Pool frog Rana lessonae in Britain: past
nativeness unresolved.
On the verification of sub-fossil bone
identification.
Published 27th October 2005
Tom E S Langton and John A Burton
A report by:
Herpetofauna Consultants International Ltd.
Triton House
Bramfield
Halesworth
UK Suffolk IP19 9AE
01986 784596 t.langt@virgin.net
Summary
A review has been conducted on Green frog sub-fossil bone identification, as reported in recent
publications, notably;
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.
The opinion of the world expert and european authority Dr Gottfried Boehme is quoted in the
above paper to confirm the authors identification of a Pool frog sub-fossil bone and appears to
have been misrepresented.
The paper gives the impression that the identification of a Pool frog Rana cf lessonae subfossil
type ilium (the “hip” bone with a socket for the top of the upper leg bone) had been confirmed by
Dr Boehme and is a “prudent” identification. It is revealed in this report that the specimen shown
to Dr Boehme by the author (Gleed Owen) was only narrowed down by Dr Boehme to the bone
being probably of either an Edible frog or Pool frog type.
Further, the statement in the above paper that Dr Boehme agreed to the validity of Gleed Owen’s
method of visual appraisal of the shapes and textures of ilial morphology is refuted. It has not
been shown to be a proven and verifiable technique.
The identification methods of Gleed Owen (2000) are therefore not “prudent” as indicated in the
text of the paper and cannot be trusted. Re-examination of relevant material by an independent
expert is strongly recommended, with an appropriate peer review.
As a consequence of the questionable identification process, subsequent published scientific and
government documentation incorrectly suggests that Pool frog was once a native British species
e.g. Beebee et al (2005) and Buckley and Foster (2005). The claim of confirmation of
identification of pool frog sub-fossil bones has been used extensively in 2005 and as the main
(only) physical evidence to argue that there is evidence that Pool frog was a natural post-glacial
coloniser of England that survived until modern times (Beebee et al 2005) and in justification of
substantial nature conservation funding for, and including the release of Pool frogs in Norfolk in
August 2005 by English Nature and others.
The appropriate conclusion for available material is that bones in mid-Saxon British
archaeological sites indicate that frogs from the Green frog complex are represented but that it
has not been shown which of the three types most likely: Edible (Rana esculanta), Pool frog
(Rana lessonae) and Marsh frog (Rana ridibunda) were present, nor if their origin was natural or
as the result of human introduction and it is noted that bones located are all from sites associated
with human settlement.
It is considered that, in the present state of knowledge, cranial bones from a non-human site, and
from pre-Roman deposits would be needed to confirm the native status of any Green frog species
in Britain. If one or more Green frogs were present after the last ice age there should be ample
material in remaining natural sites for this matter to be determined.
Background
The GReen Frog Information Programme (GRIP) was established by HCI in 2004, following
concern at the announcement in BBC Wildlife Magazine that English Nature was promoting the
release of Pool frog Rana lessonae into the wild in Norfolk. A number of private landowners in
Eastern England have combined to attempt to clarify the scientific basis behind the reporting. The
aims of GRIP are:
1. To collate and assess information on Green frogs in Britain with respect to recent views
expressed that the reports of Green frog in Fowlmere, Cambridgshire and the Thompson
Common area of Norfolk and elsewhere (as sub-fossils) indicates Pool frog Rana lessonae to
have been a recent native to England.
2. To advise and represent landowners with respect to any change or future proposed change to
guidelines, listings or legislation concerning Green frogs in Britain.
3. To assist in developing a forward-looking strategy for Green frogs in Britain.
This report follows the GRIP report (Burton and Langton 2005) that indicated serious and multiple
inconsistencies in an English Nature research report. This English Nature report had suggested
that archival/historic evidence suggests that two frog colonies first recorded in the nineteenth
century (now extinct) in East Anglia were Pool frog colonies of natural origin. This was shown in
the GRIP report to be a biased summary of the available evidence, that the conclusions of the
report cannot be justified and require re-evaluation.
