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In pursuit of Romanian frogs (part III: brown frogs)

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


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An Agile frog Rana dalmatina, encountered in damp woodland at Râpa Rosie, Romania. Note the pointed snout and the extremely long, slender hindlimbs.

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Time to look at more of the frogs I encountered in Romania. In the previous article I discussed Western Palaearctic water frogs (the species of Pelophylax). Here in Europe, water frogs live alongside another group of ranid frogs – the brown frogs, the only frogs unambiguously and unquestionably associated with the generic name Rana. Well, I say ‘live alongside’, but brown frogs are decidedly more terrestrial than water frogs (as you might expect), often being found far from water (though often in damp microhabitats).

A particularly handsome Common frog R. temporaria, photographed in England.

The best known and most widespread of them is the Common frog or Grass frog R. temporaria, the ‘original’ frog as recognised by Linnaeus. It’s the most cold tolerant of brown frogs, occurring throughout pretty much the whole of the UK and Scandinavia, even as far north as the Arctic Circle, and it’s the frog that I know best – indeed, it’s the only one that occurs where I live, and in the places where I grew up.

So imagine my excitement on being confronted, on several separate occasions, with a different brown frog while in Romania. Whoop. Yes, I was delighted and thrilled to see particularly long-legged, sharp-snouted brown frogs at several locations, including close to the Valea Cheii River and also deep in the woods at Râpa Rosie (an area with fantastic badlands, good for finding Cretaceous dinosaur fossils and also for lizards). These, very obviously, were Agile frogs R. dalmatina.

Like the Common frog and other brown frogs, the Agile frog has a dark ‘mask’ behind its eyes and surrounding its tympanum (eardrum). This feature is mentioned in every single text that discusses the identification of Old World frogs, but I wonder what (if any) advantage might be associated with its presence. Is it something to do with camouflage, heat absorption, or protecting the eyes from glare, or what? I don’t think anybody’s ever looked into this (though I’d love to be wrong). The tympanum in the Agile frog is particularly large compared to that of other brown frogs, and positioned closer to the eye than is usual.

Administering the leg length test to an Agile frog: note how far ahead of the snout the heel is. This does not hurt or damage the frog!

Its especially long hindlimbs readily differentiate it from other brown frogs – the legs look long and slim even when folded up, but the easiest way to determine how long the legs are in a frog is to stretch out one of the animal’s legs forward and in parallel with its body to see where the heel is relative to the snout. I discussed this previously in connection with water frogs; evidently, you can use it on different genera. In most brown frogs the heel doesn’t extend beyond the snout, but in the Agile frog the legs are so long that the heel is well beyond the snout – as you can see in the photo here. Frogs are so flexible that this is unlikely to hurt them: it definitely doesn’t damage them in any way. Anyway, these particularly long legs of course make the Agile frog an especially good leaper (leaps of 2 m have been measured), and – I presume – explain its English name. The webbing between its toes is reduced compared to that of other Rana species, and much reduced compared to that of more aquatic ranids like the Pelophylax water frogs.

The same frog as the one shown above with its leg stretched out, photographed after release.

Early on in the year, Agile frogs lay clusters of 600-1000 eggs; their tadpoles are olive brown in colour and are unusual compared to those of other Rana species in reaching as much as 6 cm in total length – this makes a big Agile frog tadpole one of the largest tadpoles of the European region. SVL in an adult is normally about 6 cm, but individuals may reach 12 cm in the southern part of the species’s range (Laňka & Vít 1986).

Like the Common frog and also the Moor frog R. arvalis, the Agile frog is widespread across Europe, occurring from the Pyrenean region, north to Denmark and southern Sweden, and eastward to the shores of the Black Sea. It occurs around the coasts of the Mediterranean and is unusual among brown frogs in being found on Sicily and various of the Dalmatian and Ionian islands.

Toward and away from the Mediterranean: a history of Agile frogs

Phylogenetic work indicates that the Agile frog is closely related to the Italian agile frog R. latastei and Stream frog R. graeca (Barbadillo et al. 1997, Veith et al. 2003), both of which are endemic to the Mediterranean region. In turn, this ‘Agile frog clade’ is closest to an ‘Anatolian clade’ of brown frogs.

