Overview

Dasmys robertsii – Mullin, Taylor & Pillay, 2004

ANIMALIA – CHORDATA – MAMMALIA – RODENTIA – MURIDAE – Dasymys – robertsii 

Common Names: Robert’s Shaggy Rat, African Marsh Rat, Common Dasymys (English)

Synonyms: None 

Taxonomic Note: There are at least 14 Dasymys species recognised based on morphological evidence (Monadjem et al. 2015). Mullin et al. (2005) provided a biogeographical framework for 11 of these morphological species, showing that many of the range-restricted endemics were associated with endemic hotspots for other species of mammals. Of relevance to the assessment region, two species have been split from D. incomtus: D. capensis has been elevated to full species status based on cranial morphology and its isolated distribution in the Cape region (Mullin et al. 2004). Similarly, D. robertsii (from northern South Africa) was previously known as D. incomtus but is chromosomally, genetically and morphologically distinct (Mullin et al. 2002, 2004). Finally, D. incomtus now refers only to the population restricted to eastern South Africa (from where the type locality originates), leaving the populations outside of this region without a name and are referred to as D. cf incomtus for now (Monadjem et al. 2015). This genus is in urgent need of a continent-wide review. 

Red List Status: VU – Vulnerable

Assessment Information

Assessors: Taylor, P.¹ & da Silva, J.M.²

Reviewer: Oosthuizen, M.³

Institutions: ¹University of the Free State,²South African National Biodiversity Institute,³University of Pretoria 

Previous Assessors: Pillay, N., Taylor, P., Jewitt, D., Pence, G. & Baxter, R. 

Previous Reviewer: Child, M.F.  

Previous Contributors: Roxburgh, L., Avenant, N., Avery, M., MacFadyen, D., Monadjem, A., Palmer, G. & Wilson, B. 

Assessment Rationale 

As with other Shaggy Rats, D. robertsii, is dependent on intact rivers and wetland ecosystems, as they have not been found in artificial or degraded wetlands and are thus patchily distributed within the assessment region. Furthermore, they are rare relative to Otomys spp., occurring at low densities with low reproductive rates within fragmented subpopulations. Although previously assessed as one species (D. incomtus), new data reveal three species within the assessment region: D. capensis endemic to the Cape region, D. incomtus probably endemic to the eastern areas of the assessment region, and D. robertsii occurring throughout northern South Africa and Zimbabwe. The extent of occurrence for D. robertsii is estimated to be far greater than 10,000 km². The area of occupancy, calculated by summing the amount of remaining natural vegetation around wetlands is estimated to be between 1,030–11,382 km². These values, however, do not account for degraded habitats and include potentially unoccupied patches (resulting from the poor dispersal ability of the species). Thus, we take a precautionary purview to this assessment by using the lower bound of the occupancy values.

Wetlands are continuing to be lost with agricultural and human settlement expansion, which in turn increases wetland degradation from overgrazing, water abstraction, pollution and invasive alien plant sprawl. Given the restricted range, habitat fragmentation and ongoing loss of suitable wetland habitat we list D. robertsii as Vulnerable B2ab(ii,iii,iv). We consider these genuine changes, as 32.8% of wetlands nationally have been lost between 1990 and 2013/14 (although this includes many wetlands that are simply temporarily dry due to the wetter conditions in 1990), and this trend is likely to have continued up until the present. However, data on land-use change since the last assessment is urgently needed This species should be reassessed once density estimates in suitable habitat are available and a better understanding of the population genetic structure and diversity is available. Key interventions for this species include increased trapping effort to assess population size, while considering the rarity of the species, conserving and restoring strips of natural vegetation around wetlands and riverside, and extending protected wetland habitat area and connectivity through biodiversity stewardship schemes.

Regional population effects: Dispersal may be possible through corridors or riverine vegetation within the Greater Limpopo Transfrontier Park and Mapungubwe Transfrontier Conservation Area. However, wetland habitats are fragmented, and this species is a poor disperser and thus it is unknown whether significant rescue effects are possible. 

Reasons for Change 

Reason(s) for Change in Red List Category from the Previous Assessment: No change 

Red List Index 

Red List Index: No change 

Recommended Citation: Taylor P & da Silva JM. 2025. A conservation assessment of Dasmys robertsii. In Patel T, Smith C, Roxburgh L, da Silva JM & Raimondo D, editors. The Red List of Mammals of South Africa, Eswatini and Lesotho. South African National Biodiversity Institute and Endangered Wildlife Trust, South Africa.