Introduction
In August 2005 English Nature reported the release of 170 Pool frogs at a site near Thetford,
Norfolk, captured in Sweden. They were released in deepened pond areas including old pingo
formations within a commercial tree plantation, close to a former and well-known Green frog site
at Thompson Common in Norfolk. This action has made the investigations into the evidence
relating to the identification of sub-fossil bones even more relevant as substantial nature
conservation resources will continue to be required if the project is extended
For this review, detailed aspects of bone identification are addressed. Published and unpublished
material was evaluated and sources of information for the key papers checked. The key
publication is: 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.
This paper stood out for two reasons. First, it was claimed that it is possible to definitely
distinguish Pool frog from Edible frog using subtle differences in shape of small parts of the ilium,
(the paired pelvic or “hip” bone, including the hip socket). Secondly, the methods involved a
subjective description of shapes and angles and relied on the expert opinion of a third party, who
was not an author of the paper, to confirm the identity of a single, incomplete and degraded
specimen.
The claims were of concern because it is known that bone shapes of animal species can vary
quite rapidly over periods of as short as a few hundred years, particularly in newly introduced
(small mammals in the Hebrides) or recolonising species.
Prior to the publication of Gleed Owen (Op cit), the published literature indicates that skull bones
(fronto-parietals) were the only reliable way of distinguishing between the main types of Green
frog, and as far as we can ascertain, no subsequent authors have used Gleed-Owen’s methods.
The identification of a second Pool frog ilium in 2001 (Beebee et al 2005) is also considered in
this report.
Methods
Gleed Owen (2000) has been evaluated for its content in respect of material that is described as
being Rana cf lessonae. Consideration of the identification of the two other species is not
considered in this report.
As a result of investigations into the unusual presentation of identifications, interviews were
conducted with the expert authority quoted; Dr Gottfried Boehme (retired) of the Naturhistorisches
Forschungsinstitut Musuem fur Naturkunde, Berlin. Work by Dr Boehme is the basis for
consideration of Green frog bone morphology Boehme G (1977) Boehme G and Gunther R
(1979).
Dr Boehme was interviewed by telephone on four occasions and following this a letter was written
by him for publication in this report and a suitable journal.
Results
Gleed Owen (2000) includes consideration of a mid Saxon Green frog bone fragment (dated ca
880-1040 AD) found in a sill-beam slot, that was located in the foundations of a house at
Chopdick Drove, Gosberton, near Spalding, Lincolnshire. The site includes rectangular structures
apparently of unknown use, with surrounding ditches and pits.
Gleed Owen developed his own diagnostic criteria for Green frog ilial bones. He then blind-tested
on a random selection of 48 water frog ilia from a museum collection. How the randomness was
calculated or decided is not specified. The method is reported to involve the discarding of
‘ambiguous’ specimens – it is not clear how these are judged, and it is not clear in the supporting
research reports on the subject. Of the 29 specimens not considered ambiguous, it is not clear
how they divided between the three types of Green frog but the inference is that the sample size
is very small for each.
The paper refers to Gleed Owen consulting Dr Gottfried Boehme, (presumably showing him the
fragment, or pictures of the fragment at some point in 1998/99) and the subsequent conclusions
drawn on the Gosberton Green frog bone fragment, in the paper are as follows:
“In summary, the combination of a high, robust, laterally produced tuber, a deep concavity
below the ala and the tuber, a thick junctura with a low d/t ratio, and an upstanding dorsal
acetabular margin points towards this specimen being R.lessonae. However, G Boehme
has examined this specimen and although he is certain that it belongs to either R.lessonae
or R.esculenta, he preferred not to discount the possibility that R.esculenta ilial variation
could overlap with that of R.lessonae. Thus as there is a slight element of doubt in this
diagnosis, assignment to R.cf.lessonae would be more prudent. “
Dr Boehme, when contacted as a part of this research indicated that he was not aware of the
paper and a copy was sent to him. Following this on the second contact he said that he had
checked his files and found that he had been sent a copy at some point but had not had time to
read it as it had not been his current area of research interest. He then read the paper and
consulted with his colleague on Green frog identification Dr R Gunter. He then provided a letter
commenting on the Gleed Owen (2000) paper. The text of the letter is reproduced as follows:
Statement from Dr Gottfried Boehme concerning Green frog identification.
