Tetrapod Zoology

As per bloody usual, I'm going to struggle to get a 'proper' post finished today, so yet again I'll have to settle for posting something short and sweet (PS - I failed miserably). Who am I kidding anyway - 95% of the bloggers in the blogosphere routinely produce articles that are shorter than my 'short' posts. I'm sure Tet Zoo readers know how lucky they are :) I'm reading a lot about obscure frogs and toads at the moment, hence the adjacent image. But more on that in a moment. Let me start by saying a very belated congrats to Brian Switek - better known as Laelaps - for hitting the big time and becoming a scibling (science-blogger). If you haven't checked, or visited, or bookmarked, his new Sb site then do so now: it's here. As most of you will know, you can only join Sb by invitation, so getting ushered into the club is a very special event. Well done Brian! And thanks too for awarding me with an Intellectual blogger award: according to the rules, I now have to nominate five other bloggers for awards. I'll do this in due course.

Anyway, back to the important stuff....

.... weird anurans. The animal shown here is one of the most remarkable frogs in the world (and there are over 5200 species of anurans, so there are a lot to chose from), yet it's obscure and poorly known. It's one of the horned treefrogs, horned frogs or casque-headed frogs* (Hemiphractus), a group of six species from Central and South America (most books talk about five species because they were published before a sixth, H. helioi Sheil and Mendelson, 2001 was recognised as distinct). The one shown here is the Banded horned treefrog H. fasciatus Peters, 1862 (picture from amphibiancare.com). Horned treefrogs are predominantly terrestrial animals of tropical forest floors in Panama, Colombia, Ecuador, Brazil, Ecuador, Peru, and Bolivia. They also climb in low vegetation. They are nocturnal, highly cryptic, and reportedly voiceless (so how do males attract females?) and prey on smaller anurans, lizards, and large arthropods. If disturbed, they gape to reveal a bright yellow mouth and tongue, and they are said to bite if handled, and to bite hard and then refuse to let go.

* This name (or casque-headed treefrog) is also sometimes used for a variety of other taxa, including the Brazilian Greening's frog Corythomantis greeningi, and members of the genera Argenteohyla, Osteocephalus, Tepuihyla and others.

On biting, I was always confused by Chris Mattison's (1987) statement that these frogs bite with their 'lower tusks'. Say what? No anuran has lower jaw dentition, with the notable exception of the marsupial treefrog Gastrotheca guentheri [more on marsupial treefrogs in a moment]. Actually, I've since learnt that Hemiphractus has remarkable tooth-like 'odontoids' on some of its lower jaw bones (the dentaries and angulosplenials). In contrast to the structures in G. guentheri, these aren't true teeth (Shaw 1989, Shaw & Ellis 1989). Incidentally, the presence of mandibular dentition in G. guentheri is (among tetrapods) one of the best examples of re-evolution of a lost structure - ha, take that Louis Dollo (1857-1931).

Why the wide bony head with its remarkable horn-like structures? I haven't read of an explanation - is there one? In some other frogs with excessive bony growths on the skull, thickened bones and horn-like growths are used to plug the entrance to a burrow (Jared et al. 2005), a behaviour termed phragmosis. Does Hemiphractus do this? I don't know, but I don't think so. Horned treefrogs aren't particularly large, with females reaching an SVL of 70 mm or so. Like so many frogs and toads around the world, they are now threatened by the spread of chytrid fungi, and efforts are underway in Panama and elsewhere to rescue wild individuals.

Although sometimes said to be marsupial frogs, horned treefrogs do not have a pouch: instead, the eggs simply adhere to a special patch of skin on the female's back. Here the eggs metamorphose directly into miniature adults and don't go through a tadpole phase. If the idea of frogs with pouches is new to you, there is an entire radiation of South American frogs (there are multiple species grouped into two or four genera, depending on which taxonomic scheme you follow) that possess both open pouches (as in the Flectonotus species) and enclosed marsupium-type pouches (as in the Gastrotheca species). However, not all members of this marsupial group (it's called either Hemiphractinae or Amphignathodontinae or Amphignathodontidae, depending on which phylogeny you favour) do this - some of them (e.g., some Gastrotheca species) produce aquatic larvae that grow up in ponds. Some phylogenetic studies indicate that the tadpole stage was re-evolved in these species, and that they descend from a pouch-brooding ancestor (Mendelson et al. 2000) [the photo below shows the Pygmy marsupial frog Flectonotus pygmaeus].

