Genus TRICHOSURUS

The brushtail possums all have large ears, pointed faces, close woolly fur and bushy tails. Only the common brushtail possum, Trichosurus vulpecula, was introduced to New Zealand, in several colour phases (Plate 2). It is the most widespread of the three brushtail possum species in Australia, found throughout eastern, central, and south-western wooded and forested areas.

BRUSHTAIL POSSUM

Trichosurus vulpecula (Kerr, 1792)

Synonym T. fuliginosus Ogilby, 1831. For further synonyms see Jackson.²⁰⁸

In New Zealand, the common name was formerly ‘opossum’,⁶¹,²²⁵ but this invited confusion with the entirely different American opossum, Didelphis sp. In Australia, the more frequent usage is ‘possum’³⁹⁶ and the standard common name adopted by the Australian Mammal Society is ‘common brushtail possum’³⁶¹ or, where it is the only resident species, as here, simply ‘possum’.

Description

The possum has a thick bushy tail (Latin trichosurus = hairy tail), a pointed snout and long, fox-like tapering ears (Latin vulpecula = little fox), and a darkly stained sternal gland. Size and weight vary greatly around New Zealand (Tables 2.1, 2.2).

There are two general colour forms, grey and black, each with much variation.

Greys are mainly a clear grizzled grey on the back and sides of the body, with the face pale grey, darker around the eyes and sides of the snout, and white at the base of the ears. The outer sides of the limbs are a uniform grizzled grey, often darker medially and posteriorly. The chin is dark, but the throat, chest, belly, and inner sides of the limbs are white or dirty yellow. The sternal gland stains the fur a dark rusty red, more prominently in males than females; juveniles have only a faint grey streak. The fur around the pouch of females and at the base of the tail is often stained brownish. The tail has bushy grey fur changing abruptly to black at about the midpoint (the extreme tip may be white in ~5% of animals), and naked on its ventral surface for the last 50–80 mm.

Blacks are generally deep umber-brown tinged with rufous, paler on the forequarters, sides and below, darker along the posterior back. The ears have little or no white at the base, and the tail is nearly wholly black (though occasionally also with a white tip). Staining from the sternal gland is less obvious than in greys.

Table 2.1: Geographical variation in bodyweights of adult possums in New Zealand.

Latitude in decimal degrees.

a. SF, State Forest; NP, National Park.

b. N, native forest; F, farmland with scrub; E, exotic forest.

Table 2.2: Linear measurements of the possum in New Zealand (mean ± s.d.).

Latitude in decimal degrees.

Data from Voller,⁴⁰⁵ Clout et al.,⁷² Triggs³⁹⁵ and Brockie⁴¹.

Possums moult continuously but with one major period of shedding,²⁷² which in the Orongorongo Valley is in August–November.¹⁰⁰

The iris of the eye is brown, the vibrissae long and well developed. The hand and foot both have five digits, each with a strong curved claw and pronounced palmar pads and striations. The second and third digits of the foot are syndactylous (i.e. joined for most of their length) and bear elongated claws used in grooming. The first toe is well developed, opposed to the rest of the toes, and has no claw. There is a fully formed pouch with two mammaries and forward-directed opening, corresponding to Type 5 parental care as defined by Russell.³³⁹ The testes descend permanently into the scrotum, which is pendulous and prominently situated anterior to the penis.

Possums have three basic gaits:²¹⁶,⁴³² walking, along horizontal and up gently inclined surfaces; half-bounds, jumping from branch to branch or up steeply inclined surfaces or in long grass; and bounds, up steeply inclined and vertical surfaces, such as tree trunks.¹⁶⁹ The prehensile tail maintains its grip until all feet have shifted from one branch to the next. Possums can swim, reluctantly; one was sighted off Kapiti I., rafting on a floating log.²²⁴

The skull is stout and heavily built. The nasals are smoothly convex above, with a shallow nasal notch; the forehead is flattened; the interorbital region is narrow, concave along its centre, its edges sharply ridged; the palate is fenestrated; the anterior palatine foramina run back level with the canines; the posterior palatal vacuities extend from the back of the first molar nearly to the hind edge of the palate, bounded only by a narrow transverse strip of bone; the bullae are low, flattened, and scarcely inflated.

Chromosome number 2n = 20.¹⁸¹

Dental formula I 3⁄2 C 1⁄0 Pm 2⁄1 M 4⁄4 = 34.

