TS-CRC Student project - Ecology and conservation status of the Brush-tailed Rabbit-rat (Conilurus penicillatus)

Charles Darwin University, Darwin

Ron Firth

Background | Aims | Habitat | Shelter Requirements | Movements | Population | Diet | Other Top End populations | References |

PhD student Ron Firth tracks down an elusive Brush-tailed Rabbit-rat (Conilurus penicillatus) on NT’s Cobourg Peninsula.

PhD student Ron Firth tracks down an elusive Brush-tailed Rabbit-rat (Conilurus penicillatus ) on NT’s Cobourg Peninsula. Below, the brush-tailed rabbit rat.

The elusive rabbit rat

Photos: Ron Firth

Background

The Brush-tailed Rabbit-rat ( Conilurus penicillatus ) is the only extant member of its genus: its sole congener Conilurus albipes became extinct at the end of the 19th century before any but the most superficial of studies could be carried out on its ecology (Taylor and Horner 1971). There are three recognised subspecies of the Brush-tailed Rabbit-rat; C. p. randi (New Guinea), C. p. melibius (Bathurst and Melville Is) and C. p. penicillatus (Top End and Kimberley) (Kemper and Schmitt 1992).

In all, Australia has lost 17 species of mammals in the last 200 years since European colonisation (Woinarski and Braithwaite 1990; Smith and Quin 1996). Marsupials and rodents have suffered the highest rates of extinction (Short and Smith 1994; Braithwaite and Griffiths 1996), and especially so for those in the critical weight range, 35.0 g to 5500 g (Burbidge and McKenzie 1989).

Weighing around 150g, the Brush-tailed Rabbit-rat falls clearly within the critical weight range. The limited evidence suggests that the Brush-tailed Rabbit-rat is in decline in the Top End. It has not been trapped in the Alligator Rivers Region or mainland Arnhem Land for at least a decade (Woinarski et al. 2001). Early reports by naturalists such as Dahl (1897) and Collett (1897) noted that Conilurus penicillatus was common all over Arnhem Land. Tunney collected 40 individuals in 1902-03 (Thomas 1904). In the Top End it can still be found on Bathurst and Melville Islands (Tiwi Islands), and is patchily common on Inglis Island off Northeast Arnhem land (Woinarski et al. 1999b). The only known mainland population occurs on Cobourg Peninsula, where Frith and Calaby (1974) found it to be common in the Port Essington, Port Bremer area, as did Taylor and Horner (1971).

Aims

There is little known regarding the ecology of the Brush-tailed Rabbit-rat (Bradley et al. 1987; Friend et al. 1992). Its decline from its core mainland range to promontories and islands suggests a successional pattern of extinction. Consequently some long-term ecological information on this species is vital so that management plans and strategies can be devised and implemented to prevent further decline and possible extinction. This project aims to provide information concerning habitat preference/ use, shelter requirements, movements, population dynamics, life history parameters, diet and the possible causes of decline. Due to its relative accessibility and large population of Rabbit-rats, the study is mainly being conducted at Garig Gunak Barlu National Park (Cobourg Peninsula) and at the Mardugal campground in Kakadu National Park.

Habitat

On Cobourg Rabbit-rats appear to use an array of habitats from tall eucalypt woodlands dominated by Eucalyptus miniata, E. tetrodonta and E. nesophila with a sparse understorey through to Casuarina equisetifolia beach dunes. There also seems to be an association with perennial grasses from genera such as Eriachne and Chrysopogon and animals appear to be absent from areas with predominantly annual grasses such as annual sorgum.

Radio-tracking has shown that animals spend the majority of time foraging on the ground, only returning to trees when threatened or to nest during the day. Some preliminary habitat modelling of a large distributional data set from Melville Island indicates a correlation with tall eucalypt forests.

Shelter requirements

Radio-tracking Rabbit-rats has revealed that most animals shelter during the day in tree hollows and hollow logs. Most of the nest trees have DBHs (diameter at breast height) that are on average greater than 30 cm. Species of trees used to nest in include Eucalyptus miniata, E. tetrodonta, E. nesophila, E. bleeseri, E. porrecta, Erythrophleum chlorostachys and dead trees. Hollow logs are also used as dens with animals denning in hollow logs disproportionately more than was randomly available to them. Rabbit-rats also prefer trees that are significantly wider than the majority of trees available to them. Most animals use more than one nest site over periods of a week.

Movements

The movements of animals are monitored in two ways:

  • By looking at the captures and recaptures on the trapping grids (the trapping grids consist of 20 trap lines with 20 traps in each line and spaced 20m apart, so that a grid consists of 400 traps approximately 380m x 380m).
  • By radio-tracking animals at night and locating their nest sites during the day. The radio-tracking data from 41 animals is so far demon strating that animals are relatively sedentary with home ranges on average being 0.77 hectares and ranging from 0.10 to 4.40 hectares. Males also have significantly larger home ranges than females, with the largest male home range being over four times as large as the largest females home range.

