Family Brevicipitidae

Breviceps macrops Boulenger, 1907

Desert Rain Frog, Melkpadda (A), Woestyn-blaasoppadda (A)

By L.R. Minter

Currently accepted name: Breviceps macrops
Red listing status: Vulnerable

Photo by Maguire D., 2013. URL: FrogMAP: 2091

RED LIST SPECIES

Status: Vulnerable (VU) Criteria: B2ab(ii,iii,iv,v)

Identification

B. macrops is a squat, rotund frog with short limbs and large, prominent eyes. The interorbital distance is about half the horizontal diameter of the eye (Poynton 1964). A sample of 10 specimens excavated from dunes at Port Nolloth had a mean snout–vent length of 34.5 mm (unpubl. data); Poynton (1964) recorded a maximum body length of 48.5 mm. Fleshy webbing is present between the fingers and toes, and sub-articular tubercles on the fingers are absent or weakly developed (if present, they are single, not double, cf. B. namaquensis).

The dorsum is light yellowish-brown, variably marked with fine, dark brown speckles or larger vermiculations that sometimes form a border around a pair of large, light patches in the scapular region. The ventrum is white with a large, central, unpigmented area through which the abdominal muscles, blood vessels and other internal organs can be seen.

The advertisement call is a single, long, low-pitched whistle. Calls recorded at Port Nolloth had an emphasized frequency of 1200 Hz rising slightly to 1300 Hz. They varied between 1.2 and 2.5 seconds in duration (cf. Channing 2001) and were repeated at relatively long intervals (10 seconds to 15 minutes), depending on the chorus intensity (unpubl. data). The calls were lower in emphasized frequency and much longer than those of B. namaquensis (see species account).

Distribution

Beyond the atlas region, B. macrops occurs in the southern coastal dunes of Namibia, from Lüderitz southward to Oranjemund.

In the atlas region, B. macrops occupies a narrow coastal strip along the northwestern Namaqualand coast, from Alexander Bay (2816CB) southward as far as the farm Skulpfontein north of Koingnaas (3017AB). It ranges from close to the high-water mark (Channing and Van Wyk 1987), to 10 km from the coast (S. Hanrahan pers. comm.). Distribution data for this species are scarce, as much of the area it occupies lies within diamond-mining concessions and is not easily accessible.

The atlas data are reliable, but are restricted mainly to areas lying outside the mining concessions, and are therefore incomplete.

Habitat

B. macrops is restricted to Strandveld Succulent Karoo, in the Succulent Karoo Biome. It inhabits sand dunes vegetated by low, succulent shrubs and other xerophytes. These plants usually cover the crests of the dunes which are separated by open areas of bare sand. The dunes are mostly formed by white, calcareous sand, but reddish dunes occur at some localities where the species is known to occur, for example, alongside the Holgat River (De Villiers 1988c). Annual precipitation is less than 50 mm at Alexander Bay (Low and Rebelo 1996), and 43–146 mm at Kleinsee (Channing and Van Wyk 1987). Coastal fog is common in this region.

Life history

Calling and/or surface activity has been recorded in almost all months of the year, usually during and following fog (Channing and Van Wyk 1987; De Villiers 1988c; Passmore and Carruthers 1995; S. Hanrahan pers. comm.; pers. obs.). Males call from the surface, sometimes excavating a slight depression in the sand, or moving from one call site to another while foraging (unpubl. data). The distance between calling males may be as little as 1 m, and choruses are apparently organized (Channing 2001). Calling is inhibited by dry, warm weather, but not by low temperatures (S. Hanrahan pers. comm.; pers. obs.). Amplexus, oviposition and development have not been recorded.

When not active on the surface, the frogs burrow downwards into the sand. L.R.M. (unpubl. data) excavated 10 specimens at depths of 12.5–27 cm below the surface. In all cases the “burrows” were located in low, unvegetated areas between dunes, and the frogs were found in a layer of sand that was moist enough to form a ball when compressed by hand. This moist layer was both covered and underlain by dry sand, and appears to be a reservoir of water, maintained by rain and fog. The frogs were found 2–4 cm below the upper boundary of the moist layer. The temperature drops rapidly below the surface (e.g. at midday the surface temperature was 38.3°C, but decreased to 20.5°C at 10 cm below the surface). The temperature of the sand surrounding the frogs ranged from 17.6–20.7°C.

Analysis of scats revealed that prey items include beetles and ants (Channing and Van Wyk 1987). While tracking frogs on the surface, Carruthers and Passmore (1978) found signs of activity around dung and suggested that the frogs were feeding on insects attracted to the dung. An adder, Bitis schneideri, was recorded as a predator at Lüderitz (Haacke 1975).

Conservation

Status

B. macrops was previously listed as Restricted (Branch 1988), Vulnerable (IUCN 2000) and Near Threatened (Harrison et al. 2001). Reassessment for this publication determined a status of Vulnerable, based on a restricted distribution (area of occupancy: 501–2000 km²), extensive loss of habitat to strip mining (present and projected), and a predicted population decline (>50% in the next 30 years). B. macrops does not occur within a protected area. However, a process is presently under way to proclaim the Orange River mouth at Alexander Bay a protected area. The species is protected in terms of the Cape Nature Conservation Ordinance 19 of 1974.

Threats

The major threat to B. macrops is habitat loss and fragmentation due to strip mining. The development of roads, increasing pressure from human populations and changing land use (e.g., increasing grazing, recreational use of dunes by off-road vehicles, e.g., at McDougall’s Bay), pose additional threats.

Recommended conservation actions

A detailed survey to identify the location and size of the remaining populations of B. macrops and the extent to which its habitat has been fragmented, is essential. Possible recolonization of mined-out and/or rehabilitated areas should be investigated. A study of the reproductive biology of the species is necessary to determine its fecundity and longevity, and limiting-factor research will facilitate effective management.

Management recommendations include the establishment of a long-term monitoring programme inside and outside the mining concessions, and limiting factor management. In view of the restricted distribution of B. macrops, priority should be given to the establishment of conservation areas within the range of this species. Such reserves are also needed to protect the unique ecology, fauna and flora of this area.

Current distribution map

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