Isoodon obesulus ( SHAW, 1797 )

Driessen, Michael M. & Rose Abstract, Robert K., 2015, Isoodon obesulus (Peramelemorphia: Peramelidae), Mammalian Species 47 (929), pp. 112-123 : 113-120

Isoodon obesulus ( SHAW, 1797)

Southern Brown Bandicoot

Didelphis obesula Shaw, 1797:298 . Type locality unknown; restricted to Ku-ring-gai Chase National Park, N of Sydney, N. S. W. [New South Wales, Australia], 33 o 36’ S, 151 o 16’ E, 1.5 km S of Hawkesbury River, 7.5 km from Coal and Candle Creek Road turnoff on West Head Road, based on Neotype designation by Dixon (1981:132–133).

Perameles obesula: É. Geoffroy Saint-Hilaire, 1804:64 . Name combination.

Perameles fusciventer Gray, 1841:407 . Type locality “King George Sound,” Western Australia.

Perameles affinis Waterhouse, 1846:373 . Type locality “Kangaroo Point near Hobart, Van Diemen’s Land [Tasmania].”

Isoodon nauticus O. Thomas, 1922:678 . Type locality “ Franklin’s Island , Nuyts Archipelago, S. Australia.”

Isoodon obesulus O. Thomas, 1922:678 . First use of current name combination.

Isoodon peninsulae O. Thomas, 1922:679 . Type locality “ Utingu, Northern Cape York,” Queensland.

CONTEXT AND CONTENT. Context as for genus. Isoodon obesulus has 2 extant subspecies ( Groves 2005):

I. o. nauticus O. Thomas 1922:678 . See above.

I. o. obesulus ( Shaw 1797:298) . See above; affinis (Waterhouse) , fusciventer (Gray) , and peninsulae (Thomas) are synonyms.

NOMENCLATURAL NOTES. Isoodon is derived from the Latin, “iso” meaning equal and odon referring to teeth. All teeth in Isoodon are peg-like and about similar in form. The Latin word obesus means plump or fat, whereas ulus is a diminutive suffix ( Strahan 1981). Thus, the specific name refers to its plump appearance. Other common names include short-nosed bandicoot, southern short-nosed bandicoot and, in Western Australia, quenda ( Kirsch 1968). Although considered as a synonym in Groves (2005), peninsulae may eventually be regarded as a form of I. auratus ( Gordon et al. 1990) .

DIAGNOSIS

Isoodon obesulus has a thick-set body with a short conical muzzle, small ears, short tail, and a grizzled brownish-gray dorsum streaked with black to give an agouti appearance. Digits 2 and 3 of the pes are joined (syndactylous) except at the clawed tips, and claws, if present, are strong. Among the bandicoots ( Isoodon , Perameles , Peroryctes , Echymipera , Microperoryctes , and Rhynchomeles ), Isoodon and Perameles are the most similar in appearance. Isoodon is larger and heavier than Perameles and has shorter more rounded ears that when laid forward barely reach the eye. The ears of Perameles cover the eye when laid forward.

GENERAL CHARACTERS

Isoodon obesulus is compact and robust, with a pelage of fine, short, and bristly hairs that is glossy, almost sleek, in living animals ( Fig. 1 ). Most hairs are banded (hispid), being light at the roots, blackish in the middle sections, and yellowish at the tips, producing a grizzled look to the pelage ( Wood Jones 1924). The hairs of the underfur are dark gray, but lighter at their tips. The yellowish-gray venter results from grayish bristle hairs and nearly white underfur. The comparatively short tail (ca. 25–30% of the head and body length in specimens from Tasmania) is brownish above and white below. In high density populations in Western Australia, the tail is often bitten off and reduced to a stub (C. Dickman, pers. comm.). The small rounded ears are covered with fine yellowish-gray hairs. The eye is black. The brown rhinarium is naked with a midline groove and laterally cleft nostrils.

The vibrissae are numerous and well developed. The mystacial set lies in 5 rows with hairs dark at the base and pale at the tips. The genal set contains 6 long vibrissae and there are> 12 others on each side. The manus has a digital formula of 3> 2> 4> 5> 1; the latter is rudimentary and the smallish 5 is without a claw. Digits 2, 3, and 4 have ill-defined pads, are fusiform and well developed, with strong claws. The sole of the pes, like that of the manus, is coarsely granular and naked. The digital formula of the pes is 4> 5> 2 = 3> 1; the 1st digit is rudimentary and clawless. Digits 2 and 3 are joined except at their clawed tips (= syndactyly) and the well-developed and strongly clawed digits 4 and 5 also are more or less conjoined.

The rear-opening abdominal pouch has 8 nipples arranged in an incomplete circle. Mean external measurements (mm; ranges in parenthesis) of male and female I. o. obesulus , respectively (n = 23, 12; measurement obtained from museum specimens by RKR), from Tasmania were: total length, 500.2 (325–612), 406.1 (340–428); length of tail, 132.0 (120–157), 125.1 (110– 195); length of hind foot, 69.8 (57–78), 59.0 (45–67); length of ear, 33.4 (27–38), 30.7 (25–36). Measurements of males (n = 38) and females (n = 27), respectively, from Victoria were: total length, 411.3 (310–575), 381.2 (255–460); length of tail, 118.9 (94–149), 113.0 (85–145); length of hind foot, 56.9 (44–75), 53.6 (38–64); length of ear, 29.9 (24–36), 29.7 (20–37) .