Neighbour-joining tree for Western Palaearctic brown frogs, from Veith et al. (2003). Click to enlarge.

The divergence between the ‘Agile frog clade’ + ‘Anatolian clade’ and the ‘Rana temporaria species group’ apparently happened early on in brown frog evolution, with molecular clock data indicating that it occurred round about 4 Ma ago: that is, early in the Pliocene (Veith et al. 2003). Some authors therefore argue that speciation in European brown frogs only got underway after the Messinian salinity crisis (Mensi et al. 1992, Veith et al. 2003), when arid Late Miocene conditions would have made the Mediterranean region rather inhospitable for this cool-climate, moist habitat group of frogs (needless to say, the Messinian salinity crisis – the virtual drying up of the Mediterranean and associated aridification of the whole region – evidently had a major impact on the fortunes of many European tetrapod lineages). Climatic fluctuations during the Late Pliocene seem to have driven additional speciation events, and a more equable, more frog-friendly climate presumably allowed brown frogs to now colonise the Mediterranean region: fossils identified as R. dalmatina are known from the Late Pliocene, as are other members of the Agile frog + ‘Anatolian clade’ and ‘Rana temporaria species group’, so the fossil record seems consistent with this view.

Rana cf. temporaria specimen from the Lower Miocene of Dietrichsberg, Germany. From Böhme (2001). Scale bar = 10 mm. Click to enlarge.

A possible alternative to this ‘post-Messinian’ model is that the respective lineages already existed, and had done since the Miocene, but lived in Asia and only invaded Europe during the Pliocene. Indeed, most of the older fossil brown frogs – dating to the Late Miocene – are from Kazakhstan and Japan. An Early Miocene brown frog from Germany – identified as Rana cf. temporaria (Böhme 2001) [shown here] – suggests that European brown frog lineages were in Europe prior to the Messinian salinity crisis. However, the idea that extant brown frog lineages go back prior to the Pliocene is inconsistent with the molecular clock data (Mensi et al. 1992, Veith et al. 2003): those Miocene Asian brown frogs, and the Miocene fossil from Germany, haven’t been identified with certainty* and I think a very real possibility is that they’re actually outside the crown-group that includes the modern brown frog lineages being discussed here.

* Note that they’re all ‘cf’ records: ‘cf’ stands for ‘confer’. When used in a scientific name (e.g., Rana cf. temporaria) it means something like “this specimen is very similar to Rana temporaria but does not definitely belong to that species”.

Romanian Agile frog in hand.

Whether brown frogs moved into Europe after the Miocene, or whether they were already there, it seems that the Agile frog clade had its ‘centre of origin’ somewhere round about Italy or the Balkans (I say this based on the distribution of R. latastei and R. graeca). From this region, the Agile frog spread far and wide. While not occurring today in the western Iberian Peninsula or the north or far east, fossils show that it was more widespread during the Pleistocene than today, occurring in north-western Spain and further east in Ukraine than it does at present; it also seems to have persisted in England until Middle Saxon times (c. 600-950 AD) (Gleed-Owen 2000).

So far, the Agile frog remains common across most of its range; as is the case with other amphibian species that inhabit developed countries, the fragmentation of populations caused by roads and urban areas definitely has a deleterious impact and increases the likelihood of local extinction (Lesbarrères et al. 2006). Its persistence and success relies not only on healthy freshwater breeding pools, but also on the existence and maintenance of green corridors and healthy woodlands (Ficetola et al. 2006, Hartel et al. 2009).

Aww, a wittle baby! Juvenile Agile frog, human index finger for scale.

In case it’s not obvious, I love this deep-history stuff, and it’s always frustrated me that this sort of information is so hard to find. Still, that’s why you’re here, right?

So, there we have it – more anuran stories from the beautiful country that is Romania. Is that it, done? Of course not; more later. Ahh, frogs.

