Family Bufonidae

Bufo vertebralis A. Smith, 1848

Southern Pygmy Toad, Pygmy Toad, Suidelike Dwergskurwepadda (A)

By M.F. Bates

Currently accepted name: Poyntonophrynus vertebralis
Red listing status: Least Concern

Photo by Hardaker T. & , 2013. URL: FrogMAP: 759

Distribution

B. vertebralis is endemic to the atlas region, although recent records from North West Province suggest that it may also occur in southern Botswana. Its distribution follows that of the Nama Karoo Biome in the Northern, Eastern and Western Cape provinces and the southern Free State, but it also occurs in grassland and savanna at Kimberley, north of Christiana, and in the Free State. It occurs as far east as Maseru (2927BC) and near Masite (2927CB) in western Lesotho. Recent atlas records have extended the known western limits of the species as far as the Brandvlei district (3020AD) of Northern Cape Province.

Further north, the species has been recorded in two apparently isolated populations, namely, the Koppies area (2727BA) of the northern Free State (Bates 1995), and on the border between Limpopo and North West provinces (recent atlas data) where it appears to occur in sympatry with B. fenoulheti. The confirmed presence of B. vertebralis, based on advertisement calls (atlas data), supports Jacobsen’s (1989) identification of specimens from grid cells 2428AC and 2528CA. The latter identifications were previously considered dubious because these areas were far north of the known range (Jacobsen 1989).

With regard to the apparently disjunct distribution of B. vertebralis in the Free State, Bates (1995) reported the existence of two museum specimens from “Adonasfontein, Winburg”, a locality that cannot be traced. If the latter locality is in the vicinity of Winburg (2827CA), it will bridge the large gap between southern and northern populations in the Free State.

The taxonomy of the “vertebralis group” (Poynton 1964; Poynton and Broadley 1988) requires revision to determine the exact ranges of the various species. An attempt should be made to identify morphological differences between B. vertebralis and B. fenoulheti in areas of sympatry, to assist in the identification of specimens from these areas.

The atlas data are accurate but incomplete.

Habitat

B. vertebralis inhabits primarily the Nama Karoo Biome but is also found in parts of the Savanna and Grassland biomes. The species is largely restricted to summer-rainfall areas, but has been recorded in some parts of the Nama Karoo that are transitional between summer and winter rainfall.

It occurs on a variety of substrates, from brackish soils to gravels, in open sandy and grassy areas and in Karoo scrub. It takes refuge under rocks and logs, in mud cracks, deep leaf litter, and occasionally in the abandoned mounds of Trinervitermes termites, sometimes far from open water (De Waal 1980; H. Braack pers. comm.).

Breeding habitat in the Karoo includes temporary shallow pans, pools or depressions containing rain-water, and occasionally culverts and rocky pools in seasonal water courses (new atlas data). In the Free State, tadpoles have been found in roadside pools, small dams, quarries and rock pools along rivers (new atlas data).

Life history

Choruses of B. vertebralis develop from October to March. It is an explosive breeder, congregating in large numbers at temporary pools after heavy spring or summer rains (H. Braack pers. comm.). Visser (1979b) recorded thousands of specimens crossing the national road at Victoria West after rain. The call carries a great distance and a chorus produces a deafening sound (Du Preez 1996).

Males usually call from concealed sites near the water’s edge, but when there is insufficient cover, they call from exposed positions. In large breeding aggregations, males clasp other individuals indiscriminately, regardless of gender. Satellite behaviour, in which silent males intercept and clasp females that are approaching calling males, was observed at both Beaufort West and Middelburg (H. Braack pers. comm.).

Wager (1965, 1986) collected eggs at the beginning of March; this suggests that breeding occurs after mid-summer rains. The eggs measured <1 mm in diameter and were laid in double strands 2.5 mm thick. They were entangled amongst stones and grass 2–4 cm beneath the surface. Although Wager’s aquarium tadpoles completed metamorphosis after about one month, Power (1927a) recorded a developmental period of only 16 days. When feeding, Wager’s tadpoles showed a preference for the stems of aquatic plants rather than algae, whereas Power (1927b) observed tadpoles feeding on algae and mud at the bottom of a natural pool.

Adult frogs prey on termites, ants, aphids, fly and beetle larvae, adult beetles, and mites (Bates and Irish 2002). During large-scale emergences of B. vertebralis in the breeding season, many are killed by road traffic, and Suricates Suricata suricatta and Black Crows Corvus capensis have been observed feeding on the remains. Captive Suricates and Polecats Ictonyx striatus have also been observed feeding on these toads (H. Braack pers. comm.).

Conservation

In the Free State, B. vertebralis occurs in Soetdoring, Krugersdift Dam, and Koppies Dam nature reserves (Bates 1997). It has also been collected at Oviston Nature Reserve and Mountain Zebra National Park in Eastern Cape Province. The species enjoys varying degrees of protection under provincial conservation legislation. It may have been negatively impacted by crop agriculture in Grassland-Biome portions of its range, whereas it is probably not threatened in karoid areas where land is used primarily for grazing. However, the Karoo is renowned for periodic outbreaks of the Brown Locust Locustana pardalina, and over-zealous use of insecticides could affect local frog populations. Monitoring of populations of B. vertebralis in protected areas is recommended. Additional information on the breeding biology of this endemic species is needed.

Current distribution map

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