The “fairly massive” skull of I. obesulus ( Wood Jones 1924) , with prominent ridges and elongated bullae, has a shortened rostrum compared to other bandicoots and cheek teeth are reduced in size, giving the appearance of a dentition of equal-sized teeth (= isoodon; Fig. 2 ). Cranial measurements (means; mm) for males (n = 11) and females (n = 8), respectively, from Tasmania an asymptote at about 600 (n = 7—Lobert and Lee 1990). In I. o. nauticus from Franklin Island, South Australia, 139 adult males weighed 595 g and 143 females 527 g ( Copley et al. 1990). were: greatest length of skull, 74.9, 67.5; width of cranium, 15.6, 14.5; width of zygomatic, 31.8, 28.5; length of maxillary toothrow, 24.3, 23.2; length of molariform teeth in mandibular toothrow, 25.7, 24.3. During tooth development, the replacing premolar develops terminally on a dental lamina lingual to the deciduous premolar in both jaws (Fosse and Risnes 1972). In the upper jaw this lamina is isolated, whereas in the mandible it remains connected to P2.

Sexual size dimorphism in I. obesulus becomes apparent by the 1st breeding season, and by the time they are 2 years old males may be 40–50% heavier than females. I. obesulus in Tasmania grows faster and weighs more than those from Victoria. In northern Tasmania ( Heinsohn 1966), 8 males weighed 1,166 g and 5 females 947 g, whereas in southern Tasmania ( Mallick et al. 1998a), males weighed 1,245 g (n = 49) and females 1,001 g (n = 78). By contrast, in southern Victoria, both the heaviest male (1,150 g) and female (730 g) were smaller than averagesized Tasmanian I. obesulus and growth of females reached

DISTRIBUTION

At the time of settlement by Europeans, Isoodon obesulus was common throughout much of peripheral mainland Australia, from the Sydney region southward and westward to Adelaide, with isolated populations in the extreme northeastern section of the Cape York Peninsula in Queensland ( Gordon et al. 1990), southwestern Western Australia ( Friend 1990), and on the island state of Tasmania ( Hocking 1990). Since then, distributions have been shrinking, in response to loss of habitat, introduced predators, and other factors (Fig. 3). The destructive role of introduced predators is seen in the selective predation by red foxes ( Vulpes vulpes ) on I. obesulus in Western Australia ( Dickman 1988) and by the increases in sizes of I. obesulus populations after fox control programs were initiated in the west ( Kinnear et al. 2002). In New South Wales, so few post-1970s records exist (except for some in the Sydney region, all near the Victoria border) that I. obesulus is considered to be rare ( Ashby et al. 1990). In New South Wales, it occurs primarily in Ku-ring-gai Chase and Garigal National parks just north of Sydney and in the far southeast corner including Ben Boyd National Park and Nadgee Nature Reserve (Department of Environment and Conservation [NSW] 2005). In between these areas, the species has been found in a small number of national parks and state forests. Except for populations in the Grampian and Dandenong ranges, I. obesulus is restricted to coastal or fluviatile plains across the width of Victoria (Menkhorst and Seebeck 1990). In the Grampians, populations exist at elevations up to 1,000 m. The species is absent from Victorian islands despite large tracts of apparently suitable habitat. I. obesulus is widespread in Tasmania, having been recorded either as road-killed or trapped specimens virtually across the state except in the central plateau ( Hocking 1990). The greatest concentrations seem to be in the southeastern quarter and the northern tier ( Rounsevell et al. 1991), areas with rich soils and moderate rainfall. Although persisting on only 1 island (West Sister Island) in the Furneaux group of Bass Strait islands, subfossils are known on at least 3 other islands; some survived to the 20th century on the largest, Flinders Island ( Hope 1973). Subfossil remains also are known from 2 of the western Bass Strait islands ( Hope 1973; Bowdler 1984), but not from the largest, King Island, the most distant among all islands off the north coast of Tasmania.

In South Australia, I. obesulus is now found in forest and woodland habitats on Eyre Peninsula, in the Mt. Lofty Ranges, and on the Fleurieu Peninsula, in the extreme southeastern section of the state, and on Kangaroo Island ( Kemper 1990; Paull 1995), where it is most secure, in the absence of red foxes. I. obesulus also is known as subfossils from several sites in coastal South Australia. I. o. nauticus is known from Franklin and St. Francis Islands and as subfossil material from Flinders Island, all in South Australia ( Kemper 1990). In Western Australia, the pattern is similar: shrinking distributions, surviving populations ever more coastal, and subfossil locations indicating broader distributions in the past ( Friend 1990). Populations currently are found in areas of high rainfall in the southwest and south coasts of Western Australia. In Western Australia at least, its disappearance is directly linked to habitat loss; where I. obesulus survives, it is common where wetlands and waterways provide adequate cover, areas which lie almost entirely within the 1,000 mm isohyet ( Friend 1990; Cooper 1998).

