South America has a diverse and well-studied toad fauna. The continent’s toads include some decidedly untoad-like taxa, such as the brightly coloured stubfoot toads or harlequin frogs. These remarkable little animals are superficially similar to the better known poison-dart frogs. What makes South America’s toads particularly interesting is that several of them occupy a basal position within bufonid phylogeny: in the phylogeny generated by Frost et al. (2006), Melanophryniscus, an Atelopus + Osornophryne clade, and Dendrophryniscus are all at the base of Bufonidae, while in Pramuk et al.’s (2008) phylogeny, Melanophryniscus, Atelopus, Nannophryne, Rhaebo and Dendrophryniscus are all at the base of Bufonidae. The paraphyly of these South American toads relative to all the others suggests that toads originated on the continent [waving Panamanian golden frog A. zetecki shown here; photo by Brian Gratwicke, from wikipedia].
Melanophryniscus, the South American redbelly toads, are an interesting little group, often mentioned in herpetology books because of their habit of contorting themselves such that their brightly coloured hands and feet and ventral surfaces are exposed when they’re threatened by predators. Other anurans with brightly coloured, toxic ventral surfaces (like fire-bellied toads) do this too: the distinctive, concave-backed posture they exhibit is termed ‘Unkenreflex’. Of the 24 currently recognised redbelly toad species, ten have been named since 2000. Some walk, others hop.
Stubfoot toads or harlequin toads
One of the largest toad genera is the entirely South American Atelopus (86 species are known as of December 2009), the members of which are called harlequin or stubfoot toads, or clown frogs. The ‘stubfoot’ name comes from the fact that many of these toads really do have stumpy looking feet (though the hands are often long and slim). Some species are further unusual in possessing only four toes: a character seen elsewhere among Anura in some brachycephalids and in the African four-digit toads (Didynamipus). What’s more, some species exhibit intraspecific variation in digit number: in A. exiguus from Ecuador, five toes are normal, but some individuals have only four (Coloma et al. 2000) [image below shows intraspecific variation in pedal digital count in A. exiguus; from Coloma et al. (2000). The tiny element on the medial side is the prehallux]. Rather than hopping around, harlequin toads exhibit a distinctive ambling walk [photo of Condoto stubfoot A. spurrelli by Mauricio Rivera Correa, from wikipedia].
In contrast to Bufo (sensu stricto) and other, more familiar, toad groups, the head is longer than it is broad in harlequin toads, and the snout is usually rather pointed, with the upper jaw overhanging the lower one. The tympanum and various other ear structures are absent in many species. Some – most notably the Panamanian golden frog A. zeteki [shown at top of article] – cannot communicate with sounds and use semaphore. For more on this see the Tet Zoo toad article on reproductive biology.
The best known Atelopus species are beautiful little animals. Among the better known species, the Harlequin frog, Veragoa stubfoot or Clown frog A. varius is wonderfully mottled with red and yellow-orange on a black background, while the Panamanian golden frog is (as the name suggests) golden yellow, for example [extent of variation inferred for A. varius by Savage (1972) shown above]. This colouring is aposematic: harlequin toads are highly toxic, with the skin secretions of a single Panamanian golden frog being potent enough to kill 1200 mice (Savage 2002). However, they’re not all brightly coloured: A. lynchi from Ecuador and Colombia (named in 1981) is brown and patternless, and other species might be too (these other species look brown in preservative, and might not have been so dull in life) [a dull individual of A. flavescens shown below (members of the species are normally more colourful: see comments); photo by Hugo Claessen, from wikipedia].
Many harlequin toads frequent montane streams. Their tadpoles are of the specialised, stream-dwelling sort and possess large abdominal suckers. Highly restricted ranges are typical within the group, and it’s been suggested that speciation within this group, particularly in the northern Andes, was driven by Pleistocene glaciation and orogenesis (Lötters & De La Riva 1998). Notably, the more widespread members of the group – like the Condoto stubfoot A. spurrelli and Veragoa stubfoot – are lowland species… though suggestions that these widespread ‘species’ might actually be species complexes consisting of more than one taxon complicate this otherwise tidy picture. As is the case with other speciose toad genera (most notably Bufo sensu lato), there’s a tradition of breaking down this large group into many smaller ‘species groups’, some of which do indeed seem to correspond to clades.