Population dynamics

Population information is being provided by mark-recapture data from the trapping grids. So far population estimates for both of the trapping grids are around 100 animals per grid, which gives a density of approximately six animals per hectare.

Diet

Faecal samples so far analysed macroscopically indicate that the animals primarily eat plant stems, however a substantial proportion of the faecal samples also contained grit. Animals often appear to be foraging on bare ground when they are being observed, which can account for the presence of grit in their faecal samples. Some of the scats also contained root material and small amounts of invertebrate exoskeleton fragments. I have also observed one Rabbit-rat eating a relatively large spider, a grasshopper and some herbaceous species.

Other Top End populations

I was also part of a preliminary survey of Croker Island that failed to trap any Rabbit-rats.

However, Rabbit-rats have recently been re-discovered in Kakadu National Park. In 2001, Pete Christopherson and Sandra McGregor, local Aboriginal residents of Kakadu, reported their observations of unusual looking rats at the Mardugal campground to Scientist, Dr John Woinarski. At Peter and Sandra's suggestion, John's crew conducted a trapping survey at Mardugal and confirmed that the unusual rats were the Rabbit-rat. Subsequently, in July 2001, Michelle Watson and myself trapped another 13 Rabbit-rats at the Mardugal campground, confirming that a substantial population did live in the area.

And this year, in conjunction with Cobourg, I am also conducting parallel studies in Kakadu National Park. Results so far are indicating that the population is a relatively small and isolated one.

References

Bradley, A. J., Kemper, C. M., Kitchener, D. J., Humphreys, W. F., and How, R. A. (1987). Small Mammals of the Mitchell Plateau Region, Kimberley, Western Australia. Aust. Wildl. Res. 14, 397-413.

Braithwaite, R. A. and Griffiths, A. D. (1996). The Paradox of Rattus tunneyi: Endangerment of a Native pest. Wildlife Research 23, 1-21.

Burbidge, A. A., and McKenzie, R. L. (1989). Patterns in the modern decline of western Australia's vertebrate fauna: causes and conservation implications. Biological Conservation 50, 143-198.

Dahl, K. (1897). Biological notes on the North-Australian Mammalia. The Zoologist Series 4 1, 189-216.

Friend, G., Kemper, C., and Kerle, A. (1992). Rats of the tree tops. Landscope 8, 10-15.

Frith, H. J., and Calaby, J. H. (1974). Fauna survey of the Port Essington district, Cobourg Peninsula, Northern Territory of Australia. Technical Paper No. 28. (CSIRO Division of Wildlife Research: Canberra.)

Kemper, C. M., and Schmitt, L. H. (1992). Morphological Variation between Populations of the Brush-tailed Tree-rat (Conilurus penicillatus) in Northern Australia and New Guinea. Aust. J. Zool. 40, 437-52.

Short, J., and Turner, B. (1994). A test of the Vegetation Mosaic Hypothesis: A Hypothesis to Explain the Decline and Extinction of Australian Mammals. Conservation Biology 8, (2) 439-449.

Short, J., and Smith, A. (1994). Mammal Decline And Recovery In Australia. Journal of Mammalogy 75, (2) 288-297.

Smith, A. P., Quin, D. G. (1996). Patterns And Causes Of Extinction And Decline In Australian Conilurine Rodents. Biological Conservation 77, 243-267.

Taylor, J. M., and Horner, B. E. (1971). Reproduction in the Australian Tree-rat Conilurus penicillatus (Rodentia: Muridae). CSIRO Wildl. Res. 16, 1-9.

Thomas, O. (1906). On a collection of mammals made by Mr. J. T. Tunney in Arnhem Land, Northern Territory of South Australia. Novitates Zoologicae 11, 222-229.

Woinarski, J. C. Z., and Braithwaite, R. W. (1990). Conservation Foci for Australian Birds and Mammals. Search 21, 65-8.

Woinarski, J. C. W., Palmer, C., Fisher, A., Southgate, R., Masters, P., and Brennan, K. (1999). Distributional patterning of mammals on the Wessel and English Company Islands, Arnhem Land, Northern Territory, Australia. Australian Journal of Zoology 47, 87-111.

Woinarski, J. C. Z., Milne, D. J., and Wanganeen, G. (2001). Changes in mammal populations in relatively intact landscapes of Kakadu National Park, Northern Territory, Australia. Austral Ecology 26, 360-370.

Contacts

Mr Ron Firth
PhD Student
Tel: 08 8946 6814

Fax: 08 8946 6847

Charles Darwin University
Darwin, NT 0909