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Western Chestnut Mouse (Pseudomys nanus)

The Western Chestnut Mouse is a small nocturnal native rodent of northern Australia. It uses spinifex, coastal vegetation, tussock grass cover and some low eucalypt woodland habitats, so clearing, fire and removal of dense ground cover can affect shelter and breeding habitat on development sites.

Conservation status

Northern Territory, Territory Parks and Wildlife Conservation Act 1976: Near threatened.

Breeding season

Timing varies with rainfall, and breeding can occur throughout the year when conditions are favourable. The pattern below is an indicative northern Australian guide, not a substitute for site records and pre-clearing searches.

Month ratings are indicative only. Confirm local timing from rainfall, recent records, trapping results and pre-clearing searches.

  • Breeding: Breeding often follows the wet or latter monsoon season and can extend into spring. It may occur in any month after favourable rainfall.
  • Young: Juveniles and young non-breeding adults may be prominent in spring after seasonal population increase. The documents do not give a nest-departure or independence period.
  • Movement and dispersal: No migration period or dispersal distance is provided. Local movement is likely to track rainfall, food growth and the availability of dense cover.
  • Highest clearing risk: Risk is highest during and after wet-season rainfall when breeding, nesting and juvenile activity may increase. Clearing remains a risk in all months because breeding can be year-round and burrows and nests can be occupied at any time.

Identification

The Western Chestnut Mouse is a small, lightly built native mouse. It is most likely to be detected indirectly because it is nocturnal and spends the day in cover or burrows.

Adults have light brown fur on the back, light orange-brown fur on the flanks and a white underbelly. The limbs are short, the ears are small and rounded, and the tail is thin compared with the thicker tail of a Common Rock-rat.

On Barrow Island, the species is described as an island form that was formerly called the Barrow Island Chestnut Mouse. Genetic work cited in the source found that it is an island form rather than a true subspecies.

Head and body: 80 to 140 mm

Tail: 70 to 120 mm

Weight: Not provided in the supplied documents

Identification: Similar species

The Western Chestnut Mouse can be difficult to separate from the Delicate Mouse, Sandy Inland Mouse and Lakeland Downs Mouse. The chestnut mouse is about 100 mm long, with a tail of similar length, and has grizzled or chestnut-coloured fur with a paler ring around each eye.

The Delicate Mouse is lighter coloured and considerably smaller. The Sandy Inland Mouse resembles a House Mouse and occurs in drier inland areas. The Lakeland Downs Mouse has a shorter tail, about 60 to 70 per cent of its body length.

Females have four teats. This helps distinguish the species from the introduced House Mouse during examination of a captured animal.

Identification: Field signs

Look for small grass nests in dense ground cover and entrances to burrows. Burrows may form an extensive network and can reach about 1 m deep.

The supplied documents do not provide reliable descriptions of Western Chestnut Mouse tracks, scats, calls or eyeshine. Do not identify the species from a generic small-mammal sign alone. Use trapping, handling by an authorised person and diagnostic features where confirmation is required.

  • Inspect dense tussock grass, spinifex and coastal vegetation for small grass nests before clearing.
  • Check soil surfaces and cut faces for small burrow entrances and connected burrow runs.
  • Treat an occupied nest or burrow network as potential fauna habitat until a fauna spotter catcher has assessed it.

Distribution: Range by state and region

The species occurs across northern Australia, from north-west Western Australia through the Northern Territory to north-west Queensland. Records and descriptions in the supplied documents are concentrated in northern and north-western regions.

In Western Australia, the species occurs in the Kimberley and on some Pilbara islands, including Barrow Island. On Barrow Island it is described as widespread across the island.

In the Northern Territory, it occurs in northern regions and has been recorded in the Brunchilly area. The Brunchilly report noted that the available range information did not extend as far south as that project area, so those records were considered possible identification errors.

The supplied mammal guide extends the range eastwards to north-west Queensland and islands in the Gulf of Carpentaria.

Western Australia: Kimberley and some Pilbara islands, including Barrow Island

Northern Territory: Northern regions, with historical records near Brunchilly requiring verification

Queensland: North-west Queensland and some Gulf of Carpentaria islands

Bioregions mentioned or indicated: Kimberley, Pilbara, northern Northern Territory and Gulf of Carpentaria regions

Distribution: Population information

No national population estimate, local abundance estimate or population trend is provided in the supplied documents. The Barrow Island population is described as widespread, but this does not provide a population size.

