Cacosternum Boulenger, 1887
cacos, dainty frogs, blikslanertjies (A)
Currently accepted name: Cacosternum sp.
Red listing status:
The genus is characterized by the presence of a reduced pectoral girdle, in that all species lack an ossified omosternum and the clavicle is absent (De Villiers 1930, 1931). This is reflected in the name, which is derived from the Greek: kakos = bad (i.e., poorly developed); stérnos = breast.
These are “slender, rather elongated little frogs” (Lambiris 1989a: 109). The dorsal colouration and markings are cryptic and highly variable. Externally, the presence of well-defined, irregular, dark patches on a smooth white abdomen, in combination with the absence of a medial lingual process, mid-tarsal tubercle, webbing between the digits, and a visible tympanum, are indicative of the genus in southern Africa.
Cacosternum is essentially a non-tropical genus centered in southern Africa (Poynton 1964). The genus contains nine currently recognized species. Taxonomic research underway at the time of writing indicates that three or more new species await description (M. Burger, E. Scott unpubl. data), C. n. nanum and C. n. parvum should be elevated to the status of full species, and C. poyntoni should probably be assigned to the synonymy of C. nanum nanum (unpubl. data). Five species are endemic to the atlas region, three extend beyond its borders and one, C. leleupi (Laurent 1954), is known from only 12 specimens collected in Shaba (Katanga) Province, Democratic Republic of Congo.
Three closely related species, C. namaquense, C. karooicum and C. capense, occur predominantly in the winter rainfall regions of Western Cape Province, with one of these, C. namaquense, just reaching the extreme south of Namibia (unpubl. data). C. nanum nanum occurs along the coast from the southern Western Cape Province to northern Kwa-Zulu Natal and Swaziland, extending marginally into Mozambique, while another taxon, C. n. parvum, occurs along the high-altitude escarpment from Lesotho to the Limpopo Province.
The remaining species are all closely related to C. boettgeri (unpubl. data), which extends from the Western Cape northward to Namibia (excluding Namaqualand) in the west, and the Zimbabwean plateau, eastern Botswana and southern Zambia in the east. Populations of C. boettgeri that occur in the uplands of Kenya and Tanzania possess a different call from the southern African boettgeri and may represent a different species. The C. boettgeri complex displays a typical “arid corridor” distribution, similar to that postulated by Poynton (1995) for the dwarf members of the genus Bufo.
The genus Cacosternum inhabits all southern African biomes. Many species seem to prefer open areas with short grassy vegetation.
C. namaquense and C. karooicum are restricted to arid areas, favouring rocky places where they can shelter in cracks and crevices, beneath rocks and under exfoliating granite. Most other species are found in both arid and mesic habitats.
Cacosternum species are adapted to breed in shallow temporary pools, typically in flooded ruts and inundated grasslands, ditches and the shallow backwaters of open grassy pans. These habitats have fewer aquatic predators and are warmer than permanent pools, which facilitates faster development of the larvae (Van Dijk 1971a). The ability to breed in shallow pools has enabled Cacosternum to survive in regions that have become increasingly arid in the last few million years (Van Dijk 1977). Members of this genus do not breed in lakes or large permanent ponds.
Calling males usually require some vegetation cover, although they have occasionally been found calling on sand banks around pans with no fringing vegetation. In some instances, males shelter in animal footprints or small cracks in the mud, or call from underneath rocks.
Cacosternum lay many small, pigmented eggs. Amplexus is axillary, as in most other genera of the family Petropedetidae. During amplexus, the pair clings to emergent vegetation, depositing a mass of eggs, then moves further along and lays another small clutch. Females can lay up to 400 eggs at a time, deposited in individual clutches of 8–50 eggs (Wager 1986). Eggs hatch 2–3 days after laying. The frequency with which the females become gravid per year is unknown.
The growth rate to metamorphosis is extremely rapid and is unrivalled by any other anuran (Duellman and Trueb 1986; Boycott et al. 2002). Mortality of tadpoles is high, especially in the arid zone species, due to the temporary nature of the breeding pools (Van Dijk 1971a; Wager 1986).
During the dry season, Cacosternum is known to aestivate in cracks in the ground near ponds, sometimes at depths of 30–45 cm (Methuen and Hewitt 1914), and in the bases of tufts of restio roots (Rose 1962), in termitaria (Passmore and Carruthers 1995) and even in the burrows of Sungazer lizards Cordylus giganteus (Jacobsen 1989). C. boettgeri and C. nanum have been observed emerging from mud banks at the sides of small dams in considerable numbers after the first heavy downpour of the rainy season (Wager 1986). The arid zone specialists, C. namaquense and C. karooicum, are dorsoventrally flattened and probably aestivate in cracks in rocks, being active only after rains (Boycott et al. 2002).
These small frogs feed on termites (Passmore and Carruthers 1995), ants (Jacobsen 1989), flies (Rose 1962) and mosquitoes as the latter come to the water to lay their eggs (Wager 1986). Given their numbers, Cacosternum species must play a significant role in the biological control of small insects. In turn, they are preyed upon by many animals, particularly Hammerkops, egrets, larger frogs, small carnivorous mammals and snakes.
All Cacosternum species rely on crypsis to avoid being seen by predators. Most species are agile and quickly dash for safety when disturbed, but the more sluggish C. capense relies mainly on noxious secretions from its dorsal glands to deter predators.
In the previous Red Data book (Branch 1988), C. capense was listed as Restricted and C. poyntoni as Indeterminate. In this publication, the status of C. capense is Vulnerable (Harrison et al. 2001), C. poyntoni is Data Deficient, and C. karooicum and C. striatum are also Data Deficient (see species accounts).
FrogMAP. 2020. Cacosternum Boulenger, 1887. Animal Demography Unit. Accessed from http://frogmap.adu.org.za/?sp=1140; on 2020-09-27 09:09:36.
Minter L.R., Burger M., Harrison J.A., Braack H.H., Bishop P.J. & Kloepfer D. (eds). 2004. Atlas and Red Data book of the frogs of South Africa, Lesotho and Swaziland. SI/MAB Series no. 9. Smithsonian Institution, Washington, D.C. Published by the Smithsonian Institution and the Avian Demography Unit (now Animal Demography Unit).