Family Bufonidae

Bufo Laurenti, 1768

toads, true toads, skurwepaddas (A)

By M. Cunningham

Currently accepted name: FAMILY Bufonidae
Red listing status:

Description

Bufo is Latin for “toad”. Toads are among the most distinctive and familiar of frogs. They vary from moderately small to very large and are generally robustly built, with short legs, an upright stance, thick dry glandular skin, and a bony skull with a short, stout snout. Bufo lack teeth and are further distinguished from frogs in other families by internal anatomical features of musculature and bones, and by unique structures such as Bidder’s organ, an outgrowth of the testis that develops into a fully functional ovary if the testes are removed (Griffiths 1959).

Among the most obvious external characteristics of Bufo are the parotoid glands, which are prominent swellings in the head and neck region behind the eye. These glands secrete a complex mix of compounds that protect the skin, and toxic chemical defences when toads are physically aggravated (Lyttle et al. 1996). The shape, location and size of parotoid glands are important in distinguishing different species of this genus (Poynton and Broadley 1988).

Bufo eggs and larvae are also easily recognizable. The eggs are black and are generally laid in two strings that are often entangled around vegetation. The tadpoles are small, numerous and black, and tend to associate with each other in disorganized swarms. Both the eggs and tadpoles of Bufo are toxic when consumed by vertebrates, possibly as an adaptation to predation by other tadpoles and fish (Crossland and Alford 1998; Crossland and Azevedo-Ramos 1999).

Although these traits characterise Bufo, none of them is truly diagnostic for the genus, that is, all these characters are shared to a greater or lesser extent with the other 32 genera in the family Bufonidae, each of which possesses additional traits that distinguish it from Bufo. It is likely that future revisions will alter the composition of this genus to address this problem (Graybeal and Cannatella 1995).

Distribution

Bufo is among the largest and most cosmopolitan of frog genera. At present, 252 species are recognized and these occur naturally throughout the world, with the exception of Australia, Antarctica, the Arctic, New Guinea, Madagascar, New Zealand and smaller islands of the Pacific and Indian oceans (Frost 2002). The Central and South American species Bufo marinus, was introduced to Australia and many tropical islands in the 1930s as a potential biological control of sugar cane beetles. The experiment was unsuccessful but left teeming numbers of this invasive species that continue to threaten indigenous wildlife, including native frogs (Crossland and Alford 1998). Bufo gutturalis, a species in the atlas region, was similarly introduced to the Mascarene Islands around the same time, and currently thrives in Mauritius, where the introduced B. marinus appears to have gone extinct (Owadally and Lambert 1988).

The greatest diversity of Bufo occurs in Africa, Central America, South America and Southern Asia (Blair 1972). There are 64 African species, of which only two extend beyond the continent into the Mediterranean region (Frost 2002). In Africa, the genus is ubiquitous, with the possible exception of hyper-arid parts of the Sahara desert (Poynton 1996). No single species is distributed throughout the continent, although some, especially B. maculatus, range over a vast area (Tandy and Keith 1972). Bufo, along with Afrana, is the most widespread genus in the atlas region and at least one species is expected to occur in every quarter-degree grid cell. There are 13 Bufo species in the atlas region, eight of which are endemic.

Habitat

Bufo occurs in all biomes of the atlas region, across all habitat gradients, from desert along the northwestern coast to rocky semi-arid Karoo shrublands, to the alpine grasslands of Lesotho, the amber waters of fynbos swamps in the southern and southwestern Cape, and the sub-tropical savannas of northern KwaZulu-Natal. There are no specialized forest-inhabiting Bufo within the atlas area and the occurrence of toads in this biome is peripheral and sporadic.

All species within the atlas region are terrestrial and breed in standing or slow-flowing water. The actual breeding sites vary among species from small rainwater puddles to deep permanent ponds and rivers. Shelter sites are generally under rocks, fallen wood or leaves, and although all Bufo dig to avoid desiccation, these retreats are invariably shallow when compared with those of true burrowing frogs.

Life history

Within the atlas region, Bufo are seasonal breeders and go through a period of inactivity in the non-breeding season. Species vary in the timing of reproduction, with late-winter/spring breeders (B. angusticeps, B. pantherinus and B. robinsoni) predominant in the winter-rainfall region in the southwest, and the remaining species breeding in summer, including populations of B. rangeri in the southwest. Species also differ in the length and continuity of the breeding season, from the seven-month calling season of B. garmani to the abrupt irruptions and chorusing of B. pantherinus after good rains in August or September.

Most Bufo migrate to breeding sites from the surrounding area, and some individuals move considerable distances to join choruses and find a mate. Many species show fidelity to their chosen feeding or breeding sites and will return to the same location even when moved several kilometres away. Bufo species generally call at night, usually in the company of other individuals, although at times of peak breeding, choruses may continue throughout the day.

Males usually actively search for mates and physically attempt to displace males that are already in amplexus. This system of scramble competition is generally associated with a short breeding season. Another mating system involving female choice has been observed in B. rangeri (Cherry 1993). In this case, males establish aural territories and compete by calling (see B. rangeri account).

Females lay from several hundreds to tens of thousands of eggs, depending on the species. These hatch into tadpoles within a week and after a short larval period – less than a month for smaller species (B. vertebralis and B. angusticeps groups) and around 5–6 weeks for larger species (B. pardalis, B. rangeri and B. gutturalis groups) – the small tadpoles metamorphose into tiny toadlets (Wager 1986). Juvenile toads suffer high mortality while growing to maturity, which may not be reached for several years.

Conservation

Most Bufo species in the atlas region have relatively broad distributions and are abundant throughout much of their range. Two exceptions are the range-restricted species, B. pantherinus and B. amatolicus, both classified Endangered here (cf. Harrison et al. 2001). B. robinsoni was previously thought to be restricted to a very limited habitat type, but in gathering atlas data this species was found to be more widespread and has been placed in the category Least Concern pending taxonomic revision (see species account). Additional taxonomic studies are required for B. gariepensis to determine whether localized montane forms, such as B. g. nubicolus from the Drakensberg rim, are distinct and threatened taxa.