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In pursuit of Romanian frogs (part I: Bombina)

The views expressed are those of the author and are not necessarily those of Scientific American.


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A fairly dark Yellow-bellied toad.

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.

Four anurans are visible in this photo (and one fish).

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…

You can find light and dark individuals in very close proximity, as here.

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.

This cladogram depicts a sort of rough consensus of views on anuran phylogeny, though note that studies differ on the relative positions of some of the clades. Pipoids are more basal than leiopelmatids according to some studies, for example. Following Ford & Cannatella (1993), Anura is the node-based clade that includes all extant frogs and toads; Mesobatrachia is the node-based clade that includes pipoids and pelobatoids (Ford & Cannatella regarded these two as sister-taxa, but if they're far apart - as shown here - then Mesobatrachia is more inclusive); Pipanura is the node-based Mesobatrachia + Neobatrachia clade (and, as shown here, is thus redundant with respect to an inclusive version of Mesobatrachia); and Bombinanura is the node-based clade that includes bombinatorids + alytids and neobatrachians. The name Costata Lataste, 1879 was used for the alytid + bombinatorid clade by Frost et al. (2006). Frost et al. (2006) also used the new name Sokolanura for the Costata + Acosmanura clade, though they didn't say whether it's node-, branch- or apomorphy-based. If it's node-based it's synonymous with Bombinanura. Acosmanura Savage, 1973 was used by Frost et al. (2006) for the Pelobatoidea + Neobatrachia clade.

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.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at darrennaish.wordpress.com. He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at tetzoo.com! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. UK_wildlife 2:54 am 07/17/2011

    Now there is a species I’d love to see in the wild (and photograph obviously). I had no idea about the spines and the stinging sensation if you get it into contact with a mucous membrane.

    Ive been told these (as we all species kept as pets) turn up in the UK from time to time, though Ive not heard of any viable populations….yet!

    Link to this
  2. 2. Andreas Johansson 4:10 am 07/17/2011

    Is that to say there’s a huge gap in the genus’ distribution between Europe and China?

    Also, what’s up with “Bombinatoridae” – where does the -tor- come from?

    Link to this
  3. 3. naishd 7:26 am 07/17/2011

    Thanks for comments. Neil: yes, there are (or were) wild Yellow-bellied toads in Devon and south London. Beebee & Griffiths say that individuals from these colonies were spreading to new locations as recently as the 1980s, but I don’t know what the latest is. Incidentally, there’s no reason to suspect that they were ever native to the UK.

    Andreas: yes, Bombina has a classic disjunct distribution, with a gap apparently being present between western Russia/Kazakhstan and far eastern Asia. There are Pliocene and Pleistocene fossil Bombina from Russia, but I’m not sure if they help ‘fill’ this gap.

    Why Bombinatoridae? Bombina Oken, 1816 is nowadays regarded as synonymous with Bombinator Merrem, 1820, and the first family-level name given to this group was erected for Bombinator, not for Bombina. Bombinatorina Gray, 1825, Bombinatoroidea Fitzinger, 1826 and Bombinatoridae Gray, 1831 all long pre-date the first use of Bombinidae… Fejérváry, 1922.

    Darren

    Link to this
  4. 4. DMA12 9:20 am 07/17/2011

    “The Bombina species aren’t really toads in the strict sense.”

    Aren’t toads a polyphyletic group of frogs.

    Link to this
  5. 5. John Scanlon FCD 9:42 am 07/17/2011

    Darren, that nomenclatural clarification was practically a bombinatorial explosion….

    “…I can’t talk about what we found (yet…)” – madtsoiid snakes among other stuff, one hopes!

    Link to this
  6. 6. Lasius 10:13 am 07/17/2011

    Interestingly the Unkenreflex is even named after these toads, as “Unke” is the German term for members of the genus Bombina.

    Link to this
  7. 7. falcon121 10:56 am 07/17/2011

    Someone correct me if I’m wrong, but toads in the strict sense means the family Bufonidae, even though toad is a common name for any anuran that looks similar.

    Link to this
  8. 8. Andreas Johansson 3:46 pm 07/17/2011

    @9 naishd: Thank you

    Link to this
  9. 9. naishd 5:56 pm 07/17/2011

    On the term ‘toad’: we all know that ‘frog’ and ‘toad’ don’t mean anything specific. ‘Frogs’ are all those smooth-skinned, leaping anurans at least vaguely similar to Rana temporaria while ‘toads’ are all those warty-skinned, non-leaping anurans at least vaguely similar to Bufo bufo. ‘Toads’ in the strictest, specific sense are thus only those members of Bufonidae – hence, anything that’s called a ‘toad’ but is not really a bufonid is not a “toad in the strict sense” (so, falcon 121 is pretty much right).

    As for new madtsoiids – I couldn’t possibly comment!

    Darren

    Link to this
  10. 10. Undularbore 11:40 am 07/28/2011

    Great photos Darren, the frog on his back in defense is, well, cute as heck! I have followed you from SB and registered to comment. Good to read your blogs again, Darren.

    Link to this
  11. 11. Dartian 2:57 am 08/3/2011

    Darren:

    A very neat feature of the Bombina species is that they have heart-shaped pupils. I have no idea why

    I’m just guessing here, but perhaps it has something to do with the fact that these anurans spend so much time in the water? Could it be an adaptation for seeing both under and above the water’s surface at the same time?

    You might think from books that the Yellow-bellied and Fire-bellied toads are the only two species within Bombina.

    Actually, I’d say that nowadays the Oriental fire-bellied toad is fairly widely known too, due to it’s popularity in the pet trade.

    Link to this
  12. 12. Dartian 3:00 am 08/3/2011

    Hmph, so ‘blockquote’ doesn’t work here then? Great.

    Link to this
  13. 13. David Marjanović 3:36 pm 08/8/2011

    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.

    Just to make sure… it’s the act of contortion, the (supposed) reflex, which is termed the Unkenreflex, not the posture.

    And yes, Unke = Bombina. And BTW, a lungless species of Barbourula was discovered in Borneo a year or three ago; that’s the first lungless anuran.

    Link to this

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