Today we’re going to look at one of the most remarkable groups of frogs in the world. And as of July 2015, there are over 6540 anuran species, so that’s a lot to choose from.

Banded horned treefrog Hemiphractus fasciatus, photo by Brian Gratwicke of the Panama-based Amphibian Rescue & Conservation Project. Image CC BY 2.0

Yes, we’re looking at the horned treefrogs, horned frogs or casque-headed frogs* (Hemiphractus), a group of six species of Central and South America, the most recently named of which is H. helioi Sheil and Mendelson, 2001. Long-time readers might recognise this text as a re-vamped version of something that appeared on Tet Zoo ver 2 back in 2007.

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

I saw drawings of these frogs some time before I ever saw a photo (or a live one) and had doubts about them looking as ridiculous as they do in my drawing here... but, nope, they really do look this ridiculous.... or awesome, take your pick.

Horned treefrogs are predominantly terrestrial animals of the tropical forest floors of Panama, Colombia, Ecuador, Brazil, Peru, and Bolivia. They also climb in low vegetation. They’re nocturnal, highly cryptic, and reportedly voiceless (raising the question of how males attract females... does anyone know?). They prey on smaller anurans, lizards, and large arthropods. If disturbed, they gape to reveal a bright yellow mouth and tongue, and they’re said to bite if handled, and to bite hard and refuse to let go.

Bitey frogs. 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 frog Gastrotheca guentheri (more on marsupial frogs 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.

Beautiful image of Hemiphractus in the wild by the incomparable Carel Brest van Kempen. Used with permission.

Why the wide bony head with its remarkable horn-like structures? 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 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're threatened by the spread of chytrid fungi, and efforts are underway in Panama and elsewhere to rescue wild individuals.

Spix's horned treefrog Hemiphractus scutatus mother with froglets on her back. Image by Santiago Ron, CC BY-ND 2.0.

Marsupial frogs without a marsupium, or something else? Where does Hemiphractus sit within the frog family tree? These frogs are uncontroversially regarded as part of the huge anuran clade Hyloidea (the great group that includes treefrogs, glassfrogs, poison dart frogs, toads and so on). Within this clade, they've most frequently been regarded as part of the marsupial frog group and, furthermore, within the treefrog group Hylidae. A complication here is that all Hemiphractus species lack a pouch. Instead, their eggs adhere to a special patch of skin on the female’s back where 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... well, there’s an entire radiation of South American frogs that possess both cleft-like dorsal pouches (as in the Flectonotus species) and fully enclosed marsupium-type pouches (as in the Gastrotheca species).

Montage of some (not all) hyloid lineages, Hemiphractus in the middle. These illustrations and many others are for my in-prep giant textbook... on which go here.

Most of these frogs don't have free-swimming tadpoles – instead, the eggs develop directly into froglets within the mother's pouch (this is a strategy that's evolved on numerous times within amphibians. It's termed direct development). But not all marsupial frogs are like this. Some of them (e.g., some Gastrotheca species) have free-living, pond-dwelling larvae. Phylogenetic studies indicate that the tadpole stage was re-evolved in these species, and that they descend from a pouch-brooding, direct-developing ancestor (Mendelson et al. 2000).

Anyway, 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 the pouch.

Yes, there are frogs that keep their eggs (and froglets) in a pouch on the back. This illustration from 1874 (in the public domain) shows the best-known example of this group: the Common marsupial frog Gastrotheca marsupiata (though note that this 'species' may well be a species complex).

However, whether Hemiphractus is anything to do with the marsupial frogs proper has been contested. In a major study of hylid treefrogs, Faivovich et al. (2005) did not support the monophyly of a Hemiphractus + 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. Frost et al. (2006) didn’t support hemiphractine monophyly and scattered the members of the group about the hyloid tree. They also found horned treefrogs to be a distinct lineage near the base of the huge hyloid clade Nobleobatrachia (home to brachycephalids, hylids, dendrobatids, bufonids and many others).

More recently, Pyron & Wiens (2011) also found Hemiphractus to be outside of Hylidae proper; however, they did support the monophyly of a clade that includes marsupial frogs and Hemiphractus. Furthermore, Hemiphractus is surrounded in their phylogeny by pouch-bearing lineages: by Flectonotus on one side, and by Stefania and Gastrotheca on the other. While it remains possible that those two lineages (Flectonotus, and Stefania + Gastrotheca) could have evolved their pouches independently, the simplest possibility is that a pouch was present in their common ancestor... in which case Hemiphractus is a secondarily pouchless frog. Why might these bizarre and incredible frogs have lost their pouches? Questions questions questions...

Phylogenetic hypothesis for hyloids (though with a great many lineages not shown). Note that hemiphractids are not hylids, and are outside the clade (Athesphatanura of Frost et al. (2006)) that includes hylids. The diagram here is a simplified version of a more complex version being prepared for my vertebrate history textbook.

Finally... you will know that there's an amphibian conservation crisis going on right now: about 32% of species are regarded as endangered (that's more than twice as many birds or mammals are endangered). Populations and species are being lost, amphibian-killing chytrid fungi are spreading and appearing in new areas and new species, and habitat loss, the restaurant trade and the pet trade are all taking a serious toll on amphibian numbers. You can help by raising awareness and - if you can - by donating to the several worthy causes. Consider helping Panama's Amphibian Rescue & Conservation Project, the EDGE Amphibians conservation programmes, or Save the Frogs. If you're on twitter, there are lots of amphibian researchers to follow - I've been setting up a list here.

For previous Tet Zoo articles on frogs and toads, see...

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 greeningiJournal 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.

Pyron, R. A. & Wiens, J. J. 2011 A large-scale phylogeny of Amphibia including over 2,800 species, and a revised classification of extant frogs, salamanders, and caecilians. Molecular Phylogenetics and Evolution 61, 543-583.

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.