National population size: Not reported

Population trend: Not reported

Known local status: Widespread across Barrow Island; four historical records were listed from one Northern Territory tenement

Habitat: Vegetation and structure

The Western Chestnut Mouse uses dense ground vegetation and sheltered sites in northern Australian environments. Habitat descriptions vary between regions, so assessment should focus on the actual vegetation structure and soil conditions at the site.

Primary habitat includes spinifex and coastal vegetation. Other described habitat includes low eucalypt woodland with dense tussock grass cover.

Important structural features are continuous grass or spinifex cover, dense tussocks, litter and soil suitable for burrowing. The species can occur across broad areas where these features remain available.

  • Map spinifex, dense tussock grass, coastal vegetation and low eucalypt woodland before clearing.
  • Record gaps in ground cover, recent fire, bare ground and areas of compacted or disturbed soil.
  • Retain connected patches of dense ground cover where practicable, particularly around active nests and burrow networks.

Habitat: Climate and elevation

The documents associate breeding and population increase with rainfall, flooding, wet season conditions and rapid vegetation growth. No elevation limit or precise climatic boundary is provided.

Elevation range: Not provided

Climate association: Northern Australian environments, with activity and breeding responding to rainfall and seasonal vegetation growth

Foraging habitat: Food and feeding sites

The Western Chestnut Mouse forages at night, mainly among grasses and ground vegetation. Available information on its diet is limited and differs between the supplied sources.

The Barrow Island guide states that the diet consists mostly of native grasses and seeds. A Northern Territory profile states that the diet was unknown at the time of reporting, so specific food plants should not be assumed without local evidence.

Foraging habitat is likely to be concentrated in spinifex, dense tussock grass, coastal vegetation and other areas with seed-producing ground cover.

Recorded food: Mostly native grasses and seeds

Foraging period: Night

Foraging range: Not reported

  • Retain native grass and spinifex cover around known nests, burrows and capture locations.
  • Avoid removing seed-bearing ground vegetation before fauna assessment and, where possible, undertake clearing when animals can be detected and relocated under approval.
  • Do not rely on supplementary food or water to compensate for removal of native foraging habitat.

Breeding habitat: Breeding biology

Breeding is strongly associated with rainfall and increased plant growth. Dense ground cover and nearby burrows or nests provide the conditions needed for young.

Breeding is reported during the latter part of the monsoon season and can extend into spring. Other information indicates that breeding may occur in any month when rainfall and food conditions are favourable.

Rapid population turnover has been reported, with young non-breeding adults and juveniles prominent in spring after favourable seasonal conditions.

Breeding season: Often the wet or latter monsoon season, extending into spring; potentially year-round when conditions are favourable

Litter size: Not provided

Gestation: Not provided

Age at maturity: Not provided

Lifespan: Not provided

Breeding habitat: Breeding habitat features

Breeding habitat includes dense tussock or spinifex cover, grass nests and burrow systems in suitable soil. Burrows can provide protection from fire and shelter young from surface conditions.

  • Inspect dense grass and spinifex before slashing, grading or burning.
  • Protect active nests and burrow entrances from vehicle traffic, trenching, stockpiling and soil compaction.
  • Schedule disturbance outside periods of recent rainfall and high juvenile activity where the project program allows, while recognising that breeding can occur throughout the year.

Sheltering habitat: Nests and burrows

The species uses grass nests and underground burrows. Shelter is closely linked to dense ground cover and soil that can support burrowing.

The species sleeps during the day in a grass nest. It also uses an extensive network of burrows, reported to reach about 1 m in depth.

Burrows may help the species survive fire, but the supplied documents do not describe the number of entrances, chamber dimensions or home-range size.

Day shelter: Grass nests

Burrow depth: Up to about 1 m

Other refuge dimensions: Not provided

  • Search beneath dense grass and spinifex for nests before vegetation removal.
  • Inspect burrow entrances in sandy or friable soil, tussock bases and other sheltered ground-cover patches.
  • Fence or clearly mark confirmed nests and active burrow networks until the site ecologist or fauna spotter catcher has completed an assessment.

