One of the dirty little secrets of biology is that many groups of organisms have never been ‘defined’ in the phylogenetic sense: a group grows over time as people add new species to it, but they only do this because it ‘feels’ about right, not because there’s any rigorous way of knowing whether those species really belong there or not. Many tetrapod groups – classic examples include Ranidae, Muscicapidae, Colubridae and Scincidae – lack characters that might allow their monophyly to be demonstrated, and modern studies show that various of their constituent members aren’t more closely related to each other than they are to members of other groups. Some herpetologists argue that amphibians have been particularly affected by this sort of thing, given that their taxonomy and systematics have been (comparatively speaking) little studied, and that a form of social conservatism has often dominated decisions about taxonomy (Frost et al. 2006, p. 12). Despite the number of species involved, however, toads don’t seem affected by this, in part because workers have often used what appears to be a synapomorphy when identifying an anuran as a toad: the presence of a Bidder’s organ. Yes, here’s another article on toads (aka bufonids) [adjacent image shows a few of the species conventionally lumped together within Bufo (from top to bottom): Common toad B. bufo (courtesy of Neil Phillips), Southern panther toad Amietophrynus pantherinus (from wikipedia), and Slender-legged toad Leptophryne borbonica (courtesy of John C. Murphy)].
Bidder’s organ is weird. It’s a small structure located close to the gonads: pinkish and with a granular surface texture, it occurs in both sexes and is composed of immature egg cells. Davis (1936) wrote that the Bidder’s organ ‘has long been the subject of speculation and debate, which was occasionally acrimonious and unscientific to an extraordinary degree’, and (so far as I can tell) its origin, function and role in males remains unexplained [Bidder's organ in a male toad shown below, on the right. The organs of a non-bufonid hyloid is shown on the left for comparison. From Davis (1936)].
It has often been stated that the presence of a Bidder’s organ is a bufonid synapomorphy. However, both Melanophryniscus (the South American redbelly toads) and the two Truebella species from Peru lack a Bidder’s organ, and Melanophryniscus was found by Frost et al. (2006) and Pramuk et al. (2007) to be the sister-taxon to all other toads (it has previously (Cannatella 1986) been regarded as one of the ‘atelopids’, and close to Dendrophryniscus and Oreophrynella). This means that the presence of a Bidder’s organ is actually a synapomorphy for the bufonid clade that includes all bufonids more derived than Melanophryniscus (and presumably Truebella); it is not characteristic for the group as a whole. Several other characters are also shared by the members of this clade (viz, all toads except Melanophryniscus and Truebella), including the production of egg strings and the presence of juxtaposed atlantal cotyles (Frost et al. 2006), but other characters exist that seem to be synapomorphies of Bufonidae as a whole. Toad larvae have a diastema in the papillation on the lower lip, and the lungs of larvae are either absent or rudimentary, for example (Frost et al. 2006).
The Bufo problem
By far the most problematic taxon in bufonid systematics is Bufo which, in ‘traditional’ classifications, is a virtually globally distributed genus that contains more than 250 species. As is so often the case with huge, traditionally recognised genera of this sort, Bufo of tradition (Bufo sensu lato from hereon) encompasses a pretty substantial degree of morphological disparity, the extremes of which easily ‘look distinct enough’ to belong to different genera. The giant Cane toad [shown here, from wikipedia], for example, is a thoroughly different beast from the tiny Lugh toad (conventionally B. lughensis) of eastern Africa (SVL less than 33 mm): they look about as different from one another as either does from any of the traditionally recognised non-Bufo bufonid genera.
It’s becoming increasingly recognised that many of the genera (and other higher taxa) recognised ‘traditionally’ have been maintained because of convenience and social inertia, not because good (or any) evidence supports their monophyly. And Bufo s. l. is so morphologically diverse, and so unwieldy and massive, that anuran workers have long taken to referring to ‘species groups’ within the assemblage (about 40 such groups have been recognised). Alan Channing, in 1978, said that ‘splitting a large cosmopolitan genus like Bufo can only aid our understanding of the group’ (Channing 1978, p. 396), and from the 1980s onwards it began to be realised that Bufo s. l. is paraphyletic.
Phylogenetic studies have sounded the death knell for the retention of Bufo s. l.: it is evidently radically paraphyletic with respect to many other long recognised toad genera. Either we sink just about all recognised bufonid genera into Bufo (hardly a useful thing to do), or we split Bufo s. l. into its constituent clades, and name those clades accordingly. This latter process is now well underway, as we’ll see in the following articles [adjacent image: Asian common toad Duttaphrynus melanostictus with unusually prominent cranial crests. Duttaphrynus is one of numerous genera conventionally included within Bufo s. l. Photo by Andrew Johnson ©, used with permission].
Coming next: SEX!
For previous articles in the toad series see…
And 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 – –
Cannatella, D. C. 1986. A new genus of bufonid (Anura) from South America, and phylogenetic relationships of the Neotropical genera. Herpetologica 42, 197-205.
Channing, A. 1978. A new bufonid genus (Amphibia: Anura) from Rhodesia. Herpetologica 34, 394-397.
Davis, D. D. 1936. The distribution of Bidder’s organ in the Bufonidae. Zoological Series of Field Museum of Natural History 20, 115-125.
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.
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.