November 24, 2011 | 11
As you’ll know if you’ve been following Tet Zoo for any length of time, I’ve been slowly working my way through the toads of the world for the past few years – yes, all of them, more or less. Seeing as there are about 540 living toad species, this may take a while. I’m currently dealing with that section of the cladogram that seems pretty much dominated by African lineages [the adjacent cladogram - a massively simplified version of the one generated by Van Bocxlaer et al. (2010) - is shown here]. Different studies have produced somewhat different cladograms, but there’s some support for what I’ve been calling the ‘African clade’ [note its position in the adjacent cladogram]. Recent articles introduced the concept of the ‘African clade’ and also covered the 20-chromosome toads. Here, we embark on a look at more members of the ‘African clade’…
The dwarf mountain toads
Capensibufo, the two species of cape toads, mountain toadlets or dwarf mountain toads (C. tradouwi and C. rosei), is poorly known and restricted to South Africa. Long included in Bufo*, they were given their own generic name in 1980. Recent work suggests that they’re close relatives of the also southern African Vandijkophrynus species and also of Mertensophryne and Amietophrynus (Van Bocxlaer et al. 2010).
* For those who haven’t read the previous articles in this series – or aren’t otherwise well-versed in toad taxonomy and phylogeny – an enormous number of bufonid lineages, now known to be well removed from Bufo in the strictest sense (the Eurasian Common toad B. bufo and its close relatives), were originally lumped in to Bufo due to overall phenetic resemblance. See Bidder’s organ and the holy quest for synapomorphies.
Both Capensibufo species lack any trace of digital webbing (a character seen elsewhere among toads only in the South American Truebella species [discussed here]). They produce small clutches of relatively large eggs. Populations conventionally classified as C. rosei (read on) lack a tympanum and columella (the loss of these structures seems to be a familiar theme in toad evolution). Tolley et al. (2010) found that neither currently recognised species is monophyletic and that the numerous distinct lineages recovered within this group of toads require a modified taxonomy. It’s pretty evident that we’re gong to end up with a taxonomy where there are rather more than two species [adjacent photo shows C. tradouwi; from www.grenouilles.free.fr].
As is the case with other earless anurans, much mystery surrounds the way in which earless C. rosei individuals find each other during the breeding season. Intriguingly, the inguinal area in these animals becomes swollen and pink at this time, leading some herpetologists to suggest that odiferous chemical attraction is involved. Tandy & Keith (1972) discussed a 1965 case in which hundreds of (mostly male) C. rosei were discovered packed closely together “like sardines” under rotting reeds fringing shallow breeding pools on Table Mountain. They described this aggregation as “the densest we have ever observed in anurans” (p. 133).
The one with the doughnut-headed tadpoles
Mertensophryne, the African forest toads or dwarf toads or snouted frogs, are remarkable in that their tadpoles have a doughnut-shaped structure on the top of the head. One species now included in Mertensophryne, the Chirinda forest toad M. anotis of Zimbabwe and Mozambique, was given the new generic name Stephopaedes – meaning ‘crown-bearing young’ – specifically for this character (Channing 1978: Channing was removing this taxon from Bufo sensu lato, and it had not been included in Mertensophryne when he was writing). That tadpole is shown in the adjacent diagrams (from Channing 1978).
The young of some African forest toads develop incredibly quickly. In a case reported by Müller et al. (2005), those of the Taita dwarf toad M. taitana hatched from their eggs after two days, and had fully metamorphosed within 11 days! This is one of the most rapid metamorphisms reported for any anuran.
What on earth is that doughnut - or crown – on the tadpole’s head for? Dissection shows that it contains a rich net of capillaries not present elsewhere on the body, and it’s kept in contact with the air while the rest of the tadpole is submerged. Given that these tadpoles inhabit stagnant pools, it’s therefore conceivable that the crown might act as an accessory respiratory surface, and this is supported by the fact that the tadpoles keep the crown in contact with the water surface when the water becomes warm and hence low in oxygen content (Müller et al. 2005). It’s also been suggested that the crown might help to keep surface scum away from the eyes and nostrils (Broadley, in Channing 1978).
Mertensophryne was originally a monotypic taxon (Schmidt’s snouted frog M. schmidti was the first member of the genus to be named), but the fact that a group of African toads exhibit Mertensophryne-like characters (such as a digital reduction [only four toes on the feet], absence of the columella and unusual structures on the heads of the tadpoles) has led some workers to suggest that these species belong in the genus too. These latter species were conventionally included in Bufo and known as the ‘Bufo taitanus group’. If Frost et al. (2006) are right in expanding Mertensophryne in this way, then it’s an important African radiation that includes at least 14 species (note that other authors, including Grandison (1980), Graybeal (1997) and Müller et al. (2005), had previously proposed an affinity between Mertensophryne, Stephopaedes and the ‘Bufo taitanus group’). While the characters mentioned a moment ago might plausibly be synapomorphies of this enlarged Mertensophryne, subtle differences in the anatomy of the ‘doughnut’ imply that it might not be homologous across the group [Adjacent photo shows M. micranotis, photographed in Tanzania. Photo from the Vladimir Dinets Homepage].