Distinguishing Green frogs from ilia
The discovery published by C. Gleed Owen of Rana lessonae (pool frog) from
Chopdike Drove, Gosberton, Lincolnshire published in Amphibia Reptilia and
quoting my opinion requires qualification. In my opinion the range of the
variations in the features of the ilium of Rana esculenta is wide and the features of
Rana lessonae come within it. Differentiating between brown frogs and green frogs
based on the proportions of the ilio-ischiadica is essentially safe but distinguishing
between green frogs is difficult and prone to error. Although the Chopdike Drove
find shows ilium features which can occur with Rana lessonae, I do not think that a
single fragmented ilium is adequate for identification of this animal. The frontoparietalia of Rana lessonae however do have clear features, which make it possible
to effect a fairly good differentiation between species and hybrid and this bone
should be used. Therefore it is not prudent, as suggested in the paper to refer to the
Gosberton bone as coming from Rana cf. lessonae as it is possible that it could be
from the hybrid Rana kl. esculenta.
Further, the unpublished manuscript from Beebee et al (2005) shows a further bone
from Ely that has also been identified from Gleed Owens reported 'blind testing'
from the Berlin collection and says that this enables precise identification. I can
confirm that I made my facilities available and that the individual characteristics
given by Chris Gleed-Owen for differentiation are more numerous than developed by
me. Although it is possible that this method can separate the species from the hybrid
this method has not been published and is not something that I can confirm.
The only way to confirm the findings with a degree of certainty would be to develop
a reference collection of green frogs of all types, age and sex and to record bone
morphology according to clear criteria. This would demonstrate the degree of
overlap of characteristics and the situations where definite identification can be
made. This would enable a Key to be developed for anyone to check existing and
future material. Otherwise in my view the separation of species will always be
questionable as is its use in informing applied nature conservation projects.
Dr G Boehme March 2005
Beebee et al (2005) repeats the assertion (now shown above to be incorrect) that Boehme
agreed with the identification of the Gosberton bone as Rana cf lessonae. It gives a further
description for distinguishing Pool frog ilia from those of Marsh and Edible frog as follows:
“In R lessonae, the tuber is narrower antero-dorsally, sometimes more sinuous and always
more laterally produced than in R ridibunda. The posterior margin of the tuber is also
steeper in R lessonae and the dorsal edge of the vexillum tends to be straight as opposed
to convex in R ridibunda. R esculenta is intermediate between R lessonae and R ridibunda
in ilial morphology, but can generally be separated from either species. The shape of the
corpus and the angles of its margins, in combination with tuber and vexillum shape, allow
the distinction of R lessonae from R esculenta in most cases.”
The contested identification claiming Pool frog bones is also repeated in the English Nature report
Buckley and Foster (2005)
Conclusions
The published and unpublished literature shows that anything other than very experienced
workers can get amphibian and reptile bone identifications wrong. This is due to the unknown
variation and similarity of appearances of bones within and between species and the range of
levels of preservation of sub-fossil finds.
To understand the full variations between species using bones that have not been shown to
separate in a clear cut way (in Green frogs the head bones) would require a modern bone
reference collection from a range of populations across the ranges of the species concerned,
comprising a range of adult sizes/sexes and for the bones to be subject to accurate measurement
from identifiable points.
Without this, the trends reported by Gleed Owen (2000) and in Beebee et al (2005) are as likely
to be an artefact of materials available to those studies as to be a genuine methodology and are
not robust. Any blind-testing of new proposed methods would need to be done by suitably
experienced archaeozoological workers familiar with general small animal bone morphology but
without prior experience of the material.
