I recently spent a bit of time in Romania, working with colleagues in an effort to find new Cretaceous reptile fossils. As usual, I can’t talk about what we found (yet. And we did find a lot). But what I can talk about is the modern-day wildlife I encountered while on the trip. Partly due to the fact that a lot of our field sites were in or near rivers and streams, I saw and photographed a great many anurans (frogs and toads) – so many that I haven’t yet have time to write up thoughts on all the respective species (long-time Tet Zoo readers will know that I have a bit of a soft spot for anuran biology and diversity: see links below). In this article, I discuss just one of the species I encountered.

Before I embark – a few housekeeping notes. I’m really pleased that so many of you have gone to the effort of registering and logging in here at Tet Zoo ver 3. I’ve made it very clear behind the scenes that I regard logging in for commenting as a real obstacle to the evolution of a successful, vibrant blogging network, and as a major blow as goes my own vision for Tet Zoo ver 3. Rest assured that I (and others) are doing all we can to get things changed. The people at SciAm are listening, and hopefully they’ll come around. The fact that commenting doesn’t (yet) allow your name to be linked to your site is also a problem. Oh, and if you’ve been surprised by my relative silence since Tet Zoo ver 3 went live, it’s because (due to a tightly scheduled rollout) we weren’t able to post as and when. That’s now done with, so let battle commence…

It was at several points along Transylvania’s Valea Cheii River that I observed numerous Yellow-bellied toads Bombina variegata. These small anurans (SVL* less than 50 mm) are strongly aquatic and the majority of individuals I saw were at the bottoms or edges of small pools, or sat on stream-side rocks or substrate. It was common to encounter several individuals within close proximity, and indeed the books say that this species is “often sociable” (Arnold et al. 1978).

* Snout to vent length - a standard measure used in the herpetological literature.

The Yellow-bellied toad occurs across much of central and western Europe (it’s absent from the Iberian Peninsula), extending as far east as the Carpathian Mountains. In eastern Europe, it’s a fairly familiar animal of pools and puddles on forested hillsides and mountains. In western Europe (France, in particular), it occurs in shallow, exposed pools in totally open, flat habitats like floodplains. The many individuals I saw were fairly variable in general colour, with individuals being all different shades of grey and some having varying amounts of light green on their dorsal surface.

The common name reflects the fact that the belly is patterned with bright yellow markings. These also extend onto the fingertips. Individuals in some populations are orange or even red-bellied, meaning that confusion with the closely related Fire-bellied toad B. bombina is sometimes possible. Almost entirely black ventral surfaces are also on record, and Italian populations have notably dark throats and chests.

When threatened by predators, Yellow-bellied toads famously contort themselves into a distinctive posture – termed the Unkenreflex – where they direct the belly towards the sky and fold their limbs back, thereby displaying as much of the bright ventral colouring as possible. The same behaviour is practised by the Yellow-bellied toad’s close relatives: the brightly coloured ventral skin in these animals is toxic. In fact, Bombina toads can secrete toxin anywhere on their skin – this causes a stinging sensation if you get it into contact with a mucous membrane, and it’s apparently common for children to need medical treatment after handling these animals and then rubbing their eyes (Laňka & Vít 1986).

As you might be able to see from some of my photos, the warts on Yellow-bellied toads are tipped with tiny, sharp spines. In this respect the skin surface of this species is very different from that of the Fire-bellied toad, where the warts are blunt-tipped. In Balkan Yellow-bellied toads, these spiny tips are often noticeably marked out in black. As is typical for ‘basal’ anurans, vocal sacs are absent. They also lack ear drums. Between May and August, females lay clumps of over 100 eggs. Those that hatch late in the summer typically over-winter as larvae and complete metamorphosis in the following year. Tadpoles reach a similar size to adults before completing metamorphosis. For such small animals, Yellow-bellied toads are surprisingly long-lived, with many individuals reaching their second or third decades. It seems that this longevity is due to their relative impunity to predation (Plytycz & Bigaj 1993), and indeed their bold behaviour - they lounge around in shallow pools in the middle of the day, in full sight - is in line with this impunity.

