HMW 5 - Family text: Thylacomyidae (Greater Bilby)

Texto de familia: 

Class Mammalia

Subclass Metatheria

Order Peramelemorphia

Family THYLACOMYIDAE (Greater Bilby)

 

  • Fairly small ground-dwelling marsupial with long muzzle, very long rabbit-like ears, and well-furred, long tail with terminal tuft.
  • 50–85 cm.
  • Australasian Region.
  • Arid and semi-arid regions.
  • 1 genus, 1 species, at least 1 extant taxon.
  • 1 species Vulnerable; 1 species Extinct since 1600.

 

Systematics

Species of bilbies (now only a single extant species in the family Thylacomyidae) and bandicoots (now 18 species in the family Peramelidae) have had a long evolutionary journey together. The fossil history suggests the order containing these mammals spans more than 60 million years and includes at least five families, two of which no longer have living representatives. How the various families and species of marsupials developed within Australia between the late Cretaceous (66 million years ago) and the end of the Oligocene (23 million years ago) is exceedingly vague because of the scant fossil record. The soft sediments exposed near Murgon in south-eastern Queensland contain the oldest fossil remains of Australian mammals and are thought to provide a brief snapshot of life in the early Eocene (55 million years ago). They have revealed many different kinds of fishes, frogs, turtles, snakes, birds, and also mammals that show an affinity with South American marsupials of a similar age, including a bandicoot-like animal. If the estimated age of the sediments is correct, then the bandicoot is one of the oldest known Australian marsupials, and the group may have arisen elsewhere in Gondwanaland before entering Australia.

Bilbies and bandicoots are assigned to their own order, Peramelemorphia. In historic times, ten species of bilbies and bandicoot have been recognized from Australia and 13 species of bandicoot from the New Guinea region. Two species of bandicoot occur in both New Guinea and Australia. Eighteen species in six genera form the family Peramelidae. The recently extinct Pig-footed Bandicoot (Chaeropus ecaudatus) was the sole member of the genus Chaeropus in the family Chaeropodidae. Two bilby species in the genus Macrotis form the family Thylacomyidae: the living Greater Bilby (Macrotis lagotis) and the extinct Lesser Bilby (M. leucura).

The peramelemorphs have undergone little of the adaptive radiation and specialization seen in other marsupial groups such as the didelphids, dasyurids, and macropods and mammal groups such as the tenrecs (Tenrecidae) from Madagascar, which have been isolated for comparable periods. For example, there are species of macropods and dasyurids that have adapted to life in the trees and tenrec species that have adapted to aquatic, arboreal, and terrestrial modes of life. In contrast, all bandicoots and bilbies have remained small-to-medium sized mammals, with a conservative body plan and a similar approach to exploiting the environment.

Prior to the development of DNA gene sequencing, the relationship between bilbies and bandicoots had long vexed taxonomists. M. Archer and J. Kirsch proposed that bilbies warranted separate familial status. A taxonomic revision published by C. P. Groves and T. Flannery in 1990 and based primarily on the analysis of skull characteristics proposed that bilbies were closely related to the Australian short-nosed bandicoots (Isoodon spp.) and the Long-nosed Bandicoot (Perameles nasuta) and should be included in the family Peramelidae. Other studies using molecular biology helped unravel the relationships among groups, resulting in the nomenclature outlined by Groves in D. E. Wilson and D. M. Reeder’s Mammal Species of the World in 2005, followed here. Gene sequencing of nuclear and mitochondrial DNA suggests that the Greater Bilby and the Pig-footed Bandicoot separated from a bandicoot common ancestor in the late Oligocene or early Miocene around 26 million years ago.

At one time, there were several species described in the genus Macrotis, but these have since been merged to just two species: the Greater Bilby and the Lesser Bilby, which was endemic to the deserts of central Australia and is recently extinct. Allozyme and mitochondrial DNA sequencing of samples collected from populations across the current distribution of the Greater Bilby indicated very low levels of divergence, suggesting these populations are representative of a reasonably mobile single species.

 

Morphological Aspects

The Greater Bilby differs from bandicoots by having very long ears, silky fur, elongated limbs, and a long, conspicuously crested tail (50–70% of head–body length). In describing the Greater Bilby, H. H. Finlayson stated that it “...has carried a number of structural peculiarities to grotesque lengths yet manages to reconcile them all in a surprisingly harmonious, and even beautiful, whole.”

