Hyperolius viridiflavus, photographed in the Democratic Republic of Congo by Nick Hobgood. Image CC BY-SA 3.0.

Here’s a very brief article to a group of frogs. It’s a slightly modified version of an article that initially appeared on Tet Zoo ver 2 during November 2007.

Reed, sedge and lily frogs, or hyperoliids, are a moderately large group (containing approximately 215 species) of mostly arboreal ranoids that climb in vegetation at or near the water’s edge. All are endemic to sub-Saharan Africa. Many are brightly coloured and boldly patterned, and some (like the Marbled rush frog Hyperolius marmoratus) are famous for being polymorphic – that is, individuals within the same species can look radically different, with some being striped, others being speckled, and some being plain [adjacent photo by Nick Hobgood]. Some hyperoliids (like the Kassina running frogs, one of which is shown below [photo by LA Dawson]) are mostly terrestrial and run well, rather than hop. The males of many hyperoliids have particularly large vocal sacs, these sometimes being larger than the rest of the body when inflated.

Members of the group are also unusual in possessing a disc-shaped gular gland - a unique structure that forms a sort of fatty shield on the leading edge of the vocal sac. Thanks to Mark Scherz, I've just learnt about Starnberger et al.'s (2013) study on this structure. They showed via biochemical and histological techniques that the gland emits volatile compounds, presumably as the same time as the frogs inflate the vocal sac! These frogs are therefore seemingly advertising themselves with both acoustic and chemical signals at the same time. This is not completely novel, since the release of odorous chemicals (including pheromones) has previously been documented in four terrestrial frog groups (Australian ground frogs, toads, poison-dart frogs and Madagascan mantellids) (Starnberger et al. 2013). However, association of a chemical gland with the vocal sac - and hence with vocalisation - seems to be unique to hyperoliids.

Red-legged running frog (Kassina maculata), one of 16 currently recognised Kassina species. Photo by Dawson, CC BY-SA 2.5.

Like the true hyloid treefrogs, hyperoliids possess intercalary elements (accessory digit segments located between the distal and penultimate phalanges). Elsewhere among ranoids, members of the related squeaker group (properly Arthroleptidae) have intercalary elements too, so it seems that these structures have evolved convergently several times among neobatrachians. Indeed, histological work suggests that hyperoliid and squeaker intercalary elements are not homologous.

All hyperoliids have free-living tadpoles, but various methods have evolved to protect the eggs and/or tadpoles from desiccation and predation. Many arboreal hyperoliids lay gelatinous egg masses on vegetation overhanging water (the tadpoles slide down the vegetation to the water on hatching). The leaf-folding Afrixalus frogs protect their eggs by doing what their name suggests. They might lay their eggs either above or below water, but they do the leaf-folding trick regardless.

Imposter! Big-eyed treefrog (Leptopelis vermiculatus) - a leptolepine arthroleptid, not a hyperoliid at all. Image by LA Dawson, CC BY-SA 2.5.

Hyperoliids have traditionally been thought to include the 50-odd species of leaf frog or forest treefrog (Leptopelis), also of sub-Saharan Africa and usually given their own hyperoliid ‘subfamily’ Leptopelinae. However, leptopelines now seem not be hyperoliids, but part of the squeaker group instead (Vences et al. 2003, Frost et al. 2006, Pyron & Wiens 2011). Some leptopeline species deposit their eggs in burrows or depressions created close to a pool. When the tadpoles hatch they slither to the water source. The tadpoles of L. natalensis do this, and are unusual eel-like little creatures that are said to be able to swim uncannily fast, and to jump over 70 mm out of the water (Arak 1986). L. brevirostris is particularly interesting in being a mouth-brooder: a bizarre habit that, as I’m sure you know, is also present in the hyloid Rhinoderma darwinii. A species named in 2005, L. crystallinoron, is yet another of those frogs that lacks eardrums.

We will be coming back to hyperoliids and arthroleptids in time, oh yes!

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

Refs - -

Arak, A. 1986. Frogs. In Halliday, T. & Adler, A. (eds) Animals of the World: Reptiles and Amphibians. The Leisure Circle (Wembley, UK), pp. 36-51.

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.

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.

Starnberger, I., Poth, D., Peram, P. S., Schulz, S., Vences, M., Knudsen, J., Barej, M. F., Rödel, M.-O., Walzl, M. & Hödl, W. 2013. Take time to smell the frogs: vocal sac glands of reed frogs (Anura: Hyperoliidae) contain species-specific chemical cocktails. Biological Journal of the Linnean Society 110, 828-838.

Vences, M., Kosuch, J., Glaw, F., Böhme, W. & Veith, M. 2003. Molecular phylogeny of hyperoliid treefrogs: biogeographic origin of Malagasy and Seychellean taxa and re-analysis of familial paraphyly. Journal of Zoological, Systematic and Evolutionary Research 41, 205-215.