For previous Tet Zoo articles on European anurans, see…

Refs – -

Barbadillo, L. J., García-Paris, M. & Sanchíz, B. 1997. Orígenes y relaciones evolutivas de la herpetofauna Ibérica. In: Pleguezuelos, J. M. (ed.) Distribución y Biogeografía de los Anfibos y Reptiles en Espana y Portugal. Monografías de Herpetología, vol. 3. Granada, pp. 47-100.

Böhme, M. 2001. The oldest representative of a brown frog (Ranidae) from the Early Miocene of Germany. Acta Palaeontologica Polonica 46, 119-124.

Ficetola, G. F., Valota, M. & De Bernardi, F. 2006. Temporal variability of spawning site selection in the frog Rana dalmatina: consequences for habitat management. Animal Biodiversity and Conservation, 29.2, 157-163.

Gleed-Owen, C. P. 2000. Subfossil records of Rana cf. lessonae, Rana arvalis and Rana cf. dalmatina from Middle Saxon (c. 600-950 AD) deposits in eastern England: evidence for native status. Amphibia-Reptilia 21, 57-65.

Hartel, T., Nemes, S., Cogălniceanu, D., Öllerer, K., Moga, C. I., Lesbarrères, D., Demeter, L. (2009). Pond and landscape determinants of Rana dalmatina population sizes in a Romanian rural landscape. Acta Oecologica, 35, 53-59

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

Lesbarrères, D., Primmer, C. R., Lodé, T. & Merilä, J. 2006. The effects of 20 years of highway presence on the genetic structure of Rana dalmatina populations. Écoscience 13, 531-538.

Mensi, P., Lattes, A., Macario, B., Salvidio, S., Giacoma, C. & Baletto, E. 1992. Taxonomy and evolution of European brown frogs. Zoological Journal of the Linnean Society 104, 293-311.

Veith, M., Kosuch, J. & Vences, M. 2002. Climatic oscillations triggered post-Messinian speciation of Western Palearctic brown frogs (Amphibia, Ranidae). Molecular Phylogenetics and Evolution 26, 310-327.

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!

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





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  1. 1. glynyoung 5:17 am 08/4/2011

    We have agile frogs here in Jersey and no native common frogs. Jersey has the only native frogs and toads in the (UK) Channel Islands Sadly the tiny frog population is only really maintained by active conservation practices like annual collection of spawn and captive head-starting. Never mind, at least we still have them. Introduced common frog in Guernsey and Sark seem to thrive!

    Link to this
  2. 2. naishd 5:28 am 08/4/2011

    I thought about mentioning the Jersey Agile frogs in the article but it had already become quite long enough…

    The Jersey population is weird. Rather than being woodland animals, they inhabit heathland, and they also seem to be more variable than continental animals: there are dark brown and reddish individuals as well as light brown ones like those shown in the photos above. So, the Agile frog does still occur in the British Isles – there’s quite a bit of info online about conservation efforts directed to their preservation.

    Darren

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  3. 3. BrianL 5:59 am 08/4/2011

    Regarding the Messinian salinity crisis, should I imagine the basin of the Mediterranean to have turned into very dry and desert with the odd saltplain home to breeding colonies of flamingos here and there, or as much less extreme and more steppe-like?

    Given that the Mediterranean is thought to have reflooded rather abruptly, I find it nearly impossible to imagine just how catastrophic this event must have been for any creatures that happened to inhabit the basin.

    I’ve also read (In ‘Shrikes and Bushshrikes’ by Harris and Franklin of all places)that during the Pliocene we should imagine the European fauna to have had a decidedly ‘Ethiopean’ character (as in tropical African), though of course with various now-extinct and Asian elements. Is this a valid observation? I think I remember ‘Mammoths, sabertooths and hominids’ not to be nearly as straightforward in saying this, though I could be wrong.

    I definately remember the authors stating that during the Pliocene climatic optimum, even the northern coast of Antarctica was probably icefree and perhaps even forested. One wonders what sort of fauna that area would have had at the time.

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  4. 4. UK_wildlife 12:08 pm 08/4/2011

    I liking this european amphibian stuff :)
    Just got my self a copy of Axel Kwet’s New Holland European Reptile and Amphibian guide for £4! There are some scientific names out of date but the info on each species looks good to me, especially for the price I paid!

