Family Bufonidae

Bufo robinsoni Branch and Braack, 1996

Paradise Toad, Paradys Skurwepadda (A)

By W.R. Branch and H.H. Braack

Currently accepted name: Vandijkophrynus robinsoni
Red listing status: Least Concern

Photo by Willis C.K., 2013. URL: FrogMAP: 1137

Distribution

B. robinsoni is currently known from the Richtersveld, Bushmanland and Namaqualand regions of the Northern and Western Cape provinces of South Africa. Originally thought to occur only in isolated, rocky pools within the northern mountain desert region, recent atlas surveys have extended the known range as far south as the Vanrhynsdorp area (3118BC, BD), and eastward to Ghaamsberg in the Aggeneys area (2919AC). Additional discoveries are likely from springs and gorges in the rocky granite inselbergs of Bushmanland and other inaccessible areas. It is probable that B. robinsoni will be found in southern Namibia in areas adjacent to the Richtersveld (Branch and Braack 1995), and that current gaps in its seemingly disjunct distribution will be filled by further surveys. A photograph (M. Griffin) of a specimen from Rosh Pinah, southern Namibia (2716DD), has similar colouration and habitat to B. robinsoni and may be referable to this species.

B. robinsoni occurs in sympatry with B. gariepensis, and they are closely related (Cunningham and Cherry 2000). In Namaqualand, B. robinsoni lacks the bright colouration typical in the rocky Richtersveld and Ghaamsberg areas, and has a drab colour and pattern similar to that of B. gariepensis. Conversely, juvenile B. gariepensis in rocky habitats frequently show the bright colour patterns characteristic of B. robinsoni. However, B. robinsoni is distinguished from B. gariepensis by its characteristic vocalization and various morphological features, including: poorly developed parotoid glands, weakly developed tarsal fold, small tympanum, relatively smooth skin, and large eyes.

Historical records of B. robinsoni and B. gariepensis in Namaqualand are confused in that a number of observations of tadpoles and non-calling adult specimens were automatically attributed to B. gariepensis on the assumption that B. robinsoni was restricted to the Richtersveld. However, atlas data used for the distribution map of B. robinsoni are based on the calls of males (which are strikingly different to those of B. gariepensis), and these records are reliable, although not comprehensive.

Occurrence of the two Bufo taxa in the Namaqualand region was confirmed by morphology and vocalization (Branch and Braack 1995). These authors conservatively maintained common taxonomic usage (e.g. Poynton 1964), and assigned the juvenile types of B. gariepensis to the olive-coloured, strongly tuberculate toad commonly associated with plains and valley bottoms throughout the Karoo region. This action, whilst recognizing the existence of an additional taxon restricted to the rocky areas of the Richtersveld, maintained nomenclatural stability of the widespread karroid taxon.

However, the discovery that the rock-associated taxon (B. robinsoni) is more widely distributed through Namaqualand, and is more common there than the karroid form, may necessitate a nomenclatural re-adjustment. If Smith’s juvenile types are (as now seems possible) more likely referable to the rock-living taxon, then Bufo gariepensis A. Smith 1848 is the correct name for this taxon and B. robinsoni becomes a junior synonym. B. gariepensis will then have a more restricted distribution than currently accepted and a replacement name will be required for the more widespread Karoo taxon. In such an eventuality B. tuberculosus Bocage, 1896 (type locality: Linokana, Western Transvaal and Bechuanaland) or B. granti Boulenger, 1903 (type locality: Deelfontein, near Richmond, Cape Province) are available. Resolution of this problem may be possible if DNA-sequence data can be obtained from Smith’s juvenile types of B. gariepensis, currently housed in the Natural History Museum, London.

Habitat

B. robinsoni occurs widely in winter-rainfall Namaqualand and Richtersveld, within the Succulent Karoo and Fynbos (specifically renosterveld) biomes, but also extends eastward into the summer-rainfall Nama Karoo, where it appears to be restricted to inselbergs in Bushmanland.

The species is generally found in or near rocky, sometimes mountainous, areas that offer numerous refugia in rock cracks and holes. Water bodies used for breeding may be temporary or permanent and are usually small, shallow seepages, springs, vleis, seasonal streams and rivers, rain-filled depressions, or man-made dams. In streams, breeding takes place in side-pools or slow-moving backwaters. Large perennial rivers, such as the Gariep and Olifants, are apparently avoided.

Life history

B. robinsoni is a terrestrial, partially rupicolous species that shelters in rock cracks and holes in rocky areas close to permanent or seasonal water sources. It may be locally abundant, and it shares water resources with other Namaqualand endemics, for example, Strongylopus springbokensis and Cacosternum namaquense, as well as with Tomopterna delalandii and Afrana fuscigula.

Vocalization has been recorded from March to October, usually following good rains, indicating that breeding occurs mainly during winter and spring. Males call from the bank or while “seated” in the water, from protected or exposed situations. Grappling with other males for position occurs frequently, and male aggression calls (a variant of the “meeuwing” advertisement call) are often heard. Vocalization (advertisement calls) and amplexus have been observed on several occasions in the presence of metamorphosing tadpoles, indicating that the breeding season is prolonged, or, at least, that it does not consist of a single “explosive” event.

Amplexus is axillary and, as with other bufonids, breeding males develop a black nuptial pad on the thumb. Single males have not been observed attempting to displace amplectant males and this, together with the larger female size, suggests a reproductive strategy involving selection by females.

Some 2000 eggs are laid in strings in water, attached loosely to vegetation or other objects. Tadpoles are typical of those of most southern African bufonids and development is rapid. Apparent schooling behaviour by tadpoles is caused by rapidly drying ponds; under favourable conditions the tadpoles are not gregarious. Mortality amongst eggs and tadpoles is often high because of the rapid drying of pools, or as a result of flash floods. Growth of froglets in captivity is rapid.

The diet, based on the dissection of the stomach contents of four adults, consists mainly of beetles (at least five species), with occasional termites and solpugids. An adult male was seen to feed on a male Cacosternum namaquense. Captive adults readily accepted mealworms and crickets. Predators of B. robinsoni have not been recorded.

Conservation

Branch and Braack (1995) considered B. robinsoni to be restricted to small, spring-fed pools in rocky, arid mountains of the Vandersterrberg in the Richtersveld. Owing to its very localized distribution, threats of habitat destruction and water extraction for livestock farming, and increasing needs of human populations in the region, the species was considered to be “very vulnerable”. However, additional records collected during the early phases of the atlas project indicated that B. robinsoni might have a much wider distribution. Harrison et al. (2001) therefore listed the species as Data Deficient, and its conservation status remained unresolved. Subsequently, during more extensive fieldwork associated with the atlas project, the known range of B. robinsoni has increased considerably. At present, the species is not considered to be threatened and it has been assigned the status of Least Concern (this publication).

B. robinsoni has been recorded in the Richtersveld Contractual National Park and the Goegap Nature Reserve. It is protected in terms of the Western Cape Nature Conservation Ordinance (Ordinance 19 of 1974, as amended), and by the Northern Cape Provincial Ordinance of 2000.

Although the species does not appear to be unduly threatened at present, population monitoring in conservation and other identified areas is recommended. Clarification of the taxonomic status of this species is needed (see above).

Current distribution map

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