Threats: Clearing and fragmentation

The main project risks are removal of dense ground cover, destruction of burrows and nests, inappropriate fire and introduction or increase of predators and competitors. Evidence for some threats is general to northern Australian mammals rather than specific to this species.

Clearing spinifex, tussock grass, coastal vegetation or low woodland with dense grass cover removes shelter and foraging habitat. Grading, trenching and compaction can destroy burrows, including burrows about 1 m deep.

Fragmentation can leave small isolated patches of cover and increase exposure to vehicles, predators and edge disturbance. The supplied documents do not provide a minimum patch size or movement distance.

  • Complete targeted searches for nests and burrows before clearing.
  • Keep machinery, laydown areas, spoil and access tracks out of retained dense ground cover where practicable.
  • Use clearly marked no-go areas around confirmed nests, burrow networks and capture locations.
  • Record the amount and condition of retained and removed tussock, spinifex and coastal vegetation.

Threats: Fire

Burrows can help the species survive fire, but the documents do not establish how animals respond to different fire frequencies, intensities or patch sizes. Repeated burning that removes dense tussock or spinifex can reduce shelter and food even where some burrows remain.

  • Avoid burning areas with recent records, nests or active burrows unless fire planning and fauna controls have been approved.
  • Retain unburnt patches of dense ground cover and record fire history during ecological assessment.
  • Inspect recently burnt areas and adjacent unburnt cover for signs of recolonisation before further disturbance.

Threats: Predators, competitors and disease

Cats and House Mice are recorded in the same Northern Territory fauna dataset as the Western Chestnut Mouse. The Barrow Island guide notes that native mammals benefit from the absence of introduced predators such as cats and competitors such as rabbits on that island.

The supplied documents do not identify a disease affecting the Western Chestnut Mouse. Hygiene, quarantine and control of introduced animals remain appropriate project controls, particularly at remote work camps and island sites.

  • Do not feed or attract feral cats, rodents or other introduced animals around camps and work areas.
  • Use site quarantine and waste controls to avoid moving House Mice or other introduced fauna between work areas.
  • Report sick or dead native mice to the project ecologist and follow applicable wildlife disease procedures.

Threats: Roads and hydrology

The supplied documents do not quantify road mortality, road avoidance or the effects of altered drainage on this species. Roads can nevertheless remove ground cover, compact burrow soils and increase vehicle access through occupied habitat.

Because breeding and population increase respond to rainfall, flooding and vegetation growth, changes to drainage or water movement may alter the quality and timing of foraging and breeding habitat.

  • Apply low-speed controls and fauna awareness requirements on roads through suitable habitat.
  • Avoid placing drains, culverts, stockpiles or access tracks across known burrow networks.
  • Maintain natural drainage to retained tussock, spinifex and coastal vegetation where practicable.

Survey methodology: 1. Nocturnal habitat searches

Survey effort should target this small, nocturnal rodent in spinifex, tussock grass and coastal vegetation. The supplied documents do not provide a species-specific survey guideline or standard effort, so the project ecologist should set effort using the relevant state guidance, approval conditions and the size and habitat diversity of the impact area.

Search during the night when Western Chestnut Mice are active, particularly after rainfall when breeding and population activity may increase.

Prioritise dense spinifex, tussock grass, coastal vegetation, low eucalypt woodland with dense tussock cover, burrow entrances and grass nests.

Record direct sightings, movement paths, grass nests, burrow entrances, tracks and other signs, noting that the species is small, secretive and may be difficult to distinguish from other Pseudomys mice.

Head and body length: 80 to 140 mm

Tail length: 70 to 120 mm

Typical activity: Nocturnal

  • Search each habitat type in the proposed clearing area and in nearby retained habitat.
  • Record the start and finish time, weather, recent rainfall, search route, habitat type, effort and all observations.
  • Do not rely on daytime searches alone, because the species is nocturnal and sleeps during the day in a grass nest.

Survey methodology: 2. Live trapping

Use live trapping where presence cannot be confirmed by nocturnal searches and where trapping is approved by the relevant wildlife authority.