The Greater Bilby was distinguishable from the extinct Lesser Bilby by having a more contrasting pelage and a tail with black hair on the proximal one-half that changes abruptly to white. The tail of the Lesser Bilby was uniformly white, and the pelage was grayish or fawn-gray. The dorsal pelage of the Greater Bilby is blue-gray; the ventral surfaces are paler. Two fawn hip stripes may be evident.

Fully grown Greater Bilbies of both sexes can be more than twice the size of newly recruited sexually mature individuals. Large males are often twice the size of large females: males weigh 1000–2500 g and females weigh 800–1200 g. Sexual dimorphism in size was not as pronounced in the Lesser Bilby. From the few available records of Lesser Bilbies, males weighed 360–435 g and females weighed 310–312 g.

On the hindfeet of Greater Bilbies, the fourth toe is well developed and longest, with a claw. The fifth toe is the next longest but diminished compared with species of peramelids. The first toe is absent, and the second and third toes are fused at their bases, providing a neat comb for grooming. These syndactylous hindfeet are similar in form to those found on kangaroos. The forelimbs of the Greater Bilby are strong. On the forefeet, toes one and five are atrophied and clawless. The second, third, and fourth toes have stout claws to aid digging and burrowing.

Fresh tracks can be used to identify Greater Bilbies from other species in the field. The fifth toe on the hindfoot of a Greater Bilby is less evident in a spoor imprint than in species of peramelids. The Greater Bilby moves with an overstepping, bounding gait (cantering) similar to that of dasyurids, lagomorphs, and some other groups of animals. The hindfeet are placed in front of the front feet, with the back arched. Straightening the back launches the front feet forward, producing a pattern in which the hindfeet appear parallel and the front feet are staggered.

Bilbies and bandicoots, like dasyurids, are polyprotodont (having four or more incisors on each side of the jaw) and have a battery of sharp teeth. As with bandicoots, Greater Bilbies have five incisors, one canine, three premolars, and four molars on each side of the upper jaw. The same number of canine, premolar, and molar teeth occurs on the lower jaw, but there are only three incisors. The teeth are sharp and ideal for slicing the exoskeleton of invertebrates. In the Greater Bilby and Pig-footed Bandicoot, the molar teeth have become squarer, with fewer cusps, providing a better grinding surface to deal with fibrous plant material.

The stomach of the Greater Bilby, like that of bandicoots, is relatively simple, with modest development of the cecum into a fermentation chamber. Greater Bilby scats are reasonably distinctive. The scats are quite firm when dry, have rounded ends with a smooth coating, and can contain a lot of sand. They are relatively easy to find near diggings, where they are often covered over by soil thrown up by digging activity and may persist at a feeding site for months in the absence of rain.

 

Habitat

In the past, up to four sympatric bilby and bandicoot genera occurred in parts of the Eyrean (arid and semi-arid) region of Australia. This region supports significant areas of sparse woodland, shrubland, herbland (areas dominated by herbaceous vegetation), chenopod grassland, and hummock grassland. Sand plains, dune fields, broad paleodrainage lines, calcrete (calcium-rich hardened soil crust) areas, clayey downs, and residual landforms with lateritic or stony soils form significant and recurrent parts of the landscape.

Greater Bilbies may be found in any of these landscapes. Higher prevalence of Greater Bilbies has been recorded in association with drainage lines and residual landforms. Parts of the landscape, particularly the sand plain and dune habitats, can become unsuited for breeding (and act as a sink for the species) in the absence of suitable fire and rainfall conditions. A highly dispersed population can result.

Where the distribution of the Greater Bilby overlapped with the Lesser Bilby, habitat use was evidently partitioned. The burrows of Lesser Bilbies were found only in the sand dunes and never in the swales and flats, whereas the opposite was true for Greater Bilbies. It is unclear whether Lesser Bilbies were restricted to dune habitats, but the other records of Lesser Bilbies with reasonably reliable locality data also have sand dune habitat in the vicinity.

