North American water vole

The North American water vole or just water vole (Microtus richardsoni) is the largest North American vole. It lives in the northwestern United States and southern parts of western Canada. This animal has gray-brown or red-brown fur with gray under parts, and its large hind feet help it swim very well.

Water voles enjoy eating grasses, leaves, roots, seeds, and small invertebrates. They dig burrows connected to water sources, making them semi-aquatic creatures. These burrows often open at the water's edge or underwater, and water voles usually live together in groups of 8 to 40 individuals along waterways.

They stay active all year, even tunneling through snow during the winter months. Because their populations are very small and their habitat may be disappearing, water voles are considered a sensitive species by the USDA Forest Service Region 2.

Etymology

The word "vole" started being used around 1805. It is a shorter way to say "vole-mouse," which means field mouse. The word comes from a Norwegian word, "vollmus," where "voll" means field and "mus" means mouse. It might also have been influenced by a Swedish word, "vall," which also means field.

The scientific name Microtus richardsoni has many common names, such as Richardson's water vole, Richardson vole, and giant water vole. This name honors a Scottish naturalist named Sir John Richardson.

Taxonomy and evolution

This animal was once thought to belong to a group called Arvicola, but special tests show it is closer to North American Microtus animals. The closest relative of Microtus richardsoni seems to be Microtus pennsylvanicus, known as the meadow vole. Old bone evidence suggests that M. richardsoni split from an ancient group in Siberia about 1.5 million years before Arvicola appeared in Europe. This might mean it is an early form of the Microtus group, and its looks are similar to Arvicola by chance. Today, about 60 species are recognized in the Microtus group.

Appearance and anatomy

Water voles have big hind feet, measuring between 25 and 34 mm, which help them swim quickly. Males are usually larger than females. On average, these voles are about 20–27 cm long, including their tails, which are 6–10 cm long. They weigh between 125 and 178 grams. Their fur can be grey-brown, dark brown, or reddish-brown on the top and grayish-white on the bottom. With their large teeth, big skulls, and strong jaw muscles, water voles can dig tunnels and chew through tough plant roots easily. They are the second largest arvicoline in their area, after the muskrat.

Behavior and ecology

Water voles live in two groups across western United States and Canada, from British Columbia and Alberta through parts of Oregon, Washington, Montana, Idaho, Wyoming, and Utah. They prefer areas near water in alpine or semi-alpine meadows, usually between 914–3,201 meters high. Their homes change a lot because of forests, mountains, and valleys without water, but they can move over land to meet other groups, especially if they are close by.

These voles mostly eat plants like leaves, stems, grasses, and willows, and sometimes seeds or insects. They need to eat a lot because they use a lot of energy. In winter, they dig tunnels under the snow and stay below the surface when there is about 6 cm of snow. They are mostly active at night and make small paths through plants to move around. During summer, they build tunnels and nests under plants where the mothers care for their babies.

Water voles live near water and have groups where females stay in one area and males move around to find mates. Males are usually more aggressive, especially during breeding time. They breed during the summer, and females can have about 5-6 babies at a time after a short pregnancy. The babies are born without fur and can’t see, but they grow quickly, getting fur in a few days and learning to swim in about two weeks. Mothers nurse their babies for three weeks, and the young stay with their mother for about a month before moving out on their own. Most babies grow up to have their own families the next breeding season.

Life span

Water voles usually only live through one winter and die by the end of the second breeding season. Very few of them survive to see a third winter.

Loss of habitat and endangerment

Scientists have studied the homes of water voles to see how things like rain and animals grazing affect their numbers. They found that when there is more rain, water voles have more space to live and young voles grow up faster, having more babies. However, when animals like cows graze nearby, it can hurt the voles. Light or moderate grazing means fewer young voles survive and the groups get smaller. In places where grazing is very heavy, the stream banks can become damaged and no longer a good home for voles.

Livestock can change the land in many harmful ways, such as compacting the soil, causing more runoff, and damaging the plants that voles need for food and shelter. Plants like ferns, mosses, and shrubs are important for protecting voles from danger, but these plants often disappear when livestock graze, making it hard for voles to survive.