The water vole is a mammal and a member of the rodent family, with the Latin name Arvicola terrestris. Their fur is reddish brown through to dark brown, but in Scotland they can be almost black. They can swim and dive well but have no special adaptations for their watery life style. Water voles are often mistaken for brown rats, a relatively recent arrival in Britain. Water voles have been present in Britain since the end of the last ice age, about 8000 yrs ago, whereas the brown rat arrived on ships in the 17th, or 18th century. Rats are known to kill young water voles and take over burrow systems.

The vast majority of water voles in the UK live in burrows (left) in stream and riverbanks, or along the edges of ponds and lakes. They can also make nests woven out of vegetation at the base of reeds and rushes, or nest in clumps of rushes or grass tussocks in wet fields and meadows.  In Europe, the exact same species is not as closely associated with water, and is often found living in extensive burrows in meadows and pastures, almost like a mole - something that is called 'fossorial’ behaviour.

The ideal habitat for voles is:

• Heavily vegetated stream sides;
• Slow moving water;
• Stable water levels, or that have higher ground they can retreat to during flood;
• Permanent water because water provides an escape route from predators;
• Banks of soil or clay, that allows them to burrow.

Water voles are herbivores, and will eat a wide range of plants. A national survey that looked at the feeding remains identified 227 species of plant eaten by water voles, including grasses, rushes, sedges, water plants and a wide range of herbs.  The water vole will vary its diet depending on the season, and in autumn and winter they will eat fruits such as blackberries, under ground roots and tubas, and also tree bark particularly trees like willows and some fruit trees.

The number of young that a water vole will have depends on habitat quality; good food availability means that they will have more young.  Females can have 2 to 5 litters per breeding season, March to October, and each litter will have 5 to 8 young. However, like most small mammals water voles suffer very high mortality rates. An animal born this breeding season has only a 30 - 40% chance of survival through to the winter. For it to survive the winter it needs to be over 170g, so the earlier in the year you are born the better.

In the autumn and winter the animals stop being territorial and congregate together in a core area, usually where the conditions are most favourable, for example with the lowest risk of being flooded. Water voles don't hibernate, but they spend a lot of time sheltering in their burrows or nests in the winter months, where they store food to keep them fed in periods of bad weather. In the summer months a water vole will spend about two thirds of its time out foraging for food and a third of the time in its burrow. In winter those figures are reversed.  Of the animals that congregate together in the winter, up to 70% of those don't make it through to spring to breed again. They are more vulnerable in winter because an entire population could be contained in a single burrow system, so if something happens to that burrow an entire population could be lost.

During the breeding season water voles live in colonies that are strung out along suitable stretches of watercourse, and are divided into a series of bordering territories by breeding females. Like many colonial mammal species, hierarchy is determined by interaction between animals, not so much full on fights, but rather a series of squabbles.  A female’s breeding territory varies in size depending on the quality of the habitat, anything from 30 - 150 metres in length and the better the quality of habitat the smaller the range.  Males also establish territories that are larger than those of the female, anything from 60m to 300m, again depending on the quality of the habitat.

Tracking experiments have shown that water voles move around, dispersing from the area they were born in to occupy other sites and join other populations, they in effect have a network of colonies that animals move between, so some years a site might not be inhabited, but the next year voles from other locations occupy the site. The technical name for this is meta-populations.