Place traps along habitat edges, within dense spinifex and tussock grass, near burrows and beside likely travel routes. Protect traps from heat, rain and predators, and check them at the frequency required by the permit and state guidance.

Use experienced personnel who can distinguish the species from the introduced House Mouse and other small native rodents. The Western Chestnut Mouse has light brown upper fur, light orange-brown flanks, a white underside and four teats in females.

  • Set trapping effort across representative habitat and the proposed disturbance area, rather than concentrating traps only at access points.
  • Record trap locations, trap nights, weather, bait, captures, non-target animals, trap condition and release locations.
  • Release native animals promptly at the capture location or at nearby retained habitat approved in the fauna management plan.

Survey methodology: 3. Burrow, nest and sign inspection

Inspect tussock bases, dense grass, spinifex hummocks and soil for burrows and grass nests before clearing. Burrows may extend to about 1 m and can help the species survive fire.

Map all burrows, nests and concentrated sign within the impact area and in retained habitat. Treat an occupied or potentially occupied burrow as a fauna habitat feature until checked by an authorised ecologist.

Use sign inspection to supplement trapping and nocturnal searches, not as the sole presence or absence method, because the documents do not provide validated sign-based detection rates.

  • Mark burrows and nests for avoidance or staged inspection before machinery enters the area.
  • Inspect likely shelter sites again immediately before clearing if the first survey was undertaken more than a short period before works or if rain has occurred.

Survey methodology: Minimum effort and reporting

There is no numerical species-specific effort standard in the supplied documents. The project ecologist should document why the selected number of nights, trap nights, search hours, transects and survey area are adequate for the habitat and proposed impact.

  • Survey in suitable nocturnal conditions and include the wet season or the period following useful rainfall where the program allows, because breeding and activity respond to rainfall.
  • Report survey dates, nights, hours, trap nights, transect lengths, area searched, weather, rainfall, habitat types, methods, personnel, records and limitations.
  • Submit confirmed records and negative survey results where required to the relevant state or territory wildlife database.
  • Retain voucher photographs, coordinates and identification notes for records that could be confused with other Pseudomys species.
  • Escalate an unexpected concentration of animals, occupied burrows or a range concern to the project ecologist and relevant wildlife authority before clearing proceeds.

Before habitat disturbance: Avoid and minimise habitat loss

Plan the works around the species' shelter and food resources. Its documented habitat includes spinifex, coastal vegetation, tussock grass cover and low eucalypt woodland with dense tussock grass.

Retain dense spinifex, tussock grass, coastal vegetation and low woodland with a dense grass layer wherever practicable.

Avoid clearing occupied or potentially occupied burrows, grass nests and concentrated shelter areas. The species can use burrows up to about 1 m deep.

Keep retained habitat connected to other suitable vegetation and avoid creating isolated strips or small remnants surrounded by cleared ground.

  • Use existing disturbed areas for access, laydown and infrastructure where practicable.
  • Set site-specific no-go areas around mapped burrows, nests and retained vegetation, with distances approved by the project ecologist and regulator.
  • Do not describe an area as unsuitable solely because no animals were seen during a daytime inspection.

Before habitat disturbance: Approvals and fauna planning

The species is listed as Near Threatened in the Northern Territory and is not listed under the EPBC Act in the supplied information. Confirm the current listing before works because statutory listings can change.

Obtain any state or territory scientific, wildlife handling, trapping and relocation permits before survey or capture work begins.

Prepare a fauna management plan that identifies habitat to retain, survey methods, clearing limits, fauna spotter catcher responsibilities, handling arrangements, stop-work triggers and reporting requirements.

Supplied conservation status: Near Threatened in the Northern Territory; not listed under the EPBC Act

Western Australian source status: Not listed as threatened under the Wildlife Conservation Act 1950, according to

  • Check whether an EPBC referral is required for the current project and impact footprint, even though the supplied listing states that the species is not EPBC listed.
  • Check the applicable state or territory threatened-species legislation, clearing approvals and database reporting requirements.
  • Include a contingency for rainfall, unexpected captures, injured animals and discovery of occupied burrows during works.

Before habitat disturbance: Timing and site preparation

Where practicable, schedule clearing outside periods of high activity following rainfall and the wet season, while recognising that favourable conditions can permit breeding throughout the year.