 

General Habits

Bilbies and bandicoots are mainly nocturnal, emerging from burrows or nests well after the sun has set and settling into nests well before sunrise. The Greater Bilby (and the extinct Lesser Bilby) is the only peramelemorph known to regularly construct and use burrows for shelter. Individuals use several burrows in a home range, and the choice of daytime refuge is varied at irregular intervals. Burrows exhibiting the greatest and most recent sign of digging activity are often not used and a less obvious, nearby burrow is occupied instead.

The burrows constructed by the Greater Bilby may reach a depth of more than 2 m and spiral laterally over a 4-m radius. No nesting material is used to line the burrow. In clayey soils or soils containing stony material like silcrete (silica-rich hardened soil crust) or laterite, the burrow localities are frequently reworked and multiple entrances become more common. The burrow localities in clayey and stony landscapes are often easy to recognize from a distance because the disturbed soil sports a healthy sward of grasses and forbs compared with the surrounding area. One Greater Bilby usually uses a burrow, but occasionally a male and female or two females will share a single burrow. The Lesser Bilby was reported by P. M. Byrne to use the inner extremity of its burrow during summer and to lie within a foot or so of the entrance during cold weather.

Greater Bilbies can forage in exposed regions as long as they can make a quick retreat to a burrow when necessary. The persistence of Greater Bilbies on the brown clay downs of south-western Queensland is testimony to their capacity to persist in moonscape-like environments where vegetation cover can be less than 1% during dry times. At the other extreme, Greater Bilbies are rarely encountered when vegetative ground cover exceeds 35% in hummock grasslands, possibly because it limits their capacity to move around freely and forage effectively.

 

Communication

Vocalization and visuals signals are not well developed in bilbies and bandicoots and reflect the unsophisticated social organization within the order. Greater Bilbies will sometimes hiss and gape when disturbed or when confronted with an unfamiliar individual. Anxious juveniles will squeak when they are taken away from their mothers. It has been suggested that the black and white flag-like tail of the Greater Bilby serves as a signaling function to attract attention or to distract a predator in pursuit. The lack of vocalization may be compensated through chemical rapport. A scent gland positioned behind the ear has been located in Greater Bilbies and some peramelid bandicoots. Greater Bilbies in captivity press and drag their cloaca along the ground at the entrance of a burrow. This behavior was primarily done by dominant males and probably used to express their presence and assert rank.

Greater Bilbies and bandicoots are usually relatively docile and non-aggressive when captured and handled carefully. Unlike bandicoots, Greater Bilbies exhibit very low levels of aggression or destructive fighting within or between sexes in captivity. Nevertheless, large males are not comfortable being housed together in close company. In contrast, captive bandicoots have been described as desperately pugnacious toward each other, with harassment and fighting to the death among individuals of the same and different sexes.

 

Food and Feeding

Greater Bilbies, like bandicoots, are omnivorous. Seeds, bulbs, fruits, invertebrates, vertebrates, and fungi can form a substantial part of the diet of the Greater Bilby. Seeds from quick-growing monocotyledonous and dicotyledonous plants that thrive with summer rain after fire or severe drought are prominent; the seeds of two grasses, Yakirra australiensis and Dactyloctenium radulans (both Poaceae), are particularly important to the Greater Bilby. Fruit from bush tomatoes (Solanum spp., Solanaceae) and the bulb of the sedge Cyperus bulbosus (Cyperaceae) can also form important dietary components of the Greater Bilby. This sedge can be found on the fringes of some drainage lines and salt lakes throughout central Australia.

The invertebrates consumed by Greater Bilbies include root-dwelling larvae, termites, spiders, and beetles. A number of these invertebrates are available even through drought periods. For example, termites feed on perennial grasses unpalatable to most other species, store this food, and live underground protected from environmental extremes. Likewise, a number of species of Acacia (Fabaceae) host root-dwelling larvae (Lepidoptera) during drought times. The shrub species targeted by Greater Bilbies are commonly associated with residual landforms. The fungi consumed by Greater Bilbies are predominantly from underground fruiting bodies known as hypogean sporocarps.