Isoodon obesulus in Queensland is poorly understood, but its known locations (slated for preservation in a national park) overlap with the distributional range of Isoodon macrourus (northern brown bandicoot— Gordon et al. 1990). The only other location where 2 species of Isoodon are sympatric is in the poorly surveyed Kimberley Range of Western Australia, where I. macrourus and I. auratus , the golden bandicoot, potentially overlap in large areas ( Friend et al. 1991). A recent discovery in the Lamb Range extends the distribution of I. obesulus 350 km from the nearest contiguous populations ( Pope et al. 2001).

FOSSIL RECORD

The recent ancestry of Isoodon obesulus can be traced to fossils from several late Pleistocene and Holocene localities, including Victoria Cave in southeastern South Australia, dated more than 50,000 years ago (Archer and Hand 1984) and Black’s Point Sinkhole, a 4,000-year-old record from Venus Bay, South Australia ( McDowell 1997). Two local faunas from southwestern Western Australia include fossils of I. obesulus: Mammoth Cave (70,000 –37,000 years ago) and Devil’s Lair (35,000 – 12,000 years ago). I. obesulus fossils are known from 3 caves in south-central Tasmania, all of late Pleistocene age and 1 cave with specimens dated to 2,500 years ago ( Cosgrove et al. 1990). Fossil remains, including those from Pleistocene deposits of Darling Downs, southeastern Queensland ( Price 2004), indicate that I. obesulus formerly lived throughout southeastern, central, and northwestern Queensland ( Archer 1978; Muirhead and Godthelp 1996), far more widely than at present.

Holocene fossils dated <4,000 years ago are from the Fromm’s Landing local fauna in South Australia (Archer and Hand 1984). Although extant only on West Sister Island of the Furneaux Island group and absent from Ranga Cave (8,000 years ago) deposits on Flinders Island, subfossils are known from sand dunes on Flinders, Cape Barren, and Passage Islands of Tasmania ( Hope 1973). Subfossils are known from 6 of 7 sites on the basalt plains of western Victoria, including both cave and aboriginal (beach midden) localities, Pyramids Cave deposits in eastern Victoria, and from McEachern’s Cave in extreme southwestern Victoria ( Wakefield 1964, 1967a, 1967b).

Based on 12S rRNA sequence data, Isoodon diverged from Perameles in the late Miocene, about 8 million years ago (Westerman and Krajewski 2000).

FORM AND FUNCTION

The skull of Isoodon obesulus is notable for its slender rostrum. The dental formula is i 5/3, c 1/1, p 3/3, m 4/4, total 48. Although the name Isoodon , meaning “teeth of equal size,” is not literally true, the canines and cheek teeth are smallish, giving that appearance. All teeth have roots; hence, these bandicoots can be aged by degree of tooth wear.

Comparisons of variation in size and shape of skulls from 5 recent (<100 years) and 5 subfossil (“a few thousand years”) sites in southwest Western Australia reveal skulls of modern I. obesulus to be larger, with relatively longer snouts, and showing greater geographic variation than subfossil representatives ( Cooper 2000).

The caecum is capacious and descending, and both upturned and pointed at the tip. The large intestine is short and wide (Hill and Rewell 1955).

Basal metabolic rates (BMRs) for I. obesulus held at 5°C increments of ambient temperatures (T a) from 10–35°C showed the typical mammalian responses of increasing basal metabolic rate from its low point in the thermal neutral zone (TNZ: 25–35°C) to more than double (2.4 times) that value at 10°C ( Larcombe 2002). Basal metabolic rate is 64.6% of that predicted for a placental mammal of comparable size, near the 70% value typical of all marsupials (Dawson and Hulbert 1970). The increase in oxygen consumption between 30 and 10°C was achieved by a 2.3-fold increase in tidal volume and a 30% increase in breathing rate. By contrast, the efficiency in the uptake of oxygen (26– 27%) did not change over this range of temperatures.

Maximal running speed of 2 adult I. obesulus (mean body mass = 718 g) was 14.3 km /h ( Garland et al. 1988). When undisturbed and seemingly undetected, I. obesulus sometimes hops in a manner similar to rabbits. Galloping, 1 of 4 gaits recorded on film, occurred at 3.73 m /s or 14.4 km /h). At this speed, the length of a stride approaches 1 m, or 3–4 body lengths ( Moloney 1982).

Field metabolic rates were 644 kJ/day for a 1.23 kg I. obesulus in autumn and 459 kJ/day for a 517 g individual in spring. The rates of water flux for these 2 bandicoots were 109 and 40 ml/day, respectively ( Nagy et al. 1991).

The pH of I. obesulus blood, 7.36–7.58, was in the range of 5 other marsupials, as were levels of 5 blood gases. The values of 5 serum electrolytes (Na +, K +, Ca 2+, Mg 2+, and Cl−) likewise were similar to values for 9 other marsupials. Levels of organic constituents in the blood and of serum proteins were comparable to those of the 11 other examined marsupials ( Parsons et al. 1971).