Tihen (1960) noted that species included within Mertensophryne possess a reduced presacral vertebral formula compared to many other toads – there are only seven presacral vertebrae (eight is normal for toads). Tihen (1960) also referred to extensively developed quadratojugal and palatine bones in the skull. As usual, it would be nice to know if these peculiarities relate to any specific aspect of behaviour or ecology, but (so far as I can tell) no obvious correlations have been reported.
Poynton’s toads and Van Dijk’s toads
Possibly closely related to Mertensophryne is another African toad assemblage conventionally included within Bufo, the ‘Bufo vertebralis group’. These toads possess small tympana and flattened, poorly developed parotoid glands. Frost et al. (2006) argued that DNA evidence published by Cunningham & Cherry (2004) demonstrates that the toads in this group form a clade that they named Poyntonophrynus. The name honours South African herpetologist J. C. Poynton [adjacent P. vertebralis image - by Marius Berger - from the University of Cape Town’s Avian Demography Unit page].
Vandijkophrynus Frost et al., 2006 is the new name proposed for the ‘Bufo angusticeps group’, a southern African group that seems to form a clade with Capensibufo [V. angusticeps - the Sand toad of the South African Cape Flats - is shown below. Image by Trevor and Margaret Hardaker of the excellent Our Wildlife Adventures, used with permission]. This time the generic name commemorates “Eddie Van Dijk, noted South African herpetologist and indefatigable tadpole specialist” (Frost et al. 2006, p. 220). The toads included in this group are proportionally short-legged and at least some have smooth skins. The Sand toad is one of the best known species, and – as suggested by its name – it’s mostly restricted to areas of sandy soil. It has been reported to eat snails on occasion. Vandijkophrynus toads tend to have a dark, reticulate pattern on their dorsal surface, and males have particularly large testes. Smooth skin and hypertrophied testes are also seen in Capensibufo, and consequently an affinity between these two groups of species has long been suggested. It’s recently been apparently confirmed by analysis of mitochondrial and nuclear DNA (Van Bocxlaer et al. 2010).
And that’s where we’ll end for now. More toads real soon.
For previous articles in the Tet Zoo toads series see…
For previous articles on hyloid anurans see…
Refs – -
Channing, A. 1978. A new bufonid genus (Amphibia: Anura) from Rhodesia. Herpetologica 34, 394-397.
Cunningham, M., & Cherry, M. (2004). Molecular systematics of African 20-chromosome toads (Anura: Bufonidae) Molecular Phylogenetics and Evolution, 32 (3), 671-685 DOI: 10.1016/j.ympev.2004.03.003
Frost, D. R., Grant, T., Faivovich, J., Bain, R. H., Haas, A., Haddad, C. F. B., De Sá, R. O., Channing, A., Wilkinson, M., Donnellan, S. C., Raxworthy, C. J., Campbell, J. A., Blotto, B. L., Moler, P., Drewes, R. C., Nussbaum, R. A., Lynch, J. D., Green, D. M. & Wheeler, W. C. 2006. The amphibian tree of life. Bulletin of the American Museum of Natural History 297, 1-370.
Grandison, A. G. C. 1980. Aspects of breeding morphology in Mertensophryne micranotis (Anura: Bufonidae): secondary sexual characters, eggs and tadpole. Bulletin of the British Museum (Natural History), Zoology 39, 299-304.
Graybeal, A. 1997. Phylogenetic relationships of bufonid frogs and tests of alternate macroevolutionary hypotheses characterizing their radiation. Zoological Journal of the Linnean Society 119, 297-338.
Müller, H., Measey, G. J. & Malonza, P. K. 2005. Tadpole of Bufo taitanus (Anura: Bufonidae) with notes on its systematic significance and life history. Journal of Herpetology 39, 138-141.
Tandy, M. & Keith, R. 1972. Bufo of Africa. In Blair, W. F. (ed). Evolution in the Genus Bufo. University of Texas Press, Austin, pp. 119-170.
Tihen, J. A. 1960. Two new genera of African bufonids, with remarks on the phylogeny of related genera. Copeia 1960, 225-233.
Tolley, K. A., de Villiers, A. L., Cherry, M. I. & Measey, G. J. 2010. Isolation and high genetic diversity in dwarf mountain toads (Capensibufo) from South Africa. Biological Journal of the Linnean Society 100, 822-834.
Van Bocxlaer, I., Loader, S. P., Roelants, K., Biju, S. D., Menegon, M. & Bossuyt, F. 2010. Gradual adaptation toward a range-expansion phenotype initiated the global radiation of toads. Science 327, 679-662.