Field sign

Tracks (‘pads’ or ‘runs’) are often evident where possums emerge from forest out on to pasture to feed, and are also visible in forest when possum numbers are high.³¹⁷ Trees frequently bear extensive surface scratches. Bark biting, usually a series of horizontal scars on a variety of native and introduced trees and shrubs, may have social importance as a visual sign.²²⁶ Faecal pellets are usually ~15–30 mm long, 5–14 mm wide, crescent shaped, slightly pointed at the ends, and found singly or in groups; colour and texture vary with diet. Leaves browsed by possums have torn rather than cut edges, with the midrib and lower part of the leaf often partly remaining, unlike insect browse.²⁵³ Possums are destructive feeders, leaving the ground littered with broken branches, discarded leaves, or partly eaten fruits. Dark brown urine trails may be seen, particularly if possums have been feeding on kāmahi (Weinmannia racemosa) or five-finger (Pseudopanax arboreus) leaves.³⁵

Measurements

Males and females differ little in size or weight.¹³⁹ In nine separate studies, mean adult female weight was 98.6% that of males, but the asymptotic weight (and presumably size) of males calculated from growth curves consistently exceeds that of females.⁶⁹

Variation

Subspecies. Six subspecies are recognised.²⁰⁸ Only T. v. vulpecula from south-eastern Australia, and the larger-sized and more robust T. v. fuliginosus from Tasmania, were brought to New Zealand.³¹⁴

Body size. (See Tables 2.1, 2.2.) In the North I., populations on farmland (2.36 ± 0.03 kg) are significantly lighter than those from native or exotic forests (2.54 ± 0.06 kg). North I. populations in general (n = 13; 2.45 ± 0.04 kg) are significantly lighter than South I. ones (n = 17; 3.04 ± 0.08 kg). The north–south cline in body and skull size and bodyweight is closely correlated with mean annual temperature, as predicted by Bergmann’s Rule.⁴³⁸

Colour. Black possums predominate in the Kaimai Range, Urewera NP and East Cape in the North I., and in Westland, Fiordland and western Southland in the South I., many of which are high-rainfall areas.³¹³ Greys predominate on farmland and in drier, open country on both main islands, though there are exceptions where founder effects remain; e.g. only black possums were introduced to main Chatham and Codfish islands. There have been no major changes in the broad pattern of coat colour variation since the 1950s.

The New Zealand fur trade currently recognises four different colours of skins.³⁷ Natural dark brown and red-brown are varieties of the brown/black form, while rusty and pale are varieties of grey. Red-necks, a variety of rusty, are usually grey, with a pronounced browning of the shoulder fur. The grey and black forms are probably the only true-breeding colour types; pales can be found with brown offspring, and browns with pale offspring. Most fur is now traded as plucked (loose) hair, rather than full pelts, and spun with merino wool into luxury garments (p. 58).

Dentition. Highly variable. The frequency of occurrence of the most common variants, missing canines or first premolars in the upper jaw, is often higher in New Zealand (8–45%) than in Australia (8%).¹⁰⁰ I² is often minute; other very small teeth (probably Pm1 and Pm3) may also be present in front of the lower premolar; and missing or extra incisors and molars are frequent. Only 49% of 350 skulls from Banks Peninsula had the ‘standard’ dental formula.¹⁶⁴

History of colonisation

Possums were introduced to establish a fur trade similar to that flourishing in Australia at the time. The first successful release was in forest behind Riverton, Southland, in 1858 by C. Basstian, at the same place where possums had been unsuccessfully released some 20 years earlier.³¹⁴ Most importations were made by the regional acclimatisation societies, particularly between 1890 and 1900. The total imports numbered only ~200–300 individuals; more than half (58% of recorded shipments) were from Tasmania, and 74% of all imports were the black form. Tasmanian blacks, particularly favoured for their large size and superior fur quality, were all released in the South I., except for a few taken to Paraparaumu and Lake Waikaremoana. Greys imported from mainland Australia (Gippsland and New South Wales) were released primarily in the Catlins district and near Dunedin and Auckland.

