Tetrapod Zoology

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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.

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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).

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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.

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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).

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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 S, 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., Bhme, 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.

Comments

  1. #1 Christopher Taylor
    November 18, 2007

    Claws again? Where do these frogs keep getting them from?

  2. #2 Weatherfac
    November 19, 2007

    So some frogs have vertical pupils and others horizontal? What’s the significance of that, I wonder?

  3. #3 David Marjanovi?
    November 19, 2007

    the youngest major anuran clade

    By definition, it’s just as old as Hyloidea. It only appears later in the fossil record.

    What about the… novel idea that Trichobatrachus does cutaneous breathing in reverse — adding oxygen to the water next to the eggs?

  4. #4 Anthony Docimo
    November 19, 2007

    herbivorous frogs? I’d always thought that herbivorous frogs (not to be lumped with herbivorous tadpoles) ranked up there with herbivorous snakes in plausibility.

    then again, with my luck, I missed the announcement of the discovery of an herbivorous snake. :)

  5. #5 DDeden
    November 20, 2007

    The male hairy frog’s “hair” seems to be positioned similarly to human males with male pattern baldness, (excluding the throat’s expandable vocal sac non-bearded condition), with the “hair” being positioned in the axillary and pubic/thigh regions and lower back. Remarkable apparent parallel to human males, with a completely different reason for existing but the same hydrodynamic forces, and body anatomy, vocality, poorly webbed hands, well webbed feet, claws which seem to resemble fingernails, and perhaps slight tendency towards bipedality (per photo?) resulting in parallel convergence. I could not have imagined this one.
    Thanks Darren!

  6. #6 David Marjanovi?
    November 21, 2007

    1. It isn’t even remotely similar to hair. It is outgrowths of the whole skin, like external gills.
    2. No “tendency to bipedality” at all whatsoever. It swims and hops. The photo was taken underwater. The frog hovers, it doesn’t stand.
    3. Are frog hands ever webbed?

    [from Darren: yup, those of Rhacophorus for example]

    4. The black dots could be the claws. If so, they don’t resemble fingernails. In any case the presence of claws must be a quite recent innovation in the first place.
    5. The “hair” on the butt is unlike the human pattern. Also, there is none on the back; you are seeing the “hair” from the other side of the body, which in turn is not similar to axillary hair in distribution.

    So, I don’t see any convergence. Maybe you are trying to see what you want to see…

  7. #7 Edgar
    November 27, 2007

    still curious is when frogs develop external gills cant regrow large branchial archs like the aquatic urodeles,then frogs have lost the ability(genes)to do it?

    _novel idea that Trichobatrachus does cutaneous breathing in reverse_

    sounds terribly interesting, how many requisites are needed to import a hairy frog and measure it in aquaria, or go to their land and do the measures on field(oops, apart of funding…)?

  8. #8 David Marjanovi?
    November 28, 2007

    Gills are a purely larval feature. Only neotenic urodeles retain gills throughout life. All others — including the aquatic amphiumids and cryptobranchids — lose them in their incomplete metamorphosis. There are no neotenic frogs, and probably there cannot be any.

  9. #9 Chris
    November 28, 2007
  10. #10 Darren Naish
    November 29, 2007

    Did you hear about the effort to translate the bible into lolcat-speak? What a total waste of time – go and pick up litter or something you wasters!