Stage clearing so that animals can move from the impact area into connected retained habitat before machinery arrives. Do not use broad-scale disturbance to force animals into unknown habitat.

Complete a pre-clearance survey close to the start of works, because breeding and activity respond quickly to rainfall and vegetation growth.

  • Fence and sign exclusion areas before clearing if fencing will not trap animals or block movement to retained habitat.
  • Mark burrows, nests, dense tussock patches, spinifex hummocks and retained vegetation on plans and on the ground.
  • Brief machinery operators on the exclusion areas, daily fauna checks, stop-work triggers and the requirement to follow the fauna spotter catcher's directions.
  • Inspect proposed access tracks, stockpile areas, trench alignments and temporary work areas as well as the main clearing footprint.

During habitat disturbance: Daily controls and machinery interface

Use slow, staged clearing with an authorised fauna spotter catcher present. The spotter catcher should have authority to direct machinery to stop when an animal, burrow or nest is found.

Inspect the active work area each day before machinery starts, especially after rain or overnight disturbance.

Clear in short, supervised sections towards retained habitat, leaving a visible escape route where this is safe and approved.

Keep machinery outside marked exclusion areas and stop immediately when the fauna spotter catcher gives a stop signal.

Do not allow spoil, mulch, vehicles or materials to cover mapped burrows, nests or retained grass cover before the area has been checked.

  • Maintain daily attendance records for the fauna spotter catcher, machinery used, areas cleared and fauna observations.
  • Check open trenches, pits and excavations at the frequency required by the fauna management plan and state conditions, and provide escape ramps or cover where approved.
  • Cover or secure excavations when unattended so small mammals cannot fall in.

During habitat disturbance: Animal handling and relocation

If a Western Chestnut Mouse is found, stop work around the animal and keep people and machinery back.

Only an appropriately authorised and experienced person should catch, handle, identify or relocate the animal.

Use the least stressful capture method permitted by the relevant approval. Do not handle the animal unnecessarily or hold it longer than required for identification, health assessment and release.

Release an uninjured animal into nearby retained habitat that is connected to suitable spinifex, tussock grass or coastal vegetation and has been approved in the fauna management plan.

  • Record the animal's location, condition, sex if known, identification features, capture method, release location, time held and personnel involved.
  • Place an injured animal in a suitable ventilated container and transfer it promptly to the nominated wildlife carer or veterinarian under the relevant permit.
  • Do not relocate animals to an isolated patch, across a road or into habitat that has not been checked for suitability.

During habitat disturbance: Habitat features and stop-work triggers

Treat grass nests, burrows, dense tussock bases and spinifex hummocks as potential shelter features until they have been inspected by the fauna spotter catcher.

Stop work and obtain ecological direction if an animal is found, if an occupied or potentially occupied burrow is exposed, or if animals are repeatedly detected in the active clearing area.

Stop work if clearing moves outside the approved footprint, exclusion fencing is damaged, an escape route is blocked, or the fauna spotter catcher cannot safely observe the machinery and ground ahead.

Do not excavate, collapse or fill a burrow as a routine clearing measure.

  • Photograph and map each significant burrow or nest that cannot be avoided.
  • Record all stop-work events, decisions, animal outcomes, habitat features removed and corrective actions.
  • Notify the project environmental officer and regulator when required by the approval or fauna management plan.

After habitat disturbance: Post-clearance checks

Confirm that cleared areas and retained habitat meet the approved conditions. Continue checking for animals displaced by machinery, altered shelter or newly exposed ground.

Inspect the completed clearing edge, retained vegetation, access tracks, stockpiles, pits and trenches for live or injured mice, burrows, nests and other signs.

Repeat checks after rainfall and after major changes to ground cover, because activity and breeding respond to favourable conditions.

Remove temporary fauna hazards only when the fauna spotter catcher or project ecologist confirms that no animals remain inside them.

  • Record the date, area checked, personnel, methods, weather, rainfall, detections, injuries and corrective actions.
  • Repair damaged exclusion fencing and maintain no-go markers until construction risks have passed.
  • Report any discovery outside the approved footprint as a non-conformance and investigate before further work.

