The French gourmet鈥檚 penchant for lightly saut茅ed frogs鈥� legs has
never really crossed the Channel, but the frogs certainly have. In the 1830s,
England鈥檚 landed gentry began to introduce continental frogs wholesale. The
craze was pioneered by George Berney of Morton Hall in Norfolk. He imported
thousands of French and Belgian amphibians鈥攈oused in large tiered hampers
covered with water-lily leaves to keep the frogs comfortable on their journey.
He subsequently scattered his trophies around East Anglia, and the noisy little
frogs soon made themselves at home.
But unknown to the acquisitive landowners, the hampers harboured not one but
two species of European water frogs. Tucked away among the lily pads alongside
the hybrid edible frog was the deceptively similar pool frog. Continental frogs
now outnumber the indigenous common frog in many places. No wonder then, that
until very recently modern zoologists failed to recognise the native pool frogs,
which had been lurking in ancient ponds in the heart of Norfolk since the end of
the last ice age.
Ponds and ditches throughout southern England are alive with vigorous
European water frogs these days. They are the descendants of those introduced by
19th-century landowners. But by the mid-1990s, two British herpetologists,
Charles Snell, based in London, and John Buckley of the Herpetological
Conservation Trust in Bournemouth began to feel that there was more to the
Norfolk pool frogs than anyone suspected, and urged other enthusiasts to take a
closer look.
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At the very least, Norfolk鈥檚 brown pool frogs looked different from their
decidedly green French and Belgian counterparts. Could the frogs that everyone
had written off as yet another alien introduction actually be a native species?
Was the brown-skinned pool frog Britain鈥檚 seventh native amphibian, a species
that had lived in the ancient glacial ponds of Norfolk since the last ice age?
These ponds鈥攆ormed from thawed glacial mounds and known locally as pingos
鈥攈arbour beetles that have hung on for thousands of years, so why not
frogs too?
The idea set in motion a collaborative research project to unravel the true
history of the pingo pool frogs by comparing the Norfolk frogs with their
European relatives. If these frogs were recent introductions, they should
closely resemble pool frogs in France and Belgium鈥攖he documented source of
the introductions. If the frogs were ice-age survivors, however, they might more
closely resemble the isolated relict populations in Sweden and Norway.
It should have been a simple matter of comparing living frog with living
frog. But alas, by the time zoologists realised the potential significance of
the Norfolk frogs, it was too late to halt their spiral towards extinction. We
foolishly ignored something we shouldn鈥檛 have, reckons amphibian expert Trevor
Beebee of the University of Sussex. Supported by English Nature, Snell tried to
breed pool frogs in captivity, but without success. 鈥淟ucky鈥�, the last pingo pool
frog, died in London in 1999. Despite extensive searches for a mate, researchers
couldn鈥檛 find a local female for him.
Meanwhile Beebee, a biochemist by profession, set out to discover what DNA
analysis could reveal鈥攊f only retrospectively. With colleague Inga Zeisset
he tested specimens collected from various spots around East Anglia in the 19th
century and now preserved in museum collections. At the Natural History Museum
in London he found Norfolk pool frogs pickled in alcohol. In many cases, small
samples of their tongues yielded usable DNA. The stuffed frogs at the Norwich
Museum looked grotesque鈥攂ut all the dried skin samples produced usable
DNA.
At the same time, Julia Wycherley of the Surrey Amphibian and Reptile Group
began to analyse sound patterns from three separate recordings of Norfolk pool
frogs. These were made by an enthusiast in the 1980s and are now stored in the
National Sound Archive in London.
Finally, palaeozoologist Chris Gleed-Owen began to sort through thousands of
tiny bones bagged up by archaeologists after digs in the fens of Ely and
Lincolnshire. Bone by bone, he searched for pool frog remains dating from before
the 19th century flood of frog imports.
Remarkably, all three lines of inquiry came up trumps. After countless hours
picking over the delicate bones, Gleed-Owen found two dating from between AD 800
and 1000 that almost certainly come from a pool frog. The bones were found in a
post hole, which the frog appears to have fallen into while some Anglo-Saxons
were building their house. There have been suggestions that the Vikings might
have introduced these amphibians as a source of food, but this seems
far-fetched, as archaeological research has produced no hint that anyone ever
ate them.
The DNA analysis was decisive too, clearly linking the Norfolk pool frogs to
Scandinavian populations rather than those over the Channel. Pool frogs seem to
have been able to live farther north than other water frogs鈥攖hey are the
only species native to Sweden and Norway. Even the brown colour of these pool
frogs fits the story: their dark skin allows the amphibians to soak up the weak
northern sunlight better. It now looks as if the frogs arrived in Britain under
their own steam some time after the last cold spell around 10,000 years
ago鈥攂ut before the North Sea formed and cut Britain off from Scandinavia
about 8000 or 9000 years ago.
The clinching evidence comes from Wycherley鈥檚 exhaustive study of the croaks
of pool frogs across Europe. Over several summers, she travelled the continent
to record populations of pool frogs in full throat, and found subtle differences
between the Scandinavian populations and those living further south on the
continent. Her results show that Britain鈥檚 now deceased population had something
of a Norwegian accent rather than a French one.
It鈥檚 ironic that calls should provide the last piece of the puzzle, for early
naturalists were familiar with the raucous choruses of males in pursuit of a
mate. Some seem to have recognised the fenland frogs as a distinctive English
species well before Berney and his friends began introducing foreign frogs. The
leading herpetologist of his day, dental surgeon Thomas Bell, commented on the
鈥渧ery remarkable and sonorous croak鈥� of a frog to be heard in the fens so
commonly that they were known locally as Cambridgeshire nightingales or Whaddon
organs. In 1849, he challenged the view that all the Norfolk water frogs were
recent introductions by recalling observations made by his father in the late
1700s. Bell senior had often described to his young son 鈥渢he peculiarly loud,
and somewhat musical, sound uttered by the frogs of Whaddon and Foulmere鈥� (now
known as Fowlmere).
In 1843, a Mr Bond felt impelled to write to the Zoologistto record
that the deafening croaking he heard at one East Anglian fen was nothing like
that of the softly spoken common frog. 鈥淭he sound is more of a loud snore,
exactly like that of the barn owl.鈥�
Audible more than a kilometre away, the
pool frog鈥檚 call is rather like the quack of mallard ducks鈥攁 kind of
鈥渁uwack鈥� iterated quickly like a rapid query, and intensifying towards the end.
Run together in rapid succession, the effect is of a rippling, squeaking purring
sound, says Wycherley.
No one could have guessed that when Lucky croaked his last, it would be his
croak that gave him entry to the pantheon of Britain鈥檚 native species.