Water voles colonised Britain in two waves

The water vole (Arvicola amphibius) was once a common sight along the banks of slow moving rivers, streams and other waterways in the UK. But numbers have been in decline in recent decades and it's now one of our most threatened mammals.

Now, new research from Museum scientists and collaborators adds to our understanding of their origins and genetic diversity, and could help with conservation efforts.

The researchers from the Museum, Royal Holloway University of London, Bournemouth University and the Université de Liège in Belgium, carried out a genetic analysis of water voles from across Britain and continental Europe.

Their findings showed that water voles in England are more closely related to those in eastern Europe than to those in Scotland.

The presence of two distinct populations - one either side of the border - suggests Britain's water voles arrived in two waves. One became the Scottish population and the other became the English.

'The contrast really is quite stark,' says Dr Selina Brace, the first author of the research published in the journal Proceedings of the Royal Society B. 'Genetically, all of the Scottish water voles group together, as do almost all of the English voles.

'It's exactly what you'd expect if the two populations arrived separately.'

Celtic fringe

Armed with the knowledge that there were two waves of colonisation, the researchers set out to determine when these may have occurred.

'We wanted to know - did the two vole groups arrive at the same time but spread to different parts of Britain, or did one group arrive first, only to be displaced at a later date?' explains Professor Ian Barnes, who led the research.

Unable to differentiate between the two scenarios from modern specimens alone, the team also included historical water vole specimens between 100 and 28,000 years old in their analysis. This covered a period when dramatic environmental change took place, including the formation and retreat of an ice sheet that stretched as far south as Wales.

The team successfully extracted ancient DNA from 34 of the historical specimens, and analysed their genetic similarities.

The results showed that the earliest English water voles examined - dated to between 12,000 and 28,000 years ago - were closely related to the modern-day Scottish voles. However, subsequent English voles were more similar to modern English water voles.

This implies that a single population of water voles did exist across Britain prior to 12,000 years ago, but is now confined to Scotland. Modern English water voles arrived later, displacing the earlier inhabitants in most areas south of the border, leaving only a 'Celtic fringe' of the original inhabitants.

'From the specimens we have, we can date the English voles' arrival to between 4,500 and 12,000 years ago, after the last glacial maximum,' says Prof Barnes.

At that time, the ice had retreated from Britain but the region was still temporarily connected by land to the rest of Europe.

'The land connection was broken around 8,000 years ago, so that narrows down the voles' arrival to between 8,000 and 12,000 years ago.'

Ice age link

Although the second wave of water voles all but replaced the earlier inhabitants in England, this doesn't necessarily mean that they directly out-competed them.

'Instead, a thousand-year-long cold snap, starting around 13 000 years ago, may have led to a huge reduction in water vole numbers across the whole of Britain,' explains Dr Brace. 'Then, when the climate warmed up, and the second group arrived in southern Britain from the continent, they found themselves in a region almost devoid of other water voles.

'This allowed them to spread freely, confining the original group to northern regions of Britain, such as Scotland, while replacing them in areas south of the border.'

This climate change link could explain why the water voles, and other small British mammals such as shrews, show a 'Celtic fringe' pattern of genetic diversity.

Knowing that the two populations are genetically distinct is important in shaping conservation efforts. 'Both populations should be targeted as conservation priorities to maintain as much genetic diversity as possible,' says Dr Brace.

'And if conservation efforts fail, knowing which European populations the British water voles are most similar to could help us reintroduce the species if we need to. This is something that has been considered for species that are already extinct in Britain such as beavers and lynx.'