A body temperature (T b) of 32.4°C was recorded for captive I. obesulus using deep rectal thermistors (Guiler and Heddle 1970). Results by Larcombe (2002), who held animals for ≥ 3 h at different temperatures before recording body temperature, suggest that I. obesulus is slightly (by as much as 2.6°C) hypothermic at ambient temperatures of 10, 15, 20, and 25°C. At 30 and 35°C, at the upper end of the thermal neutral zone, body temperature was 35.0 and 36.0°C, respectively. The temperature of the scrotum (24.5°C) in an anesthetized male was 4.9°C below body temperature ( Setchell 1977), but less temperature differential was detected when a thermistor was used directly on testicular vessels (Guiler and Heddle 1970).

Studies of visual pathways in I. obesulus have revealed that retinal projections to the lateral geniculate nucleus (LGd), an important processing center linking retina to visual cortex, differ from those of other polyprotodont marsupials (Haight and Sanderson 1990). One difference is the proportionately smaller volume of the lateral geniculate nucleus, compared to other marsupials, suggesting that binocular vision is poorly developed in I. obesulus .

ONTOGENY AND REPRODUCTION

In Isoodon obesulus , each testis has 6 single loops and one 3-limbed loop in the seminiferous tubule, with 15 connections between the tubules and the duct draining the testis ( deBurlet 1921; Woolley 1990). An 1,155 g male had a combined testes mass of 4.4 g, or 0.38% of body mass (Tyndale-Biscoe and Renfree 1987). A 790 g male had an estimated 48 million sperm per epididymis, or 25 million per gram of testis ( Bedford et al. 1984). Scrotal diameter (mm) reached an asymptote beyond 500 g, indicating fertility by that body size ( Reese 2001).

All peramelids are polyestrous, with spontaneous ovulation; the mean ovulation rate is 20–21 days in I. obesulus . After ovulation, the granulosa cells of the follicle are penetrated by cells of the theca interna, which fill the central cavity of the developing corpus luteum ( O’Donoghue 1916). After 4 days, growth of the corpus luteum is due entirely to hypertrophy of luteal cells (Tyndale-Biscoe and Renfree 1987).

In I. obesulus , gestation is short (<15 days by most accounts but as few as 12 days in Ullmann [1981]) and parturition occurs during the peak of the luteal phase. If lactation follows, the corpora lutea remain large and seemingly functional for about 40 days of the 60-day lactation period. The follicular phase is suppressed by lactation. After day 45, the corpora lutea decline rapidly in size, coincidental with declining progesterone levels, and new follicles grow; ovulation occurs soon after (Tyndale- Biscoe and Renfree 1987).

The 1st endoderm cells appear in I. obesulus blastocysts 1.0– 1.5 mm in diameter and the blastocysts are fully bilaminar by 1.5–1.9 mm (Hollis and Lyne 1977), estimated at day 6 (Lyne and Hollis 1977). Like other peramelids (but not other families of marsupials, except the Phascolarctidae [koala]), I. obesulus has both yolk sac (choriovitelline) and allantoic placentae. During the later stage of gestation, the chorioallantois forms a discoidal placenta with a long, thick umbilical cord (Tyndale- Biscoe and Renfree 1987).

The mean number of pouch young is variable and always many fewer than the 8 teats can accommodate: 2.1 on both Franklin Island and Belair National Park, South Australia ( Copley et al. 1990; Reese 2001), 1.57 and 2.58 in 2 years of study in Victoria (Lobert and Lee 1990), 3.0 in another study in Victoria (Stoddart and Braithwaite 1979), 2.8 in northern Tasmania ( Heinsohn 1966), and 3.05 for 23 litters in southern Tasmania ( Mallick et al. 1998a). Although litters of 2–3 are most common, as many as 5 have been reported ( Mallick et al. 1998a). Because females can have up to 4 litters per year and can live for nearly 4 years, a long-lived female could produce up to 32 young during her lifetime (Lobert and Lee 1990). Litter size decreases (by unknown mechanisms) as the pouch young age, although handling during study may contribute to this pattern. In one study, litter size increased linearly with female body mass (Stoddart and Braithwaite 1979). An analysis of growth patterns of I. obesulus , using the Gompertz growth model (Cockburn and Johnson 1988), revealed a decelerating growth rate throughout ontogeny. However, this deceleration is mitigated by its indeterminate growth. Instantaneous relative growth rates calculated during the final 42 days of lactation revealed a decrease in overall growth rates from 14.0% increase in body mass per day in early-phase, to 7.8% per day during mid-phase, to 3.5% daily increase during late-phase lactation (Duffy and Rose 2007).

In Tasmania, young are born from July to February, and females can have up to 4 litters per year ( Heinsohn 1966). In Victoria, where breeding ends in December, females can have as many as 3 litters in a year (Lobert and Lee 1990). In a study spanning 10 years in Western Australia, litters were recorded in every month ( Thomas 1990). Females of I. o. nauticus likewise lactate year-round and at least 4 litters are produced annually by some females ( Copley et al. 1990). Importantly, young born early in the year can mature and breed in the year of their birth, an unusual feature for marsupials. Thus, with rapid maturation and up to 4 litters per year, I. obesulus has a high reproductive potential, rivaling that of some rodents.

