Toads of the world: first, (some) toads of the north

If you've been following the toad series, you'll have read articles that introduce toads in general, discuss reproductive biology, and look at cranial anatomy. This can all be regarded as background introductory stuff. From hereon, we're mostly going to look at toad diversity in rough phylogenetic order: that is, starting at the base of the clade and working up to the 'top' of the tree [European common toad Bufo bufo shown here, from wikipedia. This individual has really weird nostrils].

Basal toads are all South American, and include the South American redbelly toads (Melanophryniscus) and the stumpfoot or harlequin toads (Atelopus). The South American Rhaebo toads (conventionally included within Bufo) also seem to be basal within the bufonid crown. The rest of toad phylogeny is rather controversial: some studies recover a major Old World clade that includes several Asian, African and Eurasian clades as well as a clade of New World toads including the Cane toad group (Rhinella) and North American group (Anaxyrus) (Pramuk et al. 2008). If this topology is correct, crown-toads - ancestrally South American - radiated extensively in the Old World but re-invaded the Americas during the Eocene. Others studies find Rhinella and so on to be outside of the Old World clade (Van Bocxlaer et al. 2009). We'll be coming back to the various details here later on.

While I mostly want to cover things in rough phylogenetic order, I've decided to start things by looking at the toads that - historically - have been most studied and most influential in terms of shaping our views on this huge amphibian group.

The Bufo bufo group

It all started with Bufo bufo, the Common toad of Eurasia. It's a mostly terrestrial animal with dry, warty skin, and its familiarity to European people explain why the term 'toad' later came to be used for any superficially similar anuran from elsewhere in the world, even those that are more closely related to Rana temporaria, the European common frog. B. bufo is tolerant of cool climates, it's an explosive breeder, is reasonably large (males reach 50-60 mm and females 80-90 mm: as usual with anurans, all lengths given here are snout-vent lengths, or SVLs), and is sexually dimorphic (males are greyish, females are brownish). Its eyes are of an iridescent copper colour and its pupils are horizontal, and eyes like this are seen widely within Bufonidae [male Common toad shown here, from wikipedia].

As is the case with so many species that have a large geographical range, the Common toad is a variable animal, and a consequence of this variation is that a number of subspecies have been recognised; furthermore, distinct populations in various regions have been identified as distinct species, though not without controversy. The Caucasian toad B. verrucosissimus (named in 1814) was generally regarded as a Common toad subspecies until 1987, and today there's a lot of uncertainty over its status relative to the Common toad and to the Giant toad B. spinosus, a form usually regarded as a B. bufo subspecies but raised by some authors to species status [Caucasian toad pair shown here, from Nature of Georgia].

Natterjacks and the green toad group

Another toad is really familiar if you live in western Europe: the Natterjack. Frost et al. (2006) resurrected the generic name Epidalea Cope, 1865, the type species of which is the Natterjack (traditionally B. calamita), in the hope that they would be able to use it for a clade that included the European green toad (traditionally B. viridis) and all of its relatives (known conventionally as the 'Bufo viridis group') [an American toad, conventionally known as B. debilis, is sometimes called the Green toad too, though alternative names include Sonora toad and Dwarf toad].

However, Frost et al. (2006) noted that Graybeal (1997) did not find the Natterjack and European green toad to form a clade and hence concluded that the hypothesis of a Natterjack + green toad group relationship was based entirely on general, phenetic resemblance, and was not supported by synapomorphies. According to Frost et al.'s (2006) favoured taxonomy, the Natterjack is on its own as Epidalea calamita while the former 'Bufo viridis group' is Pseudepidalea Frost et al., 2006. I'm not going to look at Pseudepidalea here, but will come back to it later (it's a particularly confusing and complicated clade. How confusing and complicated? See the figure below, from Stöck et al. (2006)) [Green toad shown here. Ooh, pretty. Photo by Richard Bartz, from wikipedia].

However, some authors have contested the distinction of the Natterjack relative to Pseudepidalea and have extended Epidalea to include the green toad group too: Speybroek & Crochet (2007) argued that Graybeal's sequence data for the Natterjack appeared to contain errors, and they pointed to the data present by Stöck et al. (2006), some of which might support a Natterjack + green toad group (see also Sos 2008). Actually, Stöck et al. (2006) is controversial in that it advocates the recognition of three 'new' European members of the green toad group: B. balearicus Boettger, 1880 from southern Italy and the western Mediterranean islands, B. variabilis (Pallas, 1769) from Asia Minor, eastern Europe, Sweden, Denmark, Greece and Germany, and an unnamed taxon from Sicily. These authors weren't the only ones to support the recognition of these possible species, and debate continues on the systematics and status of these toads. Genetic data indicates that the green toad group might be most closely related to Duttaphrynus and Peltophryne
(Frost et al. 2006), two genera that we'll come to later on.

For previous articles in the toad series see...

• Toadtastic - the invasion begins!
• Bidder's organ and the holy quest for synapomorphies
• Our sex lives in words and pictures (or, On the reproductive biology of the Bufonidae)

For previous articles on hyloid anurans see...

• Britain's lost tree frogs: sigh, not another 'neglected native'
• Ghost frogs, hyloids, arcifery.. what more could you want?
• Green-boned glass frogs, monkey frogs, toothless toads
• It's the Helmeted water toad!
• Horn-headed biting frogs and pouches and false teeth
• More wide-mouthed South American horned frogs
• We need MORE FROGS

Refs - -

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.

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.

Pramuk, J. B., Robertson, J. B., Sites, J. W. & Noonan, B. P. 2008. Around the world in 10 million years: biogeography of the nearly cosmopolitan true toads (Anura: Bufonidae). Global Ecology and Biogeography 17, 72-83.

Sos, T. 2008. Review of recent taxonomic and nomenclatural changes in European Amphibia and Reptilia related to Romanian herpetofauna. Herpetologica Romanica 2, 61-91.

Speybroek, J. & Crochet, P.-A. 2007. Species list for the European herpetofauna - a tentative update. Podarcis 8, 8-34.

Stöck, M., Moritz, C., Hickerson, M., Frynta, D., Dujsebayeva, T., Eremchenko, V., Macey, J. R., Papenfuss, T. J. & Wake, D. B. 2006. Evolution of mitochondrial relationships and biogeography of Palearctic green toads (Bufo viridis subgroup) with insights in their genomic plasticity. Molecular Phylogenetics and Evolution 41, 663-689.

Van Bocxlaer, I., Biju, S. D., Loader, S. P. & Bossuyt, F. 2009. Toad radiation reveals into-India dispersal as a source of endemism in the Wester Ghats-Sri Lanka biodiversity hotspot. BMC Evolutionary Biology 2009, 9:131 doi:10.1186/1471-2148-9-131