The sources and patterns of these introductions have partly determined the present pattern of possum variation in New Zealand. For example, there is still a strong negative correlation between the percentage of black possums in a population and the proportion of its gene pool that came from mainland Australia.³⁷⁷ Mixing between populations originating from mainland and Tasmanian introductions may still be incomplete in some areas, such as Hawke’s Bay.³⁴⁶

Australian stock accounts for only ~8% of recorded releases. The consequent spread of possums was accelerated by additional releases of the New Zealand-bred progeny of the original introductions. About half of these had official government approval by the acclimatisation societies and private individuals equally, and the rest were illegal, i.e. the unrecorded efforts of private individuals. Assisted dispersal of possums was most vigorous between 1890 and 1940. Illegal releases were particularly common from 1920 to 1940,³¹⁴ and occasionally continue.²²³

The release and dispersion of possums in New Zealand were strongly influenced by fluctuating public attitudes and government policy. Initially possums were considered to be ‘creating a valuable industry’,⁴²³ but in the southern Longwoods and the Catlins, there was extensive uncontrolled trapping for skins by 1890. To restrain the destruction of this valuable resource, the government in 1889 (urged by the Southland Acclimatisation Society) brought the possum under the Protection of Animals Act 1880. Complaints by settlers near Riverton of damage by possums to crops and pasture (apparently in support of a case for continued trapping) then prompted the government to enquire in 1891 from the governments of New South Wales, Victoria and Tasmania for ‘any information as to the destructions of the opossum in orchards, gardens, etc.’. All replies were similar to that from Tasmania, that ‘the damage done by opossums … is very small, and amply compensated by the commercial value of their skins’.⁴²³

The conflict of interests between the acclimatisation societies and farmers, orchardists and conservationists resulted in several contradictory changes of legislation. Increasing trapping of possums prompted the 1911 declaration of possums as imported game under the Animal Protection Act 1908. In 1912, protection was withdrawn after pressure from possum trappers. In 1913, further outcry from the acclimatisation societies resulted in an Order in Council granting possums absolute protection in practically all the bush-covered districts of New Zealand. Wholesale poaching remained rife, however, and thousands of skins were exported, nominally as rabbit skins.³⁸⁹

After continuing debate, in 1919 the government requested Professor H.B. Kirk to investigate possum damage to forests. Kirk²³² concluded, ‘The damage to New Zealand forests is negligible and is far outweighed by the advantage that already accrues to the community’, and ‘Opossums may, in my opinion, with advantage be liberated in all forest districts except where the forest is fringed by orchards or has plantations of imported trees in the neighbourhood’. He recommended a May–July open season under licence. In 1921, legislation laid down the procedure for the taking of possums, and prohibited the harbouring and release of possums without permission from the Department of Internal Affairs (DIA). Skins could be sold only to licensed dealers, only stamped skins could be sold, and a levy was charged for stamping. The administration of these provisions was partly controlled by the acclimatisation societies, for which they received part of the royalties.

After 1922, there was increasing evidence of the detrimental effects of possums on native forests. DIA took a firm stand and, despite Kirk’s recommendations, refused to sanction any of the repeated applications for further releases.³¹⁴ Uncontrollable illegal releases therefore increased greatly. The general tide of opinion eventually swung against protection of possums, and in 1946–47 all restrictions on the taking of possums were cancelled, penalties for harbouring and release were increased, and limited poisoning was legalised. For post-war control operations, see p. 63.

Distribution

World. The brushtail possum is endemic to mainland Australia, Tasmania and some offshore islands.³⁶² It has the widest distribution of any marsupial in Australia. Numbers are increasing in many urban areas in Australia³⁵⁸ and in Tasmania following conversion of woodland to pasture.⁸⁸ In the arid regions of Australia, however, possum populations are being reduced by fox and cat predation and by changes in the fire regime.¹⁵⁷

New Zealand (Fig. 2.1). Possums are found throughout the North I., except for the upper slopes of Mts Taranaki and Ruapehu, and in a few parts of northern Auckland province. Northland was completely colonised only in the 15 years before 2004. Possums are widespread in the South I., except in the wettest parts of south Westland and western Fiordland, and in the upper catchments of a few rivers in South Canterbury and north-west Otago.⁶⁸ Dispersal continues, e.g. possums invaded the upper Copland and Karangarua Valleys only in the late 1970s.³⁷⁰ In the 1990s, possums were scarce south of Haast and absent south of the Arawata River, except around Milford Sound, but were spreading down the west side of Lake Te Anau and into the south-west of the South I. By 2017 possums were widespread through most of Fiordland, having colonised the south-western area over the past 20 years. They were found at high densities in coastal areas north of Milford Sound, and at moderate densities around Lakes Te Anau, Manapouri, Hauroko and Monowai. They remained patchy and at low densities along the western area between Doubtful Sound and Puseygur Point, and were still absent from most of the Mt Forbes peninsula, south of Doubtful Sound. Their distribution on the previously unoccupied western side between Doubtful Sound and Milford is not well known.⁴³⁰

Figure 2.1: Historical range expansion of possums up to 1998, with dates and locations of some of the original releases.⁶⁸ For distribution on islands, see Table 2.3.