Current archaeozoological research on Green frog bones in England is inconclusive. It is most
likely to indicate that between one and three frog of the types Edible, Pool and Marsh frog may
have been present in England, at least in places close to human settlements from around 850 AD
and there is evidence of this from other sites in England at later dates, including London
(Armitage and West 1985). There is as yet no firm evidence as to whether Green frogs were
present naturally or introduced or even of both origins but lack of records before this date, despite
numerous and multiple frog bone finds, suggests that introduction of Green frogs by humans is as
likely as past-native origin and arguably more so. Further, there is no evidence for
archeozoological sites of Green frogs existing away from human settlements.
Evidence for and accounts of frog-leg meat consumption by humans in Britain has not been
collated, but evidence does exist. Green frogs would have been a significant seasonal food item
for humans in northern Europe before modern farming techniques and the native grass frog is
likely also to have been consumed to some degree. Movement of commodities including live
animals from mainland Europe to Britain greatly increased in Roman times and became very
substantial by the 11th century. Importation at or after the time of the introduction of other food
animals to England, such as Rabbit Orictolagus cuniculus and Brown hare Lepus europaeus is
still the most likely period for the origin of Green frogs in England in the absence of pre-Roman
material although introduction before these periods remains possible.
If Green frogs occurring in England are the result of introductions then all possible sources and
species should be considered, in view of the extensive maritime trade routes ranging from the
Baltic to the Mediterranean, for pre-Roman, through Roman and on to the 20th Century.
This report considers that archaeozoological aspects of the case for considering Pool frog to be a
past native species are contentious, and that their recent release in Norfolk is therefore
premature. The bones in question are possibly of “Green frog” bones, but their identification to
any particular species, and their consequent identification to a native species worthy of
reintroduction, is considered inaccurate. It is considered that, in the present state of knowledge,
cranial bones from a non-human site, and from pre-Roman deposits (preferably several), are
needed in any attempt to confirm the post-glacial native status of any Green frog species in
Britain. Further, given known changes over time of bone morphology care must be taken to
ensure that methods and findings are applicable to any sub fossil and archaeological material.
References
Armitage P L and West B with contributions from B T Clarke, T Dyson, M F W Festing and A
Locker (1985) Faunal evidence from a late medieval garden well of the Greyfriars, London.
Transactions of the London & Middlesex Archaeological Society Vol 36 1985/1987
Beebee T J C J Buckley, I Evans, J P Foster, A H Gent, C P Gleed Owen, G Kelly G, Rowe, C
Snell, J T Wycherley, and I Zeisset (2005). Neglected native or undesirable alien? Resolution of
a conservation dilema concerning the pool frog Rana lessonae Biodiversity and Conservation 14:
1607-1626.
Boehme G (1977) Zur Bestimmung quartarer Anuren Europas an Hand von Skelettelementen.
Wissenschaftliche Zeitschrift Der Humbolt-Universitat Zu Berlin, Mathematik-Naturwissenschaft,
26, 283-300
Boehme G and Gunther R (1979) Osteological studies in the European Water frogs Rana
ridibunda, Rana lessonae and Rana “esculenta” (Anura, Ranidae). Mitt. Zool. Mus. Berlin, 55,
203-215
Buckley J and Foster J (Eds) (2005) Reintroduction strategy for the pool frog Rana lessonae in
England. English Nature Research Reports No. 642 Peterborough
Burton J A and Langton T E S (2005) On the approach to investigating the historic status of the
Green/Water frog Rana lessonae in England using archive material, with particular reference to
English Nature funded Research Reports and proposals to release Pool frogs in the wild in
England. 1st report of the Green Frog Information Programme (GRIP). Herpetofauna Consultants
International. Bramfield.
Acknowledgements
Dr Derek Yalden is thanked for comments and text amendments. Barry Clarke (Herpetology
section Natural History Museum, London) provided additional advice and assistance.