Neonates weigh 350 mg, develop quickly, permanently exit the pouch at about 53 days, and are weaned a week later (Lobert and Lee 1990). Probably as a result of more efficient transfer in the chorioallantoic placenta, I. obesulus releases from the teat and its eyes open at one-half (or less) the age of marsupials of comparable size: 135–140 days in the carnivorous eastern quoll ( Dasyurus viverrinus ) and 147 days in the herbivorous longnosed potoroo ( Potorous tridactylus ). These features contribute to the rapid maturity (3–4 months) in I. obesulus .

The peramelid lactational strategy is a contributing factor to the exceptionally rapid rates of preweaning growth and development. In early-phase milk of I. obesulus (before day 30), the levels of protein, lipids, and solids (100/ml) were 10.5, 10.4, and 27.3 g, respectively, but by late-phase (days 46–60), the respective values were 11.3, 26.4, and 52.3 g. Thus, although lipid levels comprise about 65% of total energy (kJ/ml) early, by late lactation this proportion has increased to> 80%, the protein levels having remained almost constant from week to week (Duffy and Rose 2007).

Although earlier studies yielded no reports of breeding in Tasmania during the months of January to April, 2 of 4 females had pouch young in March of 1 year and 4 of 6 in March of another year ( Mallick et al. 1998a). In Tasmania, females mature in 4–5 months (but females as young as 3–4 months with ≥ 50 mm pes length had pouch young— Mallick et al. 1998a) and males in 6 months, so both sexes can breed in the year of their birth ( Heinsohn 1966). In Victoria, maturity is not reached until 7 months (Lobert and Lee 1990). The pattern of continued growth in adult life is best recorded in populations from Cranbourne, Victoria (Lobert and Lee 1990). Measurements of body mass and length of pes indicate that although females seem to reach an asymptote at about 600 g (aged 18–24 months), they do continue to grow throughout life (a maximum of 3.5 years in that population). Likewise, many males continue the progression of slow growth throughout adult life.

Although not found in I. obesulus nauticus from Franklin Island, South Australia ( Copley et al. 1990) or in small samples from Victoria (Lobert and Lee 1990), the positive correlation between maternal body mass and litter size suggests that female condition (due to seasonal changes in food availability) can have important population consequences. Breeding may cease when females have lost mass ( Heinsohn 1966) or during drought periods ( Mallick et al. 1998a), but photoperiod may be important too in determining the onset of breeding ( Heinsohn 1966; Stoddart and Braithwaite 1979). I. obesulus in temperate locations (as in Victoria and Tasmania, 37– 42°S) or at> 1,000 m elevations breeds seasonally, whereas populations from Queensland (29°S) breed year-round, perhaps in response to adequate food supplies. Several authors speculate that both number and size of litters are related to food abundance ( Heinsohn 1966; Lobert and Lee 1990; Mallick et al. 1998a).

Minimum survival rates of males are always higher than of females, the greatest difference being in the 1st breeding season (60% versus 40%—Lobert and Lee 1990). Only 2 young were recruited at 2 study populations during a 3-year drought period in Tasmania ( Mallick et al. 1998a), suggesting reduced levels of reproduction or poor survival of young under these conditions. Differential survival is evident even during pouch life. In I. obesulus nauticus from Franklin Islands, South Australia, sex ratios of pouch young favored females (1:1.41) but were near parity or favored males as adults ( Copley et al. 1990). This pattern can be explained if the 50% higher survival of males during the first 8 months of life (Lobert and Lee 1990) applies to the Franklin Island population. In the Franklin Island population, only 45 of 167 marked pouch young were trapped as juveniles, representing either mortality or trap avoidance of 73%; 22% were lost before weaning so the greater disappearance occurs during the 1st weeks of independence ( Copley et al. 1990). Seasonal survival in this island population was highest in winter, the period of greatest rainfall. Life span in the wild is 3.5–4.0 years (Lobert and Lee 1990), and in Tasmania 1 individual was at least 3.75 years old based on the time between its first and last capture ( Mallick et al. 1998a).

ECOLOGY

Isoodon obesulus is a generalist omnivore, eating adult and immature insects of several orders, spiders, isopods, earthworms, fungal sporocarps, a range of monocot and dicot roots, and ferns in Tasmania ( Mallick et al. 1998b). Common foods were eaten in proportion to their availability, except that crickets and blackberries were selected preferentially in season ( Mallick et al. 1998b). In a study in northern Tasmania, I. obesulus consumed mainly invertebrates; the only plant material eaten was the fruit of boxthorn ( Heinsohn 1966). However, in another dietary study in southern Tasmania ( Quin 1988), I. obesulus consumed many plant items, such as grasses, seeds, clover root nodules, plus fungi and a range of invertebrate foods. In Western Australia, where 77% of invertebrate prey items were> 5 mm long, insects comprised 52% of the diet by volume, arachnids and oligochaetes 6% each, and myriapods and crustaceans 5% each (Broughton and Dickman 1991). In Queensland, invertebrates were a major component of the diet of I. obesulus , contributing 35–56% of fecal contents; roots represented the most important plant food, with grasses, forbs, fruits, and hypogeous fungi also eaten but in small quantities (Keiper and Johnson 2004). The substantial activity of trehalase and cellobiase in the mucosa of the small intestine suggests that both insects (trehalose is a disaccharide found only in insects) and plant material (cellobiose is formed during cellulose breakdown) are digestible by I. obesulus ( Kerry 1969) .