The Bombina species aren’t really toads in the strict sense. Rather, they’re part of an archaic anuran clade thought to have diverged way back in the Early or Middle Jurassic (and their fossil record matches this notion). Typically, the Bombina species have been grouped together with the painted frogs (Discoglossus), barbourulas (Barbourula) and midwife toads (Alytes) in a group termed Discoglossidae or Alytidae. Though the former name is the one used most frequently, the latter one has nomenclatural priority, apparently. However, some authors treat Discoglossidae (for Discoglossus) and Alytidae (for Alytes) as separate, family-level taxa.

As indicated by one of those names (Discoglossidae), the various members have disc-shaped tongues. However, non-monophyly of the traditional, inclusion version of Alytidae has been suspected for a while (like so many traditionally recognised ‘families’, its monophyly has been assumed and untested until recently) and recent analyses have caused some authors to classify Bombina and Barbourula in a separate clade termed Bombinatoridae. Ford & Cannatella (1993) proposed that alytids were closer to neobatrachians than were bombinatorids, and used the name Discoglossanura for the alytid + neobatrachian clade. However, monophyly of an alytid + bombinatorid clade has been supported by the majority of recent authors (Gao & Wang 2001, Haas 2003, Roelants & Bossuyt 2005, Frost et al. 2006). Frost et al. (2006) used the name Costata Lataste, 1879 for the alytid + bombinatorid clade.

A very neat feature of the Bombina species is that they have heart-shaped pupils. I have no idea why, but this feature is also present in Discoglossus.

You might think from books that the Yellow-bellied and Fire-bellied toads are the only two species within Bombina. These two get a lot of coverage because they’re European, but there are others. The Appenine yellow-bellied toad B. pachypus of Italy and Sicily has mostly been regarded as a subspecies of Yellow-bellied toad, but (since about 1990) has been argued to be a distinct species by several authors. The remaining species in the group occur across eastern Asia (a part of the world where good herpetology field guides are relatively few and far between): the Lichuan bell toad B. lichuanensis from China (only named in 1993), Yunnan firebelly toad B. maxima of China and Vietnam, and Oriental firebelly toad B. orientalis (found across much of eastern Asia). Some authors also recognise the Hubei firebelly toad B. microdeligitora (named in 1960) and Guangxi firebelly toad B. fortinuptialis (named in 1978) as distinct species; others don’t.

Anyway, I was very happy to have so many close encounters with these attractive little anurans – as might be obvious, I spent a lot of time peering at them and trying to get good photos. On that note, I’ll recount more of my Romanian frogging experiences some other time. Until then…

For previous Tet Zoo articles that cover discoglossids/alytids and nearby regions of the anuran cladogram, please see...

Refs - -

Arnold, E. N., Burton, J. A. & Ovenden, D. W. 1992. Reptiles and Amphibians of Britain and Europe. Collins, London.

Ford, L. S. & Cannatella, D. C. 1993. The major clades of frogs. Herpetological Monographs 7, 94-117.

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.

Gao, K.-Q. & Wang, Y. 2001. Mesozoic anurans from Liaoning Province, China, and phylogenetic relationships of archaeobatrachian anuran clades. Journal of Vertebrate Paleontology 21, 460-476.

Haas, A. 2003. Phylogeny of frogs as inferred from primarily larval characters (Amphibia: Anura). Cladistics 19, 23-90.

Laňka, V. & Vít, Z. 1986. Amphibians and Reptiles. Hamlyn, Twickenham.

Plytycz, B. & Bigaj, J. 1993. Studies on the growth and longevity of the yellow-bellied toad Bombina variegata, in natural environments. Amphibia-Reptilia 14, 35-44.

Roelants, K. & Bossuyt, F. 2005. Archaeobatrachian paraphyly and pangaean diversification of crown-group frogs. Systematic Biology 54, 111-126.