Family Pyxicephalidae

Pyxicephalus adspersus Tschudi, 1838

Giant Bullfrog, Highveld Bullfrog, Giant Pyxi, Grootbrulpadda (A), Letlametlu (P, Sh), Marokolo (S), Lentsoeta (S)

By L.H. du Preez and C.L. Cook

Currently accepted name: Pyxicephalus adspersus
Red listing status: Near Threatened

Photo by Dyke Christopher, 2010. URL: FrogMAP: 361

RED LIST SPECIES

Status: Near Threatened (NT)

Identification

P. adspersus is the largest amphibian found in southern Africa. In Gauteng, males reach a snout-vent length of 245 mm and a mass of 1.4 kg (C.C. unpubl. data.), but in Eastern Cape and Free State provinces, they seldom exceed 140 mm (Du Preez 1996). In contrast to most other frogs, males are larger than females. The head is very broad. Two large bony spines, separated by a smaller spine, project upwards from the lower jaw. Several prominent, interrupted skin ridges are present on the back. Spade-like inner metatarsal tubercles are present on the heels, and are used for digging.

In adults the dorsum is dark olive-green, but may vary from brown to grey and even blue; short sections of the longitudinal skin ridges may be white or cream. In juveniles, a pale vertebral stripe is often present, contrasting sharply with the bright green ground colour. The abdomen is white to creamy-yellow, except in the region of the forelimbs where it is bright yellow in breeding males. Dark mottling may be present in the gular region of males (Du Preez 1996).

The advertisement call is a long, low frequency “whoop”, 1–2 s in duration and with an emphasized frequency of 200–250 Hz. By contrast, the call of P. edulis is much shorter (0.19–0.22 s in duration) with a modulated frequency that begins at c. 250 Hz and rises to 450–600 Hz in the middle of the call (Passmore and Carruthers 1995; Channing 2001).

Distribution

P. adspersus is widely distributed in the atlas region, mainly at higher elevations. It occurs in the northeastern part of the Western Cape, central and southern Eastern Cape, northern, central and eastern parts of Northern Cape, northern KwaZulu-Natal (except the low-lying parts), Free State, North West, Gauteng and Limpopo provinces, and at only a few localities in Mpumalanga Province. North of the atlas region, its range extends to central Namibia, central and northern Botswana and across the highveld of Zimbabwe (Poynton and Broadley 1985b).

P. adspersus and P. edulis are easy to distinguish from one another in the field by differences in their calls and reproductive behaviour. However, many of the morphological characters used to separate them in the past have proven to be too variable to be of diagnostic value (Channing et al. 1994a). While specimens with a snout-vent length exceeding 140 mm may be assigned to P. adspersus with some confidence, smaller individuals cannot be distinguished easily from P. edulis. Therefore, much of the historical data derived from museum and literature records had to be vetted conservatively. For example, records from northern KwaZulu-Natal were omitted from the species distribution map but retained in the genus map. Better knowledge of intra- and inter-specific variation is required before the distribution of these species in the atlas region can be accurately determined (Channing et al. 1994a; Channing 2001).

The atlas data are reasonably reliable given the problems outlined above, but are not comprehensive.

Habitat

P. adspersus inhabits a variety of vegetation types in the Grassland, Savanna, Nama Karoo and Thicket biomes. It typically breeds in seasonal, shallow, grassy pans in flat, open areas but also utilizes non-permanent vleis and shallow water on the margins of waterholes and dams. Although they sometimes inhabit clay soils, they prefer sandy substrates.

Life history

The adults are fossorial, emerging from their burrows only during the breeding season. After heavy rain they congregate in large numbers at breeding sites. Successful breeding depends on the establishment of ephemeral pools large enough to hold water for at least 30 days. Continuous light rain does not seem to prompt emergence of the frogs, but when a downpour of at least 30 mm follows, within a few days, the first light spring rains, they emerge and move to their breeding sites (L.du P. pers. obs.).

P. adspersus is an explosive breeder, finding a mate and laying eggs within 48 hours. Spawning takes place during daylight, usually the morning after a heavy rainstorm. Adult males exhibit three size-related mating strategies, namely territorial, non-territorial (breeding arena) and satellite behaviour (Cook 1996).

Territorial males are larger than non-territorial males, and are site faithful, vigorously defending their territories against intruding males. Males jump at each other with open mouths, and when a male takes hold of the leg or arm of another he will often flip the opponent into the air. One battle, in which a male tried to flip another onto its back, continued for more than 40 minutes. Males often bear battle scars, sometimes even losing an eye (L. du P. pers.obs.) or dying from punctured lungs (C.L.C. pers. obs.).

Non-territorial males gather in a breeding arena or lek where males fight amongst themselves for favourable positions in the centre of the lek (Channing et al. 1994a). On one occasion, 20 calling males were observed within an area no more than 4 m² (L. du P. pers. obs.). Females maintain a low profile as they approach the arena, with barely more than their eyes projecting above the water’s surface. The moment a female is spotted, she is intercepted by the closest male. Sometimes an approaching female dives while she is some distance from the arena and surfaces amongst the males where she is clasped by one of the larger males. Amplexus displacement, in which a second male displaces an amplexing male, is frequently observed in groups of non-territorial males.

The smaller adult satellite males are unable to defend a territory or fight for a place in the non-territorial breeding arena; instead they remain in close proximity to a territorial male, attempting to intercept females attracted to the territorial male (Cook 1996).