Isoodon obesulus is preyed upon by a range of predators. On Franklin Island, South Australia, barn owl ( Tyto alba ) and tiger snake ( Notechis scutatus ) are major predators ( Copley et al. 1990), as are cats ( Felis catus ), red foxes, eastern quolls, and feral dogs elsewhere ( Godsell 1983; Dickman 1988; Mallick et al. 1997). In the jarrah forests of Western Australia, I. obesulus was present more frequently than expected compared to other medium-sized mammals in the diet of the chuditch ( Dasyurus geoffroii ), indicating its susceptibility to predation ( Glen et al. 2010). Bones of I. obesulus were found in 7 of 72 red fox scats collected at the Royal Botanic Garden-Cranbourne near Melbourne, Victoria, and constituted 5.5% of diet items (Coates and Wright 2003). Recent field studies have shown that I. obesulus does not respond to the odors of either native or introduced predators, further indicating its vulnerability to predation (Russell and Banks 2005; Valentina et al. 2010).

Potential competitors include the eastern barred bandicoot ( Perameles gunnii ) in Tasmania ( Heinsohn 1966; Mallick et al. 1998a). The only area of mainland Australia where a congener of I. obesulus is present is in northeastern Queensland, where I. macrourus is sympatric.

In Victoria, I. obesulus inhabits heathlands, shrubland, and open forest and woodland, where it is associated with welldrained soils and “dry” heath communities (Menkhorst and Seebeck 1990). Although older heath communities seem to be preferred (18-year-old—Braithwaite and Gullan 1978; 12-yearold— Opie 1980), sufficient ground cover may be more important than species or age of dominant plants ( Lobert 1985). Fire interval also may be important in determining the quality and availability of suitable habitat (Stoddart and Braithwaite 1979). In high rainfall areas of northeastern Queensland, I. obesulus prefers relatively dry sclerophyll woodlands characterized by high abundance of grass tree ( Xanthorrhoea johnstonii ) and high shrub cover in the understory (Keiper and Johnson 2004). In Western Australia, I. obesulus living in the jarrah ( Eucalyptus marginata ) forests is significantly larger in body mass than those inhabiting reed swamps ( Cooper 1998). Factors controlling these size differences were demonstrated to be genetic, based on breeding and growth studies conducted in the laboratory ( Hale 2000).

Its widespread distribution within Tasmania suggests that I. obesulus has broad habitat tolerances. Hocking (1990) describes it as primarily occupying dry sclerophyll forest, scrub, and healthland communities, especially those with dense ground cover of shrubs ( Leptospermum and Melaleuca ), in long matrush ( Lomandra longifolia ), or in agricultural areas dominated by such introduced plants as brambles ( Rubus ), gorse ( Ulex europaeus ), or boxthorn ( Lycium ferrocissimum ). High population densities also are associated with pastures near dense cover ( Heinsohn 1966) but in southern Tasmania, most captures were recorded in forests or at their edges and few (<10%) in pasture ( Mallick et al. 1998a). The I. obesulus apparently quickly colonizes recently logged and burnt forest patches, benefiting from the dense ground cover and food resources in early seral stages ( Green 1982).

It is likely that production of invertebrate prey, made possible by abundant rainfall, affects the distribution as well as abundance of I. obesulus . Trapping results in Gippsland (southeastern Victoria) reveal that I. obesulus is found in lowland sites <50 km from the coast, sites near watercourses with moist soils, and in association with heath or woodland vegetation ( Opie et al. 1990). In Western Australia, I. obesulus distribution corresponds to the 1,000 mm isohyet, and its historic distribution (primarily near coastlines) suggests an association with adequate rainfall for production of plant and animal biomass. In brief, I. obesulus might be much less tolerant of drought than many marsupials.

Home ranges of I. obesulus vary depending on method of analysis and population density. Home ranges mostly in the 1–5 ha range (Lobert 1990) and population densities of 1–5 per ha (Lobert and Lee 1990) support Moloney’s (1982) speculation that territories, if they exist at all, are in the form of spatiotemporal systems based on avoidance. At Cranbourne, Victoria, both male and female I. obesulus were solitary and occupied home ranges of 0.8–3.0 ha at densities up to 5 per hectare (Lobert and Lee 1990). In southern Tasmania, nonoverlapping home ranges were reported for both sexes ( Mallick et al. 1998a). Home ranges of 6.95 ha for males and 3.28 ha for females in southern Tasmania ( Mallick et al. 1998a) are similar to those from northern Tasmania, 6.6 and 2.3 ha, respectively ( Heinsohn 1966). By contrast, on Franklin Island, South Australia, the values for males and females were 2.1–2.2 and 1.5–1.6 ha, respectively ( Copley et al. 1990). In Western Australia, where home ranges for males averaged 2.34 ha and those of females 1.83 ha, supplemental food led to a 35% increase in home range (Broughton and Dickman 1991).

