Polymorphism, squeakers, the hairy frog: an afrobatrachian's tale (part II)

Now, I've described quite a few isolated dinosaur bones in my time. And I've been involved in some pretty hectic media whirlwindy events ('Angloposeidon', aka 'Europe's largest sauropod', was huge news: see here, as was Eotyrannus). But I've never been associated with any PR exercise that was as well orchestrated and successful as the event that surrounded Xenoposeidon. I have lots of thoughts about what an outstanding success the entire media campaign was, but for those you'll have to check SV-POW!

After all that, does it seem at all anticlimactic to return to frogs? No, it does not: right now I find anurans pretty much among the most fantastic and amazing beasts in existence, I love writing about them, and I wish I knew more about them. Having said that, I'm keen to get through what I've started and, now that we're deep into the 'last' of the major anuran clades - Ranoidea - we only have a few groups to go before it's all over. The problem is... these 'few groups' includes tens of particularly interesting lineages, and hundreds of species distributed worldwide. Running frogs, the incredible hairy frog, the Asian flying frogs, the immense Goliath frog and South African bullfrog, the Madagascan mantellas, the Andean backpack frogs, the ultrasonic cascade frogs, Congolese fishing frogs, plant-eating frogs...

Here's what we're gonna do. In the previous article, we got through the microhylids and also started to go through the afrobatrachians, a recently recognised assemblage of African ranoids that includes the reed and lily frogs (hyperoliids), the squeakers (arthroleptids), the pig-nosed or shovel-nosed frogs (hemisotids or hemisotines), and the short-headed or rain frogs (brevicipitines or brevicipitids). Hemisotids and brevicipitids (united as the - deep breath - xenosyneunitanurans*) were dealt with last time, so here we look at their sister-taxon: Laurentobatrachia Frost et al., 2006, the clade that includes the hyperoliids and the squeakers. The name Laurentobatrachia honours herpetologist Raymond L. Laurent.

* Believe it or don't I've committed this name to memory.

Reed, sedge and lily frogs, or hyperoliids, are a large group (over 250 species) of mostly arboreal ranoids that climb in vegetation at or near the water's edge; 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 [the painting at the top of the article shows this species: it was kindly provided by my good friend Carel Brest van Kampen: blog here, website here]. Some hyperoliids (like the running frog Kassina fusca [shown in image below] from western Africa) are mostly terrestrial and run well, rather than hop. The males of many hyperoliids have particularly large vocal sacs - sometimes larger than the rest of the body when inflated - and the group is also unusual in possessing a disc-shaped gular gland (I don't know what this is for).

Like the true hyloid treefrogs, hyperoliids possess intercalary elements (for more on these see the treefrog section in the 'core hyloid' article). Elsewhere among ranoids, some squeakers have intercalary elements too, so it seems that these structures have evolved convergently several times among neobatrachians, and indeed histological work suggests that hyperoliid and squeaker intercalary elements are not homologous. All hyperoliids have free-living tadpoles, but various methods have been 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.

Hyperoliids have traditionally been thought to include the 50-odd species of leaf frog or forest treefrog (Leptopelis) of sub-Saharan Africa, usually given their own 'subfamily', Leptopelinae. However, leptopelines now seem not be hyperoliids, but part of the squeaker group (Arthroleptidae) instead (Vences et al. 2003, Frost et al. 2006). 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, lacks an eardrum. How then does it hear airborne sounds? Maybe it doesn't.

The squeakers or arthroleptids are a group of about 130 species (again from sub-Saharan Africa), so named for the calls made by the Arthroleptis species [adjacent image shows the West African screeching frog or Mottled squeaker A. poecilonotus]. Squeakers are generally small, forest-floor frogs that lay their eggs in burrows or cavities in moist soil; their eggs then undergo direct development into froglets. They tend to have vertical pupils (a rare character among neobatrachians).

Probably part of the squeaker clade are the several taxa that used to be grouped together as the astylosternines or astylosternids. These are all tremendously obscure and poorly known frogs, with the exception of Trichobatrachus robustus, the Hairy frog of tropical western Africa [shown in the adjacent image]. During the breeding season, males and males alone develop frills of hair-like papillae along their flanks and thighs. These are supposed to increase the animal's surface area (and therefore allow more cutaneous respiration to take place) and to then allow it to remain submerged for an exceptional amount of time when it is egg-guarding. This was suggested by Dean (1912), but he imagined that male hairy frogs somehow arranged the eggs around their papillae: quite how the male was supposed to get the eggs into position I'm not sure (Dean had midwife toads in mind, but they 'only' have to wrap egg strands around their hindlimbs). Incidentally, hairy frogs are also really weird in that they have claws.

So microhylids and afrobatrachians include a motley assortment of really weird, interesting frogs with - as usual - a pretty impressive diversity of habits and reproductive strategies. The remaining ranoids are yet to come, but so is much else. If you've missed the party, the anuran series started way back here. Again, if you're bored with anurans I still recommend you check to see what comes next. You might be surprised. You might not.

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.

Dean, B. 1912. On the hair-like appendages in the frog, Astylosternus robustus (Blgr.). Bulletin of the American Museum of Natural History 31, 349-351.

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.

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.