Regional Distribution and occurrence

Geographic Range 

This species is associated with rivers and wetlands within the northern and southern African savannas from Senegal in the west to Ethiopia in the east and south to the Western Cape Province of South Africa (Monadjem et al. 2015). The type specimen for the genus was collected in Durban. Mullin et al. (2002, 2005) provided molecular and biogeographical evidence for the existence of D. incomtus in the KwaZulu-Natal Province region, D. robertsii in the lowveld and northern provinces of South Africa, and D. capensis in the Western Cape Province. No Dasymys species has been recorded from Lesotho (Lynch 1994).

The newly described D. robertsii is patchily distributed in the lowveld of northern South Africa and Zimbabwe. Although habitat may be contiguous between the two regions, as the species is a wetland specialist, we suspect that dispersal rates are hindered by the fragmented nature of wetland systems. Within the assessment region, it occurs predominantly in Limpopo, Mpumalanga and Gauteng provinces (Mullin et al. 2005), which corresponds to the Limpopo watershed area. Additionally, Power (2014) recently recorded the species in the North West Province for the first time in a wetland on a tributary of the Waterkloofspruit at Kgaswane. The species is also expected to occur in the Kgomo Kgomo floodplain wetlands (Power 2014).

The extent of occurrence (EOO) is 118,431 km² and area of occupancy (AOO) is estimated between 1,030–11,382 km². While the EOO was calculated using all available records, the AOO was systematically estimated by buffering wetlands within the EOO by both 500 m (strip width used to assess habitat condition around wetlands in the National Biodiversity Assessment, as it provides a good proxy for wetland condition; Driver et al. 2012) and 32 m (minimum buffer zone of no development around waterbodies, as set in the National Environmental Management Act, Activity 9 and 11 Listing 1 of Government Notice R544 and Activity 16 Listing 3 of Government Notice R546 of 2010), using the wetland layer created by the National Biodiversity Assessment (Nel et al. 2011). The amount of remaining natural vegetation was then calculated using a 2013–14 national land-cover dataset (GeoTerraImage 2015a). However, these data do not include degraded habitat and thus, in reality, AOO is likely to be smaller. 

Elevation / Depth / Depth Zones 

Elevation Lower Limit (in metres above sea level): (Not specified) 

Elevation Upper Limit (in metres above sea level): (Not specified) 

Depth Lower Limit (in metres below sea level): (Not specified) 

Depth Upper Limit (in metres below sea level): (Not specified) 

Depth Zone: (Not specified) 

Map

Figure 1. Distribution records for Robert’s Shaggy Rat (Dasmys robertsii) within the assessment region (South Africa, Eswatini and Lesotho). Note that distribution data is obtained from multiple sources and records have not all been individually verified.

Biogeographic Realms 

Biogeographic Realm: Afrotropical 

 Occurrence

Countries of Occurrence within southern Africa 

Large Marine Ecosystems (LME) Occurrence 

Large Marine Ecosystems: (Not specified) 

FAO Area Occurrence 

FAO Marine Areas: (Not specified) 

Climate change

Shaggy Rats are grassy wetland specialists, often sharing habitats with Vlei Rats. They are dependent on marshy soils and standing water to construct their nests. Direct anthropogenic threats such as reduced run-off in rivers due to water abstraction, afforestation and drainage of wetlands are compounded by shrub encroachment and aridification of grasslands caused by climate change (Taylor et al. 2016). 

Population information

The abundances and population sizes of this species is unknown. They are rare and exist at low densities. For example, during 11 months of monthly trapping (3300 trap-nights) in 2010-2011, at Lajuma Research Centre in the Soutpansberg Mountain (Nemakhavhani, 2012), only 17 captures (0.5% trap success) were recorded out of a total of 931 captures of small mammals in total. Similarly, during nine two-monthly trapping sessions between July 2020 and November 2021 at three sites at Lajuma, five unique individuals of D. robertsii were recorded out of a total of 890 unique small mammals sampled in total (Msimango et al. 2025). As a wetland specialist that rarely emerges from the wetlands, it is trap-shy and thus difficult to monitor.  

Population Information 

Population trend: Declining, based on ongoing habitat loss and degradation.  

Continuing decline in mature individuals? Unknown 

Extreme fluctuations in the number of subpopulations: (Not specified) 

Continuing decline in number of subpopulations: (Not specified) 

All individuals in one subpopulation: (Not specified) 

Number of mature individuals in population: Unknown  

Number of mature individuals in largest subpopulation: Unknown 

Number of Subpopulations: Unknown 

Severely fragmented: Yes. Their habitat is fragmented, and they are suspected to be poor dispersers. 