Many anuran workers have regarded horned treefrogs as close kin of these marsupial frogs (e.g., Lutz 1968, Duellman 1970, Trueb 1974, Mendelson et al. 2000), hence the use of the name Hemiphractinae for the marsupial frog group. If this is correct then the 'eggs on the back' system used by Hemiphractus may have been the first step in the evolution of true pouches. However, whether Hemiphractus is anything to do with the true marsupial frogs is now contested. In a major study of hylid treefrogs, Faivovich et al. (2005) did not support the monophyly of a Hemiphractus + true marsupial frog clade, nor did they find that any so-called hemiphractines could be included within Hylidae. They suggested instead that these frogs might be part of Leptodactylidae (in its traditional form, this is essentially a huge taxonomic dustbin for any South American anurans that aren't obviously anything else). Frost et al. (2006), in their monumental tree of life contribution on lissamphibian phylogeny, didn't support hemiphractine monophyly and scattered the members of the group about the neobatrachian tree. Horned treefrogs were found to be on their own near the base of the huge neobatrachian clade Nobleobatrachia (home to brachycephalids, hylids, dendrobatids, bufonids and many others), and were awarded their own 'family', Hemiphractidae.

Why all this sudden interest in anurans? In the interests of maintaining balance, Tet Zoo has suffered from a lissamphibian deficit that I'd like to correct (this balance also partly explains the recent and continuing absence of bird posts: see the explanation on ver 1 here). However - more importantly - I'm gearing up for involvement in a new conservation effort involving the world's lissamphibians. More news on that soon.

Oh well, so much for a 'short and sweet' post dammit. Will I ever learn...

For previous Tet Zoo articles on anurans see pieces on the Helmeted water toad and Britain's lost treefrogs.

Refs - -

Duellman, W. E. 1970. The hylid frogs of Middle America. University of Kansas Museum of Natural History 1, 1-753.

Faivovich, J., Haddad, C. F. B., Garcia, P. C. A., Frost, D. R., Campbell, J. A. & Wheeler, W. C. 2005. Systematic review of the frog family Hylidae, with special reference to Hylinae: phylogenetic analysis and taxonomic revision. Bulletin of the American Museum of Natural History 294, 1-240.

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.

Jared, C., Antoniazzi, M. M., Navas, C. A., Katchburian, E., Freymüller, E., Tambourgi, D. V. & Rodrigues, M. T. 2005. Head co-ossification, phragmosis and defence in the casque-headed tree frog Corythomantis greeningi. Journal of Zoology 265, 1-8.

Lutz, B. 1968. Taxonomy of Neotropical Hylidae. The Pearce-Sellards Series 11, 1-25.

Mattison, C. 1987. Frogs & Toads of the World. Blandford, London.

Mendelson, J. R., Da Silva, H. R. & Maglia, A. M. 2000. Phylogenetic relationships among marsupial frog genera (Anura: Hylidae: Hemiphractinae) based on evidence from morphology and natural history. Zoological Journal of the Linnean Society 128, 125-148.

Shaw, J. P. 1989. Observations on the polyphydont dentition of Hemiphractus proboscideus (Anura: Hylidae). Journal of Zoology 217, 499-510.

- . & Ellis, S. A. 1989. A scanning electron microscope study of the odontoids and teeth in Hemiphractus proboscideus (Anura: Hylidae). Journal of Zoology 219, 533-544.

Trueb, L. 1974. Systematic relationships of the netropical horned frogs, genus Hemiphractus (Anura: Hylidae). Occasional Papers of the Museum of Natural History University of Kansas 29, 1-60.