Table 2.3: Distribution and history of possums on the offshore and outlying islands of New Zealand and islands within inlets, lakes and harbours.

List includes some islands of <5 ha which may still support native species vulnerable to possum browsing, and some islands accessible only at low tide.

Latitude in decimal degrees. Compiled by Cowan,¹⁰⁰ Parkes²⁹² and Dobbins¹²⁴.

?. not known; –, not now present; +, present at (date) and current status unknown; ++, established, self-sustaining population; *, eradication completed (date).

Possums in urban environments inhabit domestic and commercial buildings, domestic gardens, city parks and reserves, and other areas with suitable vegetation or artificial cover. In Hamilton, possums were detected most often and in highest abundance in vegetated gullies, were relatively rare in amenity parks, and not detected in residential areas, perhaps because of land occupier or city council control.²⁶³

Possums were introduced to or colonised 19 offshore and outlying islands, including Stewart and main Chatham, plus nine islands in harbours and inlets and five islands within lakes (Table 2.3). One introduction failed (Auckland I.); one was soon exterminated by residents (D’Urville I.). Established populations have subsequently been eradicated from four out of the five lake islands, seven out of the nine harbour and inlet islands, and 14 offshore and outlying islands of a wide range of sizes (1–2321 ha). Since 1990, five new incursions onto islands by possums have been recorded, all subsequently eradicated.

Habitat

Cover plus a suitable and varied food supply are the possums’ only requirements. Hence they are found in a diverse range of habitats excepting only the highest rainfall and most mountainous parts of south-west Fiordland.¹⁷⁴ They live in all types of indigenous forest from sea level to the tree line; in montane scrublands and tussock grasslands; throughout the introduced and indigenous grasslands; in exotic forests, shelter belts, orchards and cropping areas; in thermal regions, swamp, and pakihi country; in sand dunes and urban and city areas. Forests are the major habitat, especially mixed hardwood forests, where possum densities are higher than in beech or exotic pine forests; forest/pasture margins often support very dense populations. Possums tolerate rainfall from 350 mm to >8000 mm per year, and elevations up to 1800 m and 2400 m, respectively, in the South and North I. mountain ranges, regularly foraging above the snow line in beech forest.

Food

Possums are best described as opportunistic herbivores, feeding mainly on leaves. They also eat buds, flowers, fruits, ferns, bark, fungi and invertebrates, which at times comprise most of the diet. Kean and Pracy’s early study²²⁸ listed more than 70 native tree species, 20 ferns and a few vines and epiphytes, as well as grasses, herbs, and sedges eaten by possums. This list has subsequently been extended,¹⁷⁴,²⁷⁸ particularly to podocarps such as Hall’s totara (Podocarpus hallii) and pahautea (Libocedrus bidwillii),²⁷⁸ and a wider range of fungi.⁴³³ Weeds such as briar (Rosa rubiginosa) and willow (Salix fragilis) are important foods in dry grass/shrubland ecosystems.¹⁶⁷ Possums in alpine areas eat mostly woody plants and herbs.²⁹³ Cultivated grain and vegetable crops, horticultural produce, introduced ornamental shrubs, and flowers are also eaten.³¹⁵ Possums readily eat meat, especially native birds and their eggs,⁴⁷ and land snails;⁴⁰⁶ and they scavenge deer and pig carcasses, including those infected with bovine tuberculosis (bTB).

Possums have pronounced preferences for some plants relative to their availability, although such preferences may be inconsistent between areas.¹⁴⁸,³⁶⁹ Local diet varies because, in any one region, they tend to concentrate on only a few species and particular individual trees.⁸³,¹²⁷,¹⁴⁷ Selection is influenced by the species composition of vegetation, except that some, e.g. fuchsia and mistletoes, are preferred wherever they are available.³⁰⁵,³⁴⁸,³⁷³ Other factors include the relative abundance of a given plant in the local habitat; plant growth forms; physical and nutritional properties of leaves, e.g. leaf age, digestibility and toxins; and the availability of other foods, e.g. fruits. Possum diet and foraging patterns are also influenced strongly by secondary metabolites and antifeedant chemicals in leaves.¹¹⁸,²³⁴–²³⁶,²⁴⁸,²⁶⁹ A field assay of food or food-based products for bait development suggested protein content was the most important determinant of possum food choice,²⁰⁶ and the severity of browsing on forest trees was related to the available nitrogen concentration of foliage (a measure of in vitro digestible