After habitat disturbance: Monitoring and compliance reporting

Monitor retained habitat and any approved relocation area using the same core method used for the pre-clearance survey where practical, so results can be compared.

Use monitoring to check whether retained spinifex, tussock grass and coastal vegetation remain intact and whether burrows and grass nests are being used.

Do not claim project success from a single non-detection, because the species is nocturnal and secretive.

  • Set monitoring frequency, duration and success measures in the fauna management plan and approval conditions.
  • Report captures, mortalities, injuries, relocations, habitat loss, incidents, stop-work events and mitigation outcomes to the relevant authority where required.
  • Submit confirmed occurrence records to the relevant state or territory wildlife database.

After habitat disturbance: Offsets and residual impacts

Assess residual loss of suitable habitat, connectivity and shelter features after avoidance and mitigation have been applied.

Obtain regulator advice on offsets or other compensatory measures if residual impacts remain and an approval requires them.

Ensure any offset or conservation area contains suitable grass and spinifex cover and is protected from the same clearing impact.

  • Document the area and habitat type lost, the area retained, the condition of retained habitat and the basis for any offset calculation.
  • Include management commitments for fire, weeds, introduced predators and future disturbance where those threats are identified in the approved plan.

Rehabilitation actions: Restore ground cover and shelter

Rehabilitation should restore the low, dense ground cover and shelter structure used by the species. Prioritise native grass and spinifex recovery rather than relying on bare ground to regenerate unaided.

Re-establish native spinifex, tussock grasses and suitable coastal vegetation in cleared temporary work areas using locally appropriate provenance where required by the approval.

Retain and respread suitable topsoil and seed-bearing material where it is free of weeds and the rehabilitation method has been approved.

Reinstate natural soil levels and drainage so that small mammal burrows can form and dense grass cover can develop.

  • Do not leave compacted pads, broad bare verges or continuous cleared strips between retained habitat and rehabilitation areas.
  • Avoid planting dense exotic grass or other species that do not reflect the documented habitat of the Western Chestnut Mouse.
  • Use staged rehabilitation so that completed areas connect to retained vegetation as soon as practicable.

Rehabilitation actions: Retain natural habitat features

Retain suitable fallen grass debris, coarse ground cover and natural soil structure where they do not create a fire or construction hazard.

Do not install nest boxes as a substitute for the species' documented grass nests and burrows unless a qualified ecologist and regulator approve that approach.

Where construction has removed burrowable soil, reinstate an appropriate soil profile rather than installing artificial hollows designed for tree-dwelling fauna.

  • Map rehabilitation areas and record the plant species used, seed source, area treated and completion date.
  • Protect new vegetation from vehicles, grazing, unauthorised access and follow-up earthworks.
  • Remove temporary erosion and sediment controls only when the area is stable and fauna risks have been checked.

Rehabilitation actions: Fire, weeds and predator management

Manage fire in accordance with the regional fire plan and ecological advice, recognising that the species uses burrows up to about 1 m deep and that breeding and population activity can increase after fire when vegetation recovers.

Control weeds that reduce native spinifex and tussock cover or alter the food and shelter resources of the site.

Prevent domestic cats and other introduced predators from entering retained and rehabilitated habitat where project controls can reasonably address that risk.

  • Inspect rehabilitation areas for weed invasion and predator activity during each monitoring visit.
  • Coordinate fire planning with adjoining land managers so that retained habitat is not exposed to an unplanned, high-intensity burn.
  • Record fire, weed and predator management actions and their outcomes.

Rehabilitation actions: Rehabilitation monitoring and success measures

Monitor whether native spinifex, tussock grass and coastal vegetation become established and whether the area reconnects with retained suitable habitat.

Use the pre-clearance survey data as the reference point for monitoring habitat condition, burrows, nests and Western Chestnut Mouse detections.

Set measurable completion criteria in the rehabilitation plan, such as stable ground, established native cover, no uncontrolled weed spread, functioning connectivity and no unresolved fauna hazards.

  • Inspect rehabilitation at intervals set by the approval and after substantial rainfall, when new activity may be detectable.
  • Continue monitoring until the project ecologist and regulator accept that the agreed vegetation and fauna outcomes have been met.
  • Submit rehabilitation monitoring results and confirmed species records to the relevant state or territory wildlife database.

Sources