Plump toads, not all of which are plump
Osornophryne Ruiz-Carranza & Hernández-Camacho, 1976, sometimes called the plump toads, is apparently the sister-taxon to Atelopus. In fact the only plump toad species known prior to 1976 was originally included within Atelopus (another six species have since been named). Together with Frostius and some other South American taxa, plump toads are sometimes regarded as belonging to a toad group known as the atelopids or atelopines. A close relationship between Osornophryne and Atelopus appears intuitively unlikely, as the plump toads are – as their name suggests – pretty different in appearance from harlequin toads.
Not only are plump toads, well, plump (though, ha ha, some species are not: adjacent image shows ‘plump’ O. bufoniformis vs gracile O. guacamayo; from Gluesenkamp (1995)), they’re usually said to have a reduced phalangeal formula (though read on) and extensively webbed hands and feet. The webbing has been suggested to be an adaptation ‘for locomotion on rocky and mossy crevices’ (Gluesenkamp 1995, p. 275), and, despite the webbing and reduced phalangeal formula, the digits are long and flexible in some species (specifically the strongly arboreal O. guacamayo). Having said that plump toads have a reduced phalangeal formula, the members of the genus are variable with regard to this feature, both intragenerically and (as in Atelopus) intraspecifically (Coloma et al. 2000).
Plump toads lack parotoid glands and auditory structures (tympanum, tympanic annulus and stapes), and the skin on their heads is firmly fused to the underlying bones. They’re also unusual in having only six or even five presacral vertebrae. The typical number for toads is eight: six presacrals are also present in the South American bush toads (Oreophrynella), but it isn’t yet clear whether this character is convergent or evidence of an affinity. Fusion of the atlas and axis is another unusual feature seen in plump toads, as is inguinal amplexus. While most plump toads are terrestrial, individuals of some species have been discovered at the bottom of stream beds, while others have been discovered in arboreal habitats.
Implications for origins?
Here ends our brief look at some of the most ‘stem-ward’ of living toads: we’ll look at more of them later. Besides indicating a South American origin for crown-toads, what might the taxa discussed here tell us about the earliest members of the crown-toad clade? Aposematic colours are seen in some of these taxa, as are specialisations towards life in cool, rocky environments like upland streams and gorges. However, other basal toad clades (like Rhaebo) aren’t like this, so (as far as I can tell) there’s no obvious indication that such things as aposematism or upland habitat choice were widespread among basal crown-toads.
And – unfortunately – the fossil record doesn’t tell us much either way, as fossil representatives of the lineages leading to redbelly toads, stubfoot toads and plump toads are currently unrecognised. This is despite the fact that these animals must have originated during or before the Paleocene.
For previous articles in the Tet Zoo toads 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)
- Skulls, crests, snouts and giant poison glands: the heads of toads
- Toads of the world: first, (some) toads of the north
- The Natterjack, its life and times
- The resurrection of Anaxyrus
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 – –
Coloma, L.A., Lötters, S. & Salas, A. W. 2000. Taxonomy of the Atelopus ignescens complex (Anura: Bufonidae): designation of a neotype of Atelopus ignescens and recognition of Atelopus exiguus. Herpetologica 56, 303-324.
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.
Gluesenkamp, A. G. 1995. A new species of Osornophryne (Anura: Bufonidae) from Volcán Sumaco, Ecuador with notes on other members of the genus. Herpetologica 51, 268-279.
Lötters, S. & De La Riva, I. 1998. Redescription of Atelopus tricolor from southeastern Peru and adjacent Bolivia, with comments on related forms. Journal of Herpetology 32, 481-488.
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.
Savage, J. M. 1972. The harlequin frogs, genus Atelopus, of Costa Rica and western Panama. Herpetologica 28, 77-94.
– . 2002. The Amphibians and Reptiles of Costa Rica. University of Chicago Press, Chicago and London.