The North American water vole or just water vole (Microtus richardsoni) is the largest North American vole. It is found in the northwestern United States and southern parts of western Canada. This animal has been historically considered a member of genus Arvicola, but molecular evidence demonstrates that it is more closely related to North American Microtus species. Water voles are on the USDA Forest Service Region 2 sensitive species list because they maintain very small populations and there is high concern that their required habitat may be declining.
These animals have gray-brown or red-brown fur with gray under parts. Their large hind feet help make them excellent swimmers, and they are found in alpine or semi-alpine meadows near water. They feed on grasses, leaves, roots and seeds, also eating small invertebrates. Water voles dig burrows that are connected to water sources, and are considered a semi-aquatic species. and they usually live in colonies of 8-40 individuals along the waterway.
Etymology
The word vole originated in approximately 1805, and is short for vole-mouse, which means field mouse. Vole originated from the Norwegian word vollmus; voll, meaning field, and mus, meaning mouse. It is also possible that there were influences from Swedish vall, which translates to mean field.
Microtus richardsoni is also known as Richardson's water vole, Richardson vole, Richardson's meadow vole, Richardson meadow mouse, water rat, big-footed mouse, giant water vole, and water vole. The specific name is after Scottish naturalist Sir John Richardson.
Taxonomy and evolution
Although this animal has been historically considered a member of genus Arvicola, molecular evidence demonstrates that it is more closely related to North American Microtus species. Genetic tests have been done that suggest that the closest relative to Microtus richardsoni is Microtus pennsylvanicus (the meadow vole). Paleontological evidence suggests that M. richardsoni diverged from a Mimomys ancestral form in Siberia approximately 1.5 million years before Arvicola evolved in Europe. This could indicate that it is the primitive form of the genus Microtus, and that the morphological similarities between M. richardsoni and Arvicola are likely convergent traits.
Their main source of food is vegetation, including leaves, stems, grasses, sedges, willows, and sometimes seeds or insects. Their food source varies significantly depending on geographic location. Studies have shown that water voles have a very high metabolic rate, and therefore have to consume more food than other rodents their size. Most often, water voles feed on the subterranean portion of plants, which are available to them year round. There is no evidence that they store food for the winter. During the winter they are seen to dig tunnels through the snow, and usually do not surface above the snow once have accumulated (approx. 7 to 8 months of the year). This may be due to the very short breeding season compared to other rodents who breed for 6 months or more. The abundance of ferns, mosses and shrubs are imperative as ground cover to protect water voles from predation, and in areas of grazing these plants were scarce or non-existent, and therefore water voles weren't often found in these areas.