The diet of the Greater Bilby, based on scat volume, can range from predominantly herbivorous (up to 95%) to insectivorous (up to 80%), depending on seasonal conditions. The substantial use of material from ephemeral plants by Greater Bilbies is unusual compared with species of peramelids that have been studied. Invertebrates dominate diets of species of Perameles, Isoodon, and Echymipera, and the plant foods used are mainly from perennial plants that fruit or seed seasonally.

The ability to occupy recently disturbed habitat has also allowed Greater Bilbies to take advantage of fruits and seeds produced by ephemeral plants promoted by disturbance events such as fire and drought. Where fire is frequent within the distribution of the Greater Bilby, there their prevalence is linked to recently burned areas.

Greater Bilbies will devote much effort to find and extract a particular food morsel. Their diggings sometimes extend to a depth of 25 cm, and small shrubs can be undermined to extract larvae from root systems. In situations where the bulb of Cyperus bulbosus is harvested by Greater Bilbies, hectares of earth can be turned over, leaving the area with the appearance of a ploughed field. The diggings made by Greater Bilbies can often resemble those made by other species, except for those made at the base of shrubs for root-dwelling larvae.

 

Breeding

The mating system of the Greater Bilby is promiscuous, with a number of males moving over the home ranges of several females. There is no prolonged association between male and female. Mating occurs at night and may continue for a number of hours. Bilbies have been reported to mate belowground, in a burrow, but mating activity in the open has also been observed in captivity. DNA fingerprinting revealed that one male fathered seven of the eight offspring sampled at a Greater Bilby colony in south-western Queensland. Two of the resident males did not contribute to the offspring sampled.

Greater Bilbies and bandicoots are polyestrous, meaning they ovulate throughout the year and can produce several litters annually. The estrous cycle is estimated to be 20–26 days, with a range of 12–37 days. This is similar to the length of the estrous cycle in other marsupial species. Gestation of the Long-nosed Bandicoot and Northern Brown Bandicoot (Isoodon macrourus) is 12·5 days; the Greater Bilby has a slightly longer gestation of 14 days. Greater Bilbies and bandicoots do not have the ability to suspend development of the embryo or show embryonic diapause.

Young Greater Bilby and bandicoots are born at a relatively advanced stage for marsupials. Their brain and eye development are at a stage equivalent to some marsupials with twice the gestation length. At birth, the young make their way to a backward-opening pouch and attach themselves to one of eight nipples.

Hair on the chin of Greater Bilbies can be observed on day 40 and full pigmentation preceding pelage around day 55. Young are first seen out of the pouch 67–80 days after birth and weaned at about day 90. Mothers abandon their young in the natal burrow at the end of the weaning period. Up to three young are produced, but two are more common, with a mean litter size of 1·5 offspring in captivity. Single young weigh about 250 g, and triplets weigh 150 g each. Sex ratio of emerging pouch young is 1:1. In captivity, without the birth of a litter, some mothers will continue to suckle their young for longer than the normal length of time and, on rare occasions, will suckle the young of other females.

Sexual maturity may be attained at 6–7 months of age in females and 11–13 months of age in males. Although very small females are able to produce young, their young may not survive to exit the pouch. A female must be about 600 g before she can carry young to the point when they can exit the pouch. Some females in a reintroduced Greater Bilby population with access to supplementary food were able to breed continuously and managed to produce four litters per year.

Greater Bilbies appear to have a longer life expectancy than species of peramelids. A five-year-old captive-bred male and a 4·7-year-old female were both reproductively active until their deaths, both spending about 2·5 years under free-ranging conditions after being reintroduced. Captive individuals have been known to live more than ten years.

 

Movements, Home range and Social organization

Like bandicoots, male Greater Bilbies range over a much larger area than females.

Reintroduced free-ranging females may move up to 2 km between burrows on consecutive nights but generally move less than 1 km. Males move 2–3 km between burrows but occasionally as far as 5 km. Movements of a similar distance were recorded for individuals in a wild population in Queensland.

The highest densities reported for wild Greater Bilbies are 0·08 ind/ha on the Nullarbor Plain in 1922 and 0·12–0·16 ind/ha in south-western Queensland in 1984. Densities of this magnitude are unusual and not sustained for long periods. Periodic disappearance of the Greater Bilbies from localities is a regular occurrence in the Tanami and Great Sandy deserts. Many favored habitats of the Greater Bilby have densities less than 0·0001 ind/ha.