Six species of ectoparasitic mites from 2 families and 5 genera are reported by Green (1989): Gymnolaelaps annectans , Haemolaelaps flagellatus , H. marsupialis , Mesolaelaps antipodianus , Myonyssus decumani , and Neotrombicula novaehollandiae . An intraerythrocytic protozoan parasite, Hepatozoon , is known from I. obesulus in Western Australia ( Wicks et al. 2006). Recently, a novel virus, named bandicoot papillomatosis carcinomatosis virus type 2, was obtained from an I. obesulus in Western Australia; it caused multicentric papillomatous lesions which responded to treatment during rehabilitation ( Bennett et al. 2008).

BEHAVIOR

There are conflicting reports on when Isoodon obesulus is active within the 24-h day. Radio-tracking studies conducted over the winter in southern Victoria revealed a diurnal pattern (Lobert 1990), which conflicts with Heinsohn (1966), Moloney (1982), and others, who found I. obesulus to be strictly nocturnal in Tasmania. In one study, I. obesulus emerged 52.2 min after sundown and remained active for 6.26 h (with 1 exception— Moloney 1982).

In the laboratory, I. obesulus becomes active at dusk and has a single period of activity lasting 6–8 h ( Armstrong et al. 1990). Continuous video recordings confirmed that I. obesulus was almost entirely nocturnal, emerging about 30 min after sundown and staying active for about 7 h ( Larcombe 2003). Little time was spent grooming (8 min) or drinking (2 min) each day. With the reversal of light-dark (LD) cycles, most animals reentrained within 8–15 days ( Armstrong et al. 1990). However, 1 or 2 did not entrain, and persisted with their original cycle in this unstable entrainment for 50 days in the reversed light-dark cycles, indicating (as suggested by Lyne’s [1981] studies of 2 other peramelids) that bandicoots may have different circadian pacemakers than other mammals.

Isoodon obesulus makes a nest of shredded vegetation, usually set in a slight depression but always in a well-hidden and camouflaged location ( Stodart 1977). In dense vegetation and with no obvious entrance, such nests are difficult to detect.

Isoodon obesulus spends most of its active period feeding. Feeding activity consists of “nosing” the ground to detect subterranean food items by olfaction ( Stodart 1977; Moloney 1982; Quin 1985). Using its forepaws, it then scratches conical depressions in the soil, up to 8 cm deep ( Mallick et al. 1998b). Based on a survey of 196 excavations, I. obesulus diggings averaged 48 mm deep and 35 mm in diameter ( Moloney 1982). The role of olfaction in detecting subterranean prey was examined by pulverizing cockroaches and then burying them (and controls) in cups placed at different depths ( Quin 1985). Although olfaction was important in prey detection, auditory trials revealed that hearing was not.

Studies of prey-killing behavior in an arena revealed that frogs were universally avoided but lizards were approached and attacked within 5 s and killed 2 s later, with ingestion completed within 30 s ( Moloney 1982). Lab mice were investigated and sniffed immediately; the 1st attempts to pin with the forepaws usually were unsuccessful but the average I. obesulus pursued and administered a killing bite to the head within 53 s. Consumption followed while I. obesulus was in a crouched position. Newly hatched domestic chicks were approached, pursued, and killed in a similar manner but the average time was 2 min 57 s. Laboratory rats and 19-day old chicks were pursued but rarely killed. Vertebrates usually were eaten head first. The forepaws never conveyed food to the mouth but sometimes were used to hold down (stabilize) prey ( Moloney 1982). Although I. obesulus can drink water with rapid thrusts of the tongue, drinking is rare because dew, water in their prey, and metabolic water usually fulfill their needs. Even in the lab, I. obesulus spends little time drinking ( Larcombe 2003).

Isoodon obesulus moves about independently, although areas of use tend to be restricted, probably due to the distribution of food resources. In studies of spatial discrimination, I. obesulus performs as well as laboratory rats ( Rattus norvegicus — Buchmann and Grecian 1974), perhaps indicating a rapid ability to learn under specific conditions. Mutual avoidance among individuals, except when breeding, was observed by Moloney (1982). After a testing period of approaches, contacts by 2 male I. obesulus always led to aggression, naso-nasal sniffing, retreating, and naso-anal sniffing, followed by the assumption of the agonistic posture position by the dominant male. The aggressor then relentlessly pursued the subordinate, striking with forepaws and biting, usually on the rump ( Moloney 1982). Female–female interactions and heterosexual pairings were characterized by mutual avoidance. Estimates of home range size and population density support the view that I. obesulus is intolerant of its kind except at breeding time.

Of the 3 types of autogrooming activities ( Moloney 1982), the rapid raking movements of the hind feet to scratch the snout, ears, chest, neck, shoulders, dorsum, and flanks, movements interspersed with gnawing and licking the syndactylous claws, are most common. Face-washing behavior, wiping the muzzle with forepaws which are then licked as if to clean, never extends posterior to the ears and frequently follows eating. A 3rd behavior, one that frequently increases in association with rainfall, is gnawing and licking all accessible parts of the pelage ( Moloney 1982).

Subauricular cephalic skin glands in both sexes of I. obesulus expand enormously during the breeding season, especially in males, causing the upper surface to bulge outwards, become rugose, and form pits at the bases of hair follicles ( Stoddart 1980). When an animal is handled or frightened, the gland complex weeps profusely, suggesting a role in communication, perhaps useful at courtship.

