The inspiration for the character Ratty in Kenneth Grahame's The Wind in the Willows, the water vole was once a relatively common sight along Britain's riverbanks. A combination of pollution and the introduction of non-native mink to Britain, which had become established in the West Country by the late 1950s, saw a steep drop in numbers. Indeed, in their species appraisal in Mammals of the British Isles, Gordon Woodroffe, Xavier Lambin and Rob Strachen described the population crash as “One of the most rapid and serious declines of any British wild mammal during 20th century”. Populations declined in both number and range and in many areas they still survive only on reserves specifically managed for them and from which mink are excluded. Countrywide, however, the recent resurgence of the otter population may be resulting in a decline in mink and an increase in water voles, although it is very early days.
That which follows is a summary of Water vole natural history. The detailed article for this species will follow in due course.
The Water vole at a glance
Previously classified as Arvicola terrestris following confusion caused when Linnaeus described two species on the same page of his 1758 opus Systema Naturae; terrestris and amphibius now considered the same species and amphibius has priority. Genetic analysis suggest two ‘races’ present in UK; one in Scotland and the other in England and Wales.
Size: Adults range 12-24cm (5-9.5 in.), excluding tail; females average 18cm, males average 19cm (7-7.5 in.). Tail typically 50% body length. Adult 100-350g (3.5-12.4 oz.), ave. ca. 180g (6.3 oz.).
Appearance: Rat-sized rodent, with blunt nose and (cf. rats and mice) a short tail. Shaggy black/brown fur being darker in northern Scotland than England/Wales (75-100% of Scottish voles melanistic, progressively higher concentration as you progress north; betrays Scandinavian origin.); small ears and tail furred, unlike rats/mice. Forefeet have four toes, hindfeet have five toes.
Distribution: Found throughout England, Wales and Scotland and most of Europe, Russia and Asia. Absent from Ireland, Portugal, much of Spain (excl. far north), parts of France (excl. central belt), north-west Italy, most of Norway, Greece and the Ukraine.
Longevity/Mortality: Average life span ca. 5 months, with few living beyond 1.5 years in wild. Record in captivity 2yrs, 5 months. Dubious reports of 5 yrs in captivity. Young must reach 170g (6 oz.) to have sufficient fat reserves to survive winter.
Sexing: Females slightly smaller than males, but otherwise impossible to sex without handling.
Activity: Can be active throughout 24-hour period depending on location, although seem predominantly diurnal. Where coincidental with rats, nocturnal activity often suppressed or eradicated. Often show bouts of activity every 2-4 hrs. Most active during breeding season and least active during winter, although do not hibernate.
Habitat: A freshwater specialist traditionally associated with ditches, rivers, canals and some marshes; typically remains within 2m (6.5ft) of water. Favours steep banks with grass and layered vegetation and prefers weak current, but water all year round, in UK; often attracted to reedbeds and moats. Oten found in meadows (i.e fossorial) in continental Europe and, more recently, in parts of urban Scotland (e.g. Edinburgh).
Territory: Extends linearly along riverbank and marked by latrine sites on bank, close to water. Range size of up to 800m (0.5 mi), depending on sex and habitat quality; may use much smaller areas within total range and shift every few days. Males range more widely than females (ave. 130m and 77m, respectively). Males will violently defend territory from intruders.
Diet: Primarily vegetarian, feeds on grasses, common reeds, sedges and, occasionally, rushes and flowering plants, particularly nettles. Rare records of fish, molluscs and insects being taken; possibly as additional protein source for pregnant females. Consume up to 80% body weight per day.
Reproduction: Males resume sperm production in late Feb; conceptions start during March. Prolific breeders, producing up to 5 litters (range 1-5, ave. 3) of ca. 6 pups per season (April-Sept in England; May-August in Scotland). Gestation ca. 25 days, young weighing 3.5-7.5g (0.12-0.26 oz.) born blind and deaf in nest of rushes and grass either underground or hidden in reeds; young venture outside nest and start taking solid food ca. 14 days old and typically weaned by 22 days old when they may be evicted by mother if second brood imminent. Female will move pups from nest if water levels rise – pups carried in mother’s mouth, held high above the water. Young leave burrow about half-grown and establish home range by ca. one-month old. Sexual maturity typically reached during second year, although some reports of breeding in year of birth.
Behaviour and Sociality: Solitary or in pairs most of year but females may co-nest during winter. During breeding season box sexes typically hold exclusive territories, although overlap has been documented for both. Most young will disperse once weaned, although in Scotland some 70% may remain on parental range during first winter. Produce rasping ‘crick-crick’ alarm call and characteristic “plop” as vole enters water is believed to alert nearby voles to danger. Males rub scent glands on flank with feet and stamp ground to scent-mark, particularly during aggressive encounters with other males; females also possess glands. Fighting most likely during spring and summer months and largely between competing males; fights can be vicious (involving teeth and claws) and occur above and below water surface with victor sometimes ‘strutting’ around angrily for minute-or-so afterwards. May box with feet and roll in clinch. Female aggressive encounters begin with teeth chattering and tail beating; male fights typically more serious than female fights. Males appear more tolerant of other males during autumn/winter. Swim in characteristically buoyant manner, with top-half of body above water.
Threats and Conservation: Arguably Britain’s fastest declining mammal species; population estimated at 5-10% that of 1960s. Multifaceted decline associated with pollution, habitat loss, incidental poisoning, increased flooding and predation (particularly by invasive American mink, which are considered significant threat to population recovery). Considered species of Least Concern by IUCN owing to sizeable populations in continental Europe; UK Biodiversity Action Plan species with effort put into habitat restoration and reintroduction programmes, with mixed success. Current UK population probably still in decline, although more locally some populations increasing. As of April 2008 Water voles and their shelters fully protected under UK law, making it illegal to intentionally catch, kill or sell a vole, or intentionally/recklessly interfere with or destroy a nest. Widely predated by weasels, stoats, foxes, rats, Tawny owls, herons, pike, otters, domestic cats and, in recent years, American mink – study in Wales found 30% fox scats and 18% heron pellets contained Water vole fur.
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