 

Relationship with Humans

The term “bilby” has now become widely accepted for living and extinct species of Thylacomyidae. In the past, Europeans referred to them as rabbit bandicoots, pinkies, pintoes, thulkies, and dalgytes. Aboriginal people of arid and semi-arid Australia have used a variety of names for the Greater Bilby. Often, one or more names were used in each language group. For example, “Ninu” or “Tjalku” were names used by the Warlpiri, Pintupi, and Pitjantjatjara people; “Ahrete” by the Arrente people; and “Marrura” by a number of language groups in north-western Western Australia. The Greater Bilby was once widespread across 70% of mainland Australia south of about 17° S latitude, from the coast of Western Australia to the Great Dividing Range in eastern Australia and sign of them was commonly encountered by early explorer naturalists.

Aboriginal people used bilbies as food, and great skill was applied in locating an active burrow and digging out individuals. The tails of Greater Bilbies were used to make the beards of old men appear longer, and they were also worn by women in their hair and attached to hair-string belts. W. B. Spencer noted in the narrative on the Horn Scientific Expedition to Central Australia that “Round the neck and arm may be worn an armlet or necklet made of the white tails of rabbit bandicoot ...,” “almost every native has one or more tassels …,” and “it is evident the rabbit bandicoot is much in request and that, in order to supply demand, it must exist in large quantities.” The tail tips of Greater Bilbies were traded for goods from outside the distribution of bilbies. European settlers also used Greater Bilbies as food. They were often caught in rabbit traps in the southern part of their range before they declined. Their pelts were marketed briefly in the early 1900s, but there was little ongoing demand.

Three decades ago, there was a strong campaign and broad support for the notion that the Greater Bilby should replace the Easter Bunny as a symbol of Easter in Australia. Given the devastation caused by rabbits in the Australian rangelands, it was felt that the rabbit provided an inappropriate symbol to be associated with Easter in Australia. The striking features and appeal of the Greater Bilby and similar nature to the rabbit made it a popular choice. Chocolate bilbies produced by boutique chocolate makers still emerge around Easter, but interest in the campaign has diminished over time.

Greater Bilbies have relatively simple husbandry requirements and can be readily maintained in captive breeding facilities. They are relatively docile in captivity if handled carefully, but they are not suitable as pets. They are nocturnally active, have an unrelenting interest in scratching and digging, climb well, and are apt to burrow under fences.

Determining the population size of Greater Bilbies in the field has long frustrated ecologists. Wild Greater Bilbies rarely venture into freestanding wire cage traps containing standard baits, but habituated captive-reared bilbies can be recaptured in cage traps using a bait of budgie seed. The other options for catching individuals are time consuming and include digging up individuals from burrows, as was done by indigenous people; constructing a wire fence around an active burrow and placing wire cage traps inside it; or capturing individuals by hand with the aid of a spotlight where ground cover is sparse. Burrow presence does not consistently equate with Greater Bilby activity.

Broad-scale surveys for the sign of Greater Bilbies using standardized techniques provide an effective means to determine distribution and assess occupancy and status. The presence of Greater Bilbies can be validated in the field by detecting fresh tracks, scats, and diggings for root-dwelling larvae at the base of shrubs and with the use of remote cameras. Diggings and burrows can be useful to flag potential activity of Greater Bilby, but this sign is unreliable to validate presence because of the similarity of some excavations to those made by other species. Estimates of occupancy can be derived by revisiting monitoring sites. Viable DNA material can be extracted from fresh scats, but a large number of samples are required to estimate population size using mark-recapture techniques.

 

Status and Conservation

A localized decline in the distribution of Greater Bilbies was reported as early as 1857 and was attributed to grazing pressure from livestock along the Murray River. Broad-scale decline of Greater Bilbies became evident in the early part of the 1900s and escalated from the south until the 1970s. By this time, wild populations of Greater Bilbies had become extinct in Victoria, New South Wales, and South Australia, and the total distribution had been reduced to 20% of the former estimated extent.

