Eurasian beaver

HomeAnimalsMammalsBeavers
Latin name
Castor fiber
Class
Mammals
Group
Beavers

Familiar to many of us as the accommodating animals who take in Peter, Susan, Edmund and Lucy Pevensie in C.S. Lewis' timeless classic The Lion, the Witch and the Wardrobe, published in 1950, the beaver was once a common and widespread animal in Britain. Centuries of persecution for fur, castoreum (an oil used in perfumes, medicines, foods and even to help flavour tobacco), meat (Catholics classified them as “fish”, allowing them to be eaten on holy days) and competition with farmers for land resulted in the beaver's extinction throughout Britain and much of Europe. The last confirmed record of a beaver in Britain was one killed for a tuppence bounty along the River Wharfe in south-west York during 1789. Now, thanks to a mix of dedicated conservation projects and illegal releases, beavers are starting to recolonise parts of the UK, with populations increasing in Scotland and south-west England.

The European beaver is almost identical to it's North American relative (Castor canadensis) in appearance and ecological impact, but is genetically distinct and, owing to the difference in chromosome number, all attempts at hybridisation have failed. The American beaver appears to be descended from Eurasian species, having diverged ~7.5 MYA.

Most closely related to the kangaroo rats and pocket gophers, beavers are the second largest members of the Rodentia order, after the South American capybara (Hydrochoerus hydrochaeris). The Eurasian is slightly smaller than its American cousin but is nonetheless the largest European rodent. Its binomial name combines both the Greek (kastor) and Latin (fiber) words for “beaver”.

That which follows is a summary of beaver natural history. The detailed article for this species will follow in due course.

A Eurasian beaver (_Castor fiber_) in Perthshire, Scotland. - Credit: Raymond Leinster

The Eurasian beaver at a glance

Size: Largest Holarctic rodent with significant variation across range; mature individuals presenting with head-body lengths of 74-90cm (29-34 in.) and tails 28-38cm (11-15 in.). In UK, usually up to ~80cm (2.6 ft.) with ~30cm (1 ft.) tail. Weigh 12-40kg (26.5-88 lbs), typically ~20-25kg (44-55 lbs).

Colour/Appearance: Uniform brown, varying from yellowish to reddish and very dark (almost black) when wet. In common with other semi-aquatic mammals, coat is double layered; long waterproof guard hairs up to 6cm (2.3 in.) long protrude through shorter (2.5cm / 1 in.) soft undercoat. Tail is flattened dorsolaterally (i.e. squashed flat), forming a paddle shape about 13cm (5 in.) wide, and covered in dark scales. Short body with small, blunt head. Short legs; front paws dextrous for food manipulation and larger rear feet partially webbed. Folds of skin cover nostrils and short rounded ears upon submergence; nictitating membrane protects eyes. Fur-lined lips close behind very large continuously growing reinforced incisor teeth to prevent water egress into mouth and lungs while biting and carrying vegetation underwater. Anal glands secrete oil that’s applied, via front feet, over whole body during grooming to keep the fur water-repellent.

Distribution: A widely hunted species, the beaver has suffered local extinctions over much of its former range. Reintroduced to much of former range and now be found throughout Europe since reintroduction projects began in Sweden in early 1920s. Distribution still fragmented, extending into parts of Mongolia, China, Russia and Siberia. In Britain, the beaver appears to have been extirpated at end of 1700s or beginning of 1800s. Scottish Beaver Project released 16 Norwegian animals into Knapdale, Argyll in 2009, almost a decade after an illegal introduction occurred on River Tay. Now widespread on the Tay catchment and Perthshire. Similar illegal release on River Otter in Devon during 2014; currently under monitoring by Devon Wildlife Trust but may be as many as 15 family groups. Small scale release to enclosure in north Devon in 2011 also part of a trial. Projects underway at Ham Fen in Kent (2001) & Valley Farm in Cornwall (2017). Reports from wild in Wales, but official project still under discussion. Introduction planned for Knepp Estate in Sussex.

Habitat: Appear to do well in most riverine habitats as only essential habitat element is year-round access to fresh water. May prefer deciduous wooded river valleys, particularly floodplains. Prolific ecosystem engineer, modifying habitat to create a mosaic of flooded forest (predