Breaking the bank vole mystery

ONE of the mysteries of Irish natural history has finally been solved and the solution is rather an elegant one. We now know how and when bank voles got to Ireland.

Bank voles were first positively identified in this country in 1964 from specimens found near Listowel in north Kerry. Today they are only found in the southwest, roughly speaking below a line drawn on the map connecting Waterford with Galway. But they are spreading quite rapidly north-eastwards.

One study suggests that the rate of their advance is up to 4km a year, which seems very fast for such a small animal. Bank voles only have a body length of about 10cm, plus a shortish tail.

From this data it looks as though they arrived in the southwest some time in the first half of the 20th century. Some biologists hazarded a guess of around 1940, though we now know it was earlier than that.

A team of scientists from Ireland and abroad has been comparing the DNA of Irish bank voles with DNA held on databases all over Europe and North America. The first results, recently published in the Irish Naturalists’ Journal, were rather surprising. Our bank voles came from Germany.

Bank voles are common in Britain and up until now this had been considered the most likely origin for the Irish population.

The scientists were intrigued by this and did a bit more research which included getting in touch with the Siemens engineering company in Germany. This company was contracted by the Irish Free State to dam the Shannon and build the Ardnacrusha hydro-electric power station in the 1920s. And it still has remarkably good records from the time.

Apparently in 1925 a lot of heavy earth-moving equipment was loaded on to ships in ports in north Germany and brought by sea to the port of Foynes on the north side of the Shannon estuary. Foynes is quite close to Listowel where, thirty-nine years later, bank voles were first identified in Ireland.

Even more remarkably the Siemens records showed that the operation to ship the equipment to Ireland was done very hurriedly, so hurriedly that the buckets of many of the draglines and diggers were only emptied of soil when they arrived in Ireland.

Bank voles are active little animals that spend most of their life above ground and are quite good at climbing trees. But according to my field guide they “use runways at ground level and also underground tunnels”.

It’s easy to imagine how this could have led to their transportation in soil in the bucket of a dragline.

They are the only voles in Ireland and so far they’ve proved to be a rather a positive addition to our countryside. They are basically vegetarian, eating buds, leaves, seeds and fruit along with the odd bug.

There are records from other countries of them doing minor damage to young forestry plantations at times of very high population density but this hasn’t happened here and is probably unlikely to.

On the other hand they are an exceedingly good source of food for many predators such as kestrels, owls and harriers as well as foxes and stoats.



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