Amplexus takes place in water 2–4 cm deep and lasts for an average of 15 minutes. Oviposition usually occurs between 08:00 and 12:00, but may continue until 18:00. When spawning, the male pushes the female’s head underwater and she raises her cloaca above the water; thus the eggs are fertilized before entering the water (L. du P. pers. obs.). A spent female prompts the male to release her by shaking her head from side to side, and then moves away into deeper water.

Eggs numbering 3000–4000 are scattered in shallow water. Eggs are 1.1–1.3 mm in diameter, inside 4-mm capsules (Channing 2001). Small black tadpoles emerge from the capsules after 36 hours, and gather in schools. Over the next two days, schools fuse, creating larger schools until all the tadpoles form a single school. The tadpoles tend to congregate in shallow, warm water where they feed on algae, and complete metamorphosis 18–33 days later (Channing 2001).

Territorial males actively defend their offspring for the duration of their larval development. They scare off intruders by assuming an aggressive attitude and even jump and snap in their direction. If a school of tadpoles becomes isolated in a shrinking pool, the male will dig a channel through the mud to open water, thereby saving the tadpoles from desiccation (Kok et al. 1989). The channels average 3 m, but may reach 18.7 m in length (Cook 1996).

Adults spend dry periods in burrows, usually at depths of 0.5–1 m, depending on the type and humidity of the soil. For example, one male was found at a depth of 10 cm in clay (L. du P. pers. obs.), while others in sandy soil were found at a depth of 1.2 m. Adults sometimes excavate old crab holes to form their burrows (C.C. pers. obs.). After excavating the burrow, the frog sheds several layers of skin to form a cocoon that insulates it from the external environment, with which contact is maintained via the nostrils.

P. adspersus feeds on a variety of prey items including small birds, rats, snakes, lizards, insects, scorpions, crabs, slugs and other frogs. Branch (1976) reported that two adult bullfrogs had ingested no less than 17 newly born Rinkhals Hemachatus haemachatus, a venomous snake species. They have been observed attacking drinking birds such as Laughing Doves and Blacksmith Plovers (C.C. pers. obs.). The species exhibits cannibalism in the adult, juvenile and even tadpole stages.

Birds are major predators of bullfrogs. Records of bird predators include various raptors, for example, the African Marsh Harrier, as well as Marsh Owl, Saddlebill Stork, pelicans and egrets. Tadpoles are preyed upon by various predators, such as terrapins, the Nile Monitor Varanus niloticus, Rinkhals Hemachatus haemachatus, and several fish species, including Barbel Clarias gariepinus and Vlei Tilapia Tilapia sparmanii (in an artificial dam; C.C. pers. obs.).

The maximum longevity of a captive specimen of P. adspersus was estimated at 45 years (Channing 2001).

Conservation

Status

Jacobsen (1982) reported that P. adspersus numbers were declining in Gauteng, North West, Limpopo and Mpumalanga provinces which, at that time, constituted the Transvaal Province. Boycott (2001) declared the species to be extinct in Swaziland. Harrison et al. (2001) estimated that the area of its habitat and population sizes had declined by more than 50% over the past 100 years, particularly in regions subjected to extensive crop agriculture and/or urban and industrial development, including Gauteng, Free State and North West provinces.

In terms of its global distribution, P. adspersus does not appear to warrant threatened status. However, at the sub-regional level, the species has undergone severe population declines in certain areas, for example, >80% in the last 20 years in parts of Gauteng, and was therefore classified Near Threatened in the atlas region (Harrison et al. 2001; this publication).

The species is known to occur in the following protected areas: Karoo, Kalahari Gemsbok and Vaalbos national parks, and Karoo, Sandveld, Soetdoring, Willem Pretorius, Koppiesdam, Borokolalo, S.A. Lombard and Oviston nature reserves (Harrison et al. 2001).

Threats

Habitat loss due to crop agriculture and urbanization poses a major threat to this species. Adults migrating to, and juveniles dispersing from, breeding sites are often killed on roads. The use of insecticides and herbicides may also have a negative impact on breeding success, but requires further investigation (Harrison et al. 2001). The illegal collection of adults and juveniles for the local and international pet industries has contributed to population declines in urban areas.

P. adspersus forms part of the traditional diet of people living in certain parts of Limpopo Province where the species is now being commercially exploited. Large numbers of frogs are being indiscriminately and illegally collected at breeding sites and sold at butcheries (L.R. Minter pers. comm.).

Recommended conservation actions

Surveys to establish the location of additional breeding sites are needed, and long-term monitoring of at least some sites is imperative (Harrison et al. 2001). Research into limiting factors and the effects of herbicides and insecticides on aestivating adults and foraging tadpoles is recommended (Harrison et al. 2001).

In areas where P. adspersus is being sold as food, steps should be taken to ensure sustainable use and the enforcement of relevant conservation laws.

The longevity and slow development of individuals clearly indicate a need for sustained conservation strategies to preserve the natural structure of populations. Successful conservation of P. adspersus depends on long-term protection of suitable breeding sites along with sufficient surrounding habitat to maintain adult populations. Where long-term survival of a population is unlikely in the face of land transformations, translocation of tadpoles or newly-emerged juveniles to neighbouring localities may be attempted, but only as a last resort (Harrison et al. 2001).

This spectacular species certainly qualifies as one of the “Big Five” herptiles of the region. Conservation agencies should consider the ecotourism potential of the Giant Bullfrog.

Current distribution map

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