The water vole is the largest and most famous of the British voles (5). 'Ratty' in Kenneth Grahame's Wind in the Willows was not a rat, but a water vole; water rat is a local name for the species (5). Water voles have a short hair-covered tail, a blunt, rounded nose, and a small chubby face with small ears. They have a rich chestnut-brown coat (2), but individuals in Scotland often have black fur. The fur traps air that provides thermal insulation when swimming, and they also possess flaps of skin in the ear that prevent water from entering (5).
The haplotype found in Scotland (the larger black form mentioned above) was found to be different from the haplotype found in England/Wales (a smaller brown form). These two forms arose from two separate recolonisations after the end of glaciation - the Scottish haplotype from an Iberian refugium and the English from an Eastern European refugium. Ref: PIERTNEY, S. B., STEWART, W. A., LAMBIN, X., TELFER, S., AARS, J. and DALLAS, J. F. (2005), Phylogeographic structure and postglacial evolutionary history of water voles (Arvicola terrestris) in the United Kingdom. Molecular Ecology, 14: 1435–1444. doi: 10.1111/j.1365-294X.2005.02496.x