Irish frogs are ancient history

Since the final days of January, my after-breakfast appointment with the rain-gauge on the back lawn has provoked a quicksilver commotion in the pond. One hundred-odd frogs ducking down into the water sets a new record for the start of the breeding season, which used always to wait for my birthday on February 6th. With global warming under way, we can expect these little changes to nature's calendar.

Any day now, some dates of quite special significance in the history of Irish frogs will emerge from the radiocarbon laboratory at Oxford University. They will put an age to 10 samples of fossil frog-bones excavated from caves in counties Sligo, Clare and Cork and held - some for a century - in the collections of the National Museum.

These were caves that also held bones of Ice Age mammals such as mammoth and lemming - but no one is expecting the frogs to be that old. They might turn out, however, to have been hopping around with Ice Age survivors such as the bear and wolf, having crossed a land-bridge from England to join them about 10,000 years ago.

It is only very recently that radiocarbon dating could be applied to frog fossils. Until new methods arrived that need only tiny samples, it would have taken half a kilo of frog bones to produce a radiocarbon date. That could have used up fossil material from a whole cave system, lumping bones of all sorts of ages together and, in the process, sacrificing them for ever.

The new study, which follows on the Irish Quaternary Fauna Project, could answer once and for all the doubts cast on the frog's pedigree as a "native" Irish animal.

Since early monks such as Donatus and Giraldus vowed that Ireland had no frogs, snakes or toads, there has been almost a cultural conspiracy to deny the frog its ancient birthright. To quote a paper in the latest research bulletin of the Irish Biogeographical Society: "It seems to have been almost unanimously accepted that the common frog was introduced - despite the lack of any scientific backing, and the weight of biogeographic and palaeontological evidence in favour of natural colonisation."

The comment comes from Dr Chris Gleed-Owen, working with the Irish herpetologist Ferdia Marnell, of Duchas, the American herpetologist John Kelly Korky, and Nigel Monaghan of the National Museum. They are all militantly hopeful that dates from the Oxford lab will connect the frog with an early land-bridge, rather than with the much-cited introduction by the Normans, 9,000 years later. "It is difficult to see," their paper concludes, "how this and so many other vertebrate and invertebrate species could have arrived without a landbridge - it must have been some floating log!"

Along with such imminent validation of the frog's antiquity in Ireland, research soon to be published in the Biological Journal of the Linnaean Society should also clinch the native credentials of the natterjack toad, the rare, threatened and charming little amphibian that breeds in ponds among sand dunes on the Kerry coast.

This warmth-loving toad survived the Ice Age in Spain and spread rapidly north again through the open postglacial landscape, reaching up into Britain and Sweden and as far east as Poland. In Ireland, it was to become a prized "Lusitanian" species, like the strawberry tree and the spotted Kerry slug.

The Kerry natterjacks were first recorded by a botanist at Castlemaine Harbour, at the head of Dingle Bay, in 1805. Later in the century, the possibility began to gain ground that the toads had been dumped out in sand-ballast from boats trading with Europe. Despite an impatient rejection by Robert Lloyd Praeger and other naturalists in the mid-1900s, this has remained an enduring "alternative" explanation of their presence.

Dr Trevor Beebee of Sussex University, leading authority on the natterjack, has long believed that the toads got to Kerry under their own steam (not hopping, but running, like mice). Their wide scattering through Europe, to what are now often isolated habitats, made them an ideal candidate for genetic study.

Working with geneticist Graham Rowe, Beebee collected natterjack tadpoles and toadlets from pools across Europe and analysed their DNA. Among other things, it showed how populations were inter-related. The Kerry toads, for example, have a strong affinity with natterjacks in north-west England, while both groups are genetically different from the natterjacks of south-east England.

The research also throws light on when the toads reached the different regions; and it supports the idea that our natterjacks crossed a land-bridge from England some 10,000 years ago.

A granting of full native heritage to the natterjacks should help public interest in their conservation, a theme taken up strongly by John Kelly Korky in the IBS Research Bulletin. Threats to both toads and frogs are a topic I'll come back to shortly, but two points from Korky's survey catch the eye. First, that he could find no natterjacks in 1997 on the Inch peninsula in Dingle Bay, once a stronghold. Second, that the toads were flourishing at the golf course in the dunes of Castlegregory, where they seemed, only a decade ago, to be at particular risk.

Inquiries to the Irish Biogeographical Society, c/o Dr J.P.O'Connor, National Museum of Ireland, Kildare Street, Dublin 2.