Family Bufonidae

Bufo rangeri Hewitt, 1935

Raucous Toad, Ranger’s Toad, Lawaaiskurwepadda (A)

By M. Cunningham

Currently accepted name: Sclerophrys capensis Tschudi, 1838
Type locality: "Promontorium Bonae Spei" (= Cape of Good Hope), Western Cape Province, Rep. South Africa.
Red listing status: Least Concern

Photo by de Villiers Marienne; Andre Fourie, 2008. URL: FrogMAP: 183

Distribution

Bufo rangeri is endemic to the atlas region. Its range includes all provinces of South Africa as well as Lesotho and Swaziland. In the arid Northern Cape and North West provinces, it is restricted to the vicinity of the Vaal and Gariep rivers that form corridors, allowing the species to reach Vioolsdrif (2817DC) and Alexander Bay (2816CB) in the west. The species occurs in sub-coastal areas, extending from the Olifants River mouth (3118CA) in the west, along the coast to the vicinity of Durban (2931CC). Around Paterson (3325BD), its range extends inland onto the highveld grassland of Lesotho and Eastern Cape, KwaZulu-Natal, Free State, Gauteng and Mpumalanga provinces. North of latitude 29°S, this toad is usually restricted to altitudes >1000 m, and in the extreme north it occurs in a discontinuous series of highland patches following the northeastern escarpment from Malolotja Nature Reserve (2631AA) in western Swaziland, through Mpumalanga, to the Soutpansberg range of Limpopo Province.

B. rangeri comprises three historically isolated genetic lineages identified from mitochondrial DNA sequences (pers. obs.). One of these lineages is widespread and extends from the Free State and KwaZulu-Natal southwest to Stellenbosch (3318DD), including the type locality at Glennifer (3227DA) near Kei Road. A second lineage is restricted to the west coast and intergrades with the widespread southern lineage in the upper Berg River valley near Franschoek (3319CC). Mitochondrial DNA sequence diversity suggests an historically large and stable population within the southern lineage, whereas the western lineage appears to have been historically restricted in distribution and to have undergone range expansion, probably with climatic changes over the past 20 000 years. A third independent genetic lineage is restricted to fragmented populations along the northeastern escarpment of Swaziland and Mpumalanga. The status of populations in Gauteng and Limpopo provinces, and the boundary between the southern and northeastern lineages, are currently unresolved.

The range of B. rangeri appears to be contracting in the north and east of South Africa, in Limpopo, Mpumalanga, Gauteng and coastal KwaZulu-Natal provinces (new atlas data). This apparent range contraction seems to complement the range expansion of B. gutturalis and it is possible that these are linked, that is, B. gutturalis displaces B. rangeri and/or habitat modification affects these species differently.

B. rangeri hybridizes with B. gutturalis at sites scattered throughout eastern South Africa (see B. gutturalis account). These are not closely related species (pers. obs.), and it seems that hybridization rarely progresses beyond the first generation (Carruthers 2001; pers. obs.). There are also numerous sites where these species co-occur with little or no hybridization. B. gutturalis tends to be much more abundant than B. rangeri at joint breeding sites. Behavioural theory predicts that females of the less common species are more likely to mate with other species, but in this case, most hybrid pairings occur between female B. gutturalis and male B. rangeri (pers. obs.).

Several authors have suggested that hybridization between B. rangeri and B. gutturalis is an historically recent phenomenon and that the creation of artificial breeding sites, such as farm dams, has broken down natural separation based on breeding habitat (Carruthers 2001). B. gutturalis readily colonizes farm dams and reaches high abundance, so it is possible that modern peri-urban and agricultural development has extended the potential distribution of this species at the expense of B. rangeri. However, the two species also co-occur and hybridize in natural situations, such as around slow flowing streams or stream-side pools, for example, along a stream in the Silaka Nature Reserve (3129DA), and it is likely that agricultural development has been accompanied by a reduction in stream-side wallows created by large mammals such as buffalo. Thus it seems likely that some level of hybridization occurred prior to the past hundred years of agricultural modification. The apparent complementary range shifts in the two species may also partly reflect the different responses of a tropical savanna species and a mesic temperate species to subtle changes in climate, rather than competitive exclusion.

This species has an easily recognizable call. The atlas data are reliable and fairly comprehensive.

Habitat

B. rangeri inhabits mesic temperate areas of South Africa, Lesotho and Swaziland, a distribution that encompasses much of the Fynbos and Grassland biomes. It also occurs peripherally in the Succulent Karoo, Nama Karoo, Thicket, Forest and Savanna biomes. The species is absent from the sub-alpine grasslands of Lesotho, upper montane areas of the Western Cape, the Cape Peninsula, Saldanha Peninsula (3217DD) and Swartland (3318AD) in the southwest, and from the lowveld and drier parts of Limpopo, Mpumalanga and KwaZulu-Natal provinces and Swaziland.

These toads are particularly abundant in the artificial grasslands of agricultural areas and are frequently encountered at breeding sites around farm dams, large ponds and pools along slow-flowing streams.

Life history

Non-breeding individuals roam widely and may be encountered crossing trails and roads throughout the year, especially on humid nights. The number of individuals encountered greatly increases around the breeding season as individuals migrate to breeding sites. Males usually call from exposed sites on floating vegetation, in shallow water near banks, or among reed beds.

There appears to be geographical variation in breeding patterns of B. rangeri. In the summer-rainfall eastern region of South Africa, this species calls from September, before B. gutturalis becomes active, and breeding choruses continue through to January. In the winter-rainfall southwest, B. rangeri has a prolonged breeding season, commencing in early November and continuing until the last week in February (Cherry 1989).

In a study population near Stellenbosch, B. rangeri maintained a strict female-choice mating system in which males competed by calling but did not actively search for mates or attempt to displace amplectant males. Instead, females visited several males before choosing a partner (Cherry 1993). This mating system may be density dependent (M. Cherry pers. comm.) and observations of attempted amplexus displacement at Klawer (3118DC) and among captive individuals from Weza (3029DB, pers. obs.) suggest that mating systems may vary among populations.

As in most toads, eggs are laid in spiraling strings and often become entangled in aquatic vegetation. A clutch of eggs laid by a captive female from Stellenbosch contained 10 760 eggs (M. Cunningham and C.L. Henderson unpubl. data).

Like other Bufo species, B. rangeri shows some fidelity to breeding sites within seasons, but breeding-site shifts of up to 5 km have been recorded across two breeding seasons (unpubl. data), and in a three-year ecological study of this long-lived species, few marked individuals were recaptured in subsequent years (Cherry 1993). The population ecology of B. rangeri would be a good topic for further research.

Conservation

B. rangeri occurs in a number of conservation areas, including Cederberg Wilderness Area (3219AC), Golden Gate Highlands (2828DA) and Royal Natal national parks (2828DB), and Malolotja Nature Reserve (2631AA) in Swaziland. Although the species is secure in most places, populations along the northeastern escarpment appear to be declining. Further efforts are needed to determine the causes of this decline and to protect these populations of a unique and possibly threatened lineage restricted to Mpumalanga and Limpopo provinces and Swaziland.

Current distribution map

Undated records;  pre-1996;  1996 to 2002;  2003 to present