The aggression of captive animals toward one another frequently is accompanied by grunts and squeaks ( Wood Jones 1924). When disturbed, I. obesulus emits sneezing and spitting noises and immediately seeks cover ( Heinsohn 1966).

In intraspecific encounters, a grunting and squeaking dominant I. obesulus pursues and eventually overtakes its equally vocal subordinate and jumps on its back, striking with its claws and removing patches of hair. Subsequent scratching by the pursuer using its forefeet removes additional hair ( Wood Jones 1924). Although the hair is readily scratched off, the skin of I. obesulus is exceedingly tough (R. K. Rose, in litt.) and hence is impossible to open with the clawed feet of aggressors. In this way, dominance is established without killing or maiming the subordinate animal. Other evidence of aggression is seen in the scarred, shortened, and missing tails of some I. obesulus . The size of defurred areas is positively correlated with population density and body mass, and males have significantly more fur loss than females ( Thomas 1990). In the laboratory, hair grows back partially in 4 weeks and more fully in 8 weeks ( Wood Jones 1924).

GENETICS

In Isoodon obesulus , the diploid number (2n) = 14. Autosomes consist of 3 pairs of large metacentric or submetacentric chromosomes, 1 pair of medium-sized metacentric chromosomes, and 2 pairs of small chromosomes, one of which may have a satellited short arm ( Sharman 1961). Chromosomes are similar in both subspecies, with the X chromosome showing the greatest morphological variation (Hayman and Martin 1974).

All Peramelines have XX:XY sex-determining mechanisms but in I. obesulus , no somatic cell has the full chromosome number due to the loss of an X chromosome in females and the Y chromosome in males, leaving all somatic cells XO (Hayman and Martin 1965).

Discriminant function analyses and a phylogeny of Isoodon based on mtDNA control region sequences reveal 2 groups, I. macrourus and I. obesulus / auratus ( Pope et al. 2001) . The latter is not distinct for mtDNA, the phylogenetic divergence being no greater than between subspecies of I. obesulus . These authors speculate that I. obesulus and I. auratus once were 1 contiguous species now separated as isolated populations that have diverged in response to their diverse environments. Similarly, phylogenetic reconstruction, using mtDNA and microsatellite data, showed little support for the maintenance of I. obesulus and I. auratus as different species ( Zenger et al. 2005). No support was found, using 12S rRNA analysis, for separating animals from Tasmania and Victoria (Westerman and Krajewski 2000).

Of 29 genetic loci examined electrophoretically, 16 were variable in Isoodon ( Close et al. 1990) . The geographic patterns of I. obesulus show some interesting relationships based on these variable loci: the nominate subspecies from South Australia is most similar to animals from Western Australia, but animals from Barrow Island (Western Australia) are more similar to those from Victoria, New South Wales, and Tasmania than to those from the nearby Western Australian mainland. Cape York (Queensland) I. obesulus is almost equally distant from all others but shares 3 unique alleles with conspecifics from Western Australia. Analysis of 12S rRNA sequence data provided little support for keeping Tasmanian I. obesulus distinct from mainland forms (Westerman and Krajewski 2000), contributing to the decision of Groves (2005) to reduce the number of subspecies to 2.

CONSERVATION

Like those of other peramelids, the distribution of Isoodon obesulus has shrunk substantially since European settlement of Australia. Populations in New South Wales and South Australia are listed as endangered and vulnerable under respective State legislations, and I. o. obesulus is listed as “Endangered” under the Federal Environment Protection and Biodiversity Conservation Act of 1999 (Australian Government Web site: www.deh.gov. au/biodiversity/threatened/species). Threats include changed fire regimes, predation by introduced predators (especially foxes and cats), habitat fragmentation, and land clearing. The application of population viability analysis (PVA), used to evaluate the most effective management/conservation methods of the endangered I. obesulus in Victoria, revealed the importance of including multiple habitats when developing population viability analysis models ( Southwell et al. 2008). I. obesulus is considered secure in Queensland, Tasmania, Victoria, and Western Australia as the species is not listed in these states. However, the illegal introduction of the red fox to Tasmania in 2001 or 2002 and its potential establishment could lead to a decline in I. obesulus populations similar to those seen in the past century on mainland Australia following red fox introductions. Poison campaigns using sodium monofluoroacetate (Compound 1080) to kill red foxes have enabled I. obesulus populations to rebound in several places on mainland Australia, an indication of the efficiency of these predators ( Kinnear et al. 2002; Murray et al. 2006). Underpasses, intended to connect fragmented populations of I. obesulus separated by new highways, can lead to declines when red foxes use the same underpasses ( Harris et al. 2010). In April 2015, the International Union for Conservation of Nature and Natural Resources status for the species was listed as “Least Concern,” with a “Decreasing” population trend.

Figures

Fig. 1.—An adult male Isoodon obesulus from Tinderbox, Tasmania, in May 1979. Used with permission of the photographer, Hans Wapstra.

Fig. 2.—Dorsal, ventral, and lateral views of skull and lateral view of mandible of an adult female Isoodon obesulus (Tasmanian Museum and Art Gallery, Accession Number A705) from Lymington, Tasmania, 9 June 1965; A. Halton. Greatest length of skull is 63.9mm.