The rate of decline of the Greater Bilby has slowed since the 1970s, but its distribution has continued to decrease in the southern parts of Northern Territory and Queensland. The distribution on the southern edge of the Western Australian range has remained more stable. Wild populations of Greater Bilbies are now primarily found in parts of the Tanami Desert in the Northern Territory, and Great Sandy, Little Sandy, and Gibson deserts in Western Australia. Populations also occur in Dampier Land and the Pilbara in Western Australia. An outlying population persists in south-western Queensland.

There is more uncertainty about the distribution of the extinct Lesser Bilby, but it appears to have been more restricted to arid Australia. The species was recorded from three localities in far north-eastern South Australia, and specimens were also taken from localities north-east of Charlotte Waters and near Barrow Creek in the Northern Territory. The authenticity of the Barrow Creek records is considered by some to be questionable. Discussions with elderly Aboriginal people suggested a possible extension in distribution of the Lesser Bilby, including parts of the Gibson and Great Sandy Deserts of Western Australia and adjoining regions of the Northern Territory. Nevertheless, no subfossil evidence has been unearthed to support the proposed distributional extension. The last live specimen of the Lesser Bilby was collected in 1932 near Cooncherie in north-eastern South Australia, but in 1967, a skull was retrieved from a nest of a wedge-tailed eagle’s (Aquila audax) on the western edge of the Simpson Desert in south-eastern Northern Territory.

Bilbies and bandicoots were among the first mammals noted to disappear from the Australian landscape following European settlement, and since then, they have been negatively affected more than most medium-sized mammal species. Three Australian species have become extinct (the Pig-footed Bandicoot; the Desert Bandicoot, Perameles eremiana; and the Lesser Bilby), and only the Northern Brown Bandicoot and the Long-nosed Bandicoot are considered stable. The Greater Bilby is listed in Australia and on The IUCN Red List as Vulnerable.

With the arrival of people in Australia about 50,000 years ago, fire in the landscape became more frequent. This caused the structure and composition of the vegetation to become more dynamic and altered the extent of refuge and foraging habitat for many species. Bilbies and bandicoots also had to contend with hunting pressure from two new and wily predators, humans and the Dingo (Canis lupus dingo). Nevertheless, the suitability of the foraging habitat and the suite of responses available to avoid predators evidently remained adequate to allow all contemporary Greater Bilby and bandicoot species to survive. None is known to have gone extinct during this period and prior to European settlement.

With European settlement, clearing and land modification for agriculture and the spread of pastoralism began to widely alter the landscape. This was accompanied by the spread of introduced herbivores—particularly the European Rabbit (Oryctolagus cuniculus) and predators, primarily the Red Fox (Vulpes vulpes) and the domestic/feral cat (Felis catus). Fire patterns also changed as indigenous people moved to larger settlements. No one factor can be clearly identified as causing the recent decline of the Greater Bilby. The loss of Australia’s mammal fauna has been greatest in arid areas and least in areas of high rainfall. Species that forage on the ground were more at risk than those foraging above the ground in vegetation or on the wing (e.g. microbats, Microchiroptera). Species in the weight range of 35–5500 g, including all bandicoots and the Greater Bilby, have declined disproportionately more than larger or smaller species, and surface-nesting species like bandicoots have suffered disproportionately more than species using arboreal, rock-pile, or burrow refuges. Species with very large distributions were found to be no better off than those more restricted in distributions. In Australia, several bandicoot species now receive considerable attention from federal, state, and private conservation organizations, with the aim of reversing the declining numbers and distribution. Captive breeding populations of Greater Bilbies are also maintained by a number of institutions. Populations have been reintroduced to several predator-proof, fenced enclosures, including François Peron National Park (Western Australia), (New South Wales), Currawinya National Park (Queensland), and (South Australia), and translocated to Thistle Island (South Australia). Populations of Greater Bilbies generally increase rapidly in response to a rabbit-free and predator-free environment. Attempts to reintroduce viable free-ranging populations of the Greater Bilby at several locations have been unsuccessful except for a population at Lorna Glen (Western Australia) near the southern edge of its current distribution. This population has persisted for several years in an area where Red Foxes are scarce and feral cats are targeted in a regular baiting and removal program. More attention needs to be directed at conserving wild populations of Greater Bilbies using less intensive forms of management. Broad-scale fire management is being conducted in parts of the distribution of the Greater Bilby, but the effect on the viability of the extant populations is still unclear.