    “have persisted in England until Middle Saxon times (c. 600-950 AD)”

    Present in historic times? Sounds like a candidate for a reintroduction programme!

    Link to this
  5. 5. Neil K. 1:13 pm 08/4/2011

    It’s your lucky day! You are, in fact, wrong that no one has looked into the adaptive significance of eye stripes in frogs.

    “If eye-lines serve as sighting devices it would be expected that smaller members of a group would be more likely to have such lines than larger members because the prey-capturing apparatus of the former is relativel smaller and the prey moves more swiftly in relation to their size. Among the North American frogs depicted in Conant (8) and Stebbins (9), almost all ranids and hylids longer than 2 1/2 inches lack eye-lines, whereas almost all below this size have them (P<.01)."

    That's from Ficken, Matthiae and Horwich (1971) "Eye marks in vertebrates: aids to vision" _Science_ 173:936-939.

    The paper looks at eye markings in a variety of groups (birds, fish, snakes) and argues that they function like gun sights, helping predators to zero in on their prey. Seems a little dubious to me, but hey, P<.01 so they must be right!

    I've now got a copy of the paper if you'd like it.

    Link to this
  6. 6. Maija Karala 5:06 pm 08/4/2011

    “It’s the most cold tolerant of brown frogs, occurring throughout pretty much the whole of the UK and Scandinavia, even as far north as the Arctic Circle…”

    Actually, I found a common frog once in Utsjoki, Finland, around 350 km north of the Arctic Circle. The critter was in a tiny stream, well up in the fells. It was actually above tree line, if I remember the place correctly. In any case, it was permafrost region with occasional snowy hillsides still in July. I’ve heard that the common lizard can also be found in that region. Got to appreciate them for coping with such an extreme environment.

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  7. 7. BilBy 9:03 pm 08/4/2011

    Ah, but @Neil K, the bulk of the eyepatch is behind the eye – how could that work as a sighting device? Plus – 1971! Time for a new study!

    Link to this
  8. 8. Hai~Ren 10:22 pm 08/4/2011

    When I was a kid, I had a book about freshwater life of Britain, and it covered only 5 species: common, marsh and edible frogs, and the common and natterjack toads. Now I know that there were so many other species of anurans in the British Isles, both native and introduced.

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  9. 9. Neil K. 3:31 am 08/5/2011

    @BilBy, I had the same thought myself, but simply wanted to note that someone had–however briefly and unsatisfactorily–addressed the question of the adaptive value of anuran eye stripes.

    I dug a bit deeper into the question and there seems to be a thread of reasoning among fish workers that dark eye stripes break up the outline of the eye and thereby aid in crypsis. That seems like a sound hypothesis, but leaves open the question of why multiple frog lineages seem to have converged upon the pattern while most don’t exhibit it. I also like the “anti-glare” idea, by analogy with cheetah and osprey and American footballers. It seems this might track with those species that feed in very shallow water or at the air-water interface rather than terrestrially or sub-aquatically (do many adult frogs even feed sub-aquatically?) I guess I could start rounding up my local chorus frogs and taping pieces of paper over their masks and measure feeding performance under different lighting conditions…

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  10. 10. naishd 10:07 am 08/5/2011

    Thanks for comments. On Common frogs and the Arctic Circle, I meant that they occur within the Arctic Circle, not just at its southerly limits. It is neat. But this isn’t unique among ranids – in North America, the Wood frog Lithobates sylvaticus also occurs well within the Arctic Circle, and is well known for its ability to withstand freezing (its tissues can be mostly frozen solid, yet it can survive).

    As for dark eye marks and so on, thanks indeed, Neil, for the mention of the Ficken et al. (1971) study. I’m not sure I see much logic in the idea that stripes on the snout help predators pinpoint prey – do the predators even see the lines on their faces? And surely general spatial awareness/proprioception is sufficient for the pin-pointing of prey items? It is for other predators. Anyway, none of this helps with the dark ‘mask’ that’s mostly posterior to the eye in the frogs we’re talking about here. Anti-glare is plausible, as might be camouflaging/disguising the eye, or perhaps a thermoregulatory role.

    Darren

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