Quantitative Analysis 

Probability of extinction in the wild within 3 generations or 10 years, whichever is longer, maximum 100 years: (Not specified) 

Probability of extinction in the wild within 5 generations or 20 years, whichever is longer, maximum 100 years: (Not specified) 

Probability of extinction in the wild within 100 years: (Not specified) 

Population genetics

Fixed chromosomal and allozyme allelic characters distinguish D. robertsii from Mpumalanga and Limpopo from D. incomtus sensu stricto from KwaZulu-Natal. Within D. incomtus, populations from southern and northern KwaZulu-Natal (having the same chromosomal configuration) are partly distinct in allozymes. Allozyme divergence between seven sampled groups of D. robertsii and D. incomtus in South Africa could be explained by geographic distance consistent with the isolation by distance model (Mullin et al. 2002). High fixation indices (mean Fst 0.42) and a high proportion of private alleles in the species-group suggest a highly subdivided metapopulation structure consistent with a fragmented distribution of suitable wetland habitats, with limited current gene flow between them. Suitable habitats likely expanded and contracted with cyclical wet-dry cycles in the Holocene, providing periods of past gene flow followed by cessation of gene flow (Mullin et al. 2002). 

Based on this information, the number of genetic populations (subpopulations) are considered to be 1 (a single metapopulation), and from this we can quantify the Convention on Biological Diversity’s Global Biodiversity Framework’s complementary genetic indicator – proportion of populations maintained (PM), which would receive a value of 1.0 (1/1 populations remaining). While it is thought that D. robertsii dispersal may be possible through corridors or riverine vegetation within the Greater Limpopo Transfrontier Park and Mapungubwe Transfrontier Conservation Area, given the fragmentation of wetland habitats, and the assumed poor dispersal abilities of the species, there might be genetically distinct, isolated subpopulations. Despite this, no subpopulations are thought to have gone extinct, therefore, the indicator value would not change should genetic structure (not following an isolation by distance pattern) be found. 

Given that the species is rare and occurs at low densities, it would be difficult to infer population size estimates. Consequently, the GBF’s GBF’s headline indicator – proportion of species with an effective population size (Ne) greater than 500 – cannot be estimated. It is highly recommended a population genetic study be undertaken to directly estimate contemporary Ne for the species and to verify the population genetic structure. Given the rarity of this species, conventional sampling may be difficult, so alternative methods such as environmental DNA can be investigated. 

Habitats and ecology

This species has been recorded from a wide variety of habitats, including forest and savannah, swampland and grasslands, but they rely on intact wetlands in these areas. They have not been recorded from agricultural landscapes or dam areas. They occur specifically in reed beds and among semi-aquatic grasses in wetlands or swampy areas or along rivers and streams, as well as in grassy areas close to water wherein they co-occur with Otomys spp. (Skinner and Chimimba 2005). Shaggy Rats construct complex, intricately woven nests in holes along the banks of rivers and ponds (Pillay 2003). Nests extend into water and might serve as a bolt hole during attack from predators. Sub- and above-surface runways extend from the nest cavities and would serve as travel routes. These rodents are opportunistic omnivores, feeding predominantly on the succulent stems and fruiting heads of semi-aquatic grasses (Skinner and Chimimba 2005), supplementing their diets with insects, especially during reproduction (Pillay 2003). They are good swimmers, adapted to living in very marshy habitats where they build runways and nests in dense ground cover (Monadjem et al. 2015). Based on a captive study, the gestation period is 29 days and litter size varies from two to five (Pillay 2003, Skinner and Chimimba 2005). Maximum reproductive output during one artificial breeding season was 18 young.

Young are altricial and nipple-cling, and reproductive output is low compared to other African rodents (Pillay 2003). In the assessment region, they occur at altitudes from sea level to 1,500 m asl in the Drakensberg Mountains (Skinner and Chimimba 2005).

Ecosystem and cultural services: Potential to become flagship wetland species for biodiversity stewardship schemes. 

IUCN Habitats Classification Scheme 

Life History 

Generation Length: (Not specified) 

Age at maturity: female or unspecified: (Not specified) 

Age at Maturity: Male: (Not specified) 

Size at Maturity (in cms): Female: (Not specified) 

Size at Maturity (in cms): Male: (Not specified) 

Longevity: (Not specified) 

Average Reproductive Age: (Not specified) 

Maximum Size (in cms): (Not specified) 

Size at Birth (in cms): (Not specified) 

Gestation Time: (Not specified) 

Reproductive Periodicity: (Not specified) 

Average Annual Fecundity or Litter Size: (Not specified) 

Natural Mortality: (Not specified) 

Does the species lay eggs? (Not specified) 

Does the species give birth to live young: (Not specified) 

Does the species exhibit parthenogenesis: (Not specified) 

Does the species have a free-living larval stage? (Not specified) 

Does the species require water for breeding? (Not specified) 

Movement Patterns 

Movement Patterns: (Not specified) 

Congregatory: (Not specified) 

Systems 

System: Terrestrial, Freshwater (=Inland waters) 

General Use and Trade Information

This species is not known to be traded or utilised in any form. 

Local Livelihood: (Not specified) 

National Commercial Value: (Not specified) 

International Commercial Value: (Not specified) 

End Use: (Not specified) 

Is there harvest from captive/cultivated sources of this species? (Not specified) 

Harvest Trend Comments: (Not specified) 

Threats

There are several major threats to this species, which revolve around habitat loss and degradation. Wetlands are the country’s most threatened ecosystem, with 65% of wetland ecosystem types threatened (48% of all wetland types Critically Endangered, 12% Endangered and 5% Vulnerable) because they are highly productive and hence become transformed for agriculture (Driver et al. 2012). The 1990–2013/14 South African National Land-Cover change report found a 32.78% decline in wetlands, nationally, during the study period (GeoTerraImage 2015a). However, this is partially confounded by 1990 being generally wetter than 2013/14 and so many wetlands in the drier western regions may not be lost, but just temporarily dry. Habitat loss due to land transformation in the surrounding matrix further isolates wetlands from one another and exacerbates the degradation of individual wetlands. For example, sugarcane plantations are often planted right up to wetlands edges, not respecting the appropriate buffer (D. Jewitt pers. obs. 2015). Water abstraction or filling in of wetlands from human settlement and industrial expansion also leads to habitat loss. Compounding this is wetland degradation from overgrazing rank grasses surrounding wetlands, which leads to the loss of ground cover and decreases small mammal diversity and abundance (Bowland and Perrin 1989, 1993). Similarly, suppression of natural ecosystem processes, such as fire, can also lead to habitat degradation through bush encroachment or loss of plant diversity through alien invasive species, and is suspected to be increasing with human settlement expansion. Overall, 45% of our remaining wetland area exists in a heavily modified condition, due primarily to onsite modification from crop cultivation, coal mining, urban development, dam construction, and overgrazing (and thus erosion) and off-site modifications from disruptions to flow regime and deterioration of water quality (Driver et al. 2012).  

Conservation

In Limpopo and Mpumalanga, D. robertsii occurs in Kruger National Park and presumably other formally and privately protected areas.  There are two important types of intervention that are a priority for this species: 

• Conserve and create wetland clusters and corridors. Biodiversity stewardship schemes should be promoted if landowners possess wetlands close to core protected areas or remaining habitat patches, and the effects on small mammal subpopulations should be monitored. Protecting such habitats may create dispersal corridors between patches that will enable adaptation to climate change. 
• Conserve or restore riparian vegetation around wetlands. Retaining ground cover and rank vegetation is the most important management tool to increase small mammal diversity and abundance around wetlands. This can be achieved through lowering grazing pressure (Bowland and Perrin 1989), or by maintaining a buffer strip of natural vegetation around wetlands (Driver et al. 2012). Small mammal diversity and abundance is also higher in more complex or heterogeneous landscapes, where periodic burning is an important tool to achieve these (Bowland and Perrin 1993). Removing alien vegetation from watersheds, watercourses and wetlands is also an important intervention to improve flow and water quality, and thus habitat quality. Education and awareness campaigns should be employed to teach landowners and local communities about the importance of conserving wetlands. 

Recommendations for land managers and practitioners: 

• Working for Water managers should continue to work with private landowners in key wetland areas to remove alien vegetation. 
• Landowners and communities should be incentivised to stock livestock or wildlife at ecological carrying capacity and to maintain a buffer of natural vegetation around wetlands. 
• Enforce regulations on developments that potentially impact on the habitat integrity of grasslands and wetlands. 
• Publicise these species for conservancies as symbols of wetland conservation and thus biodiversity stewardship agreements. 

Research priorities: 

• Field surveys to more accurately delineate geographic distribution and investigating particularly whether these species occur within artificial waterbodies, agricultural landscapes and urban/rural gardens. 
• Similarly, density estimates need to be produced. Estimating current and future rates of wetlands and/or natural habitat loss within the species’ ranges. 

Encouraged citizen actions: 

• Private landowners should continue to work with Working for Water to conserve wetlands and improve ecosystem functioning. 
• Similarly, citizen scientists can collect owl pellets for deposit at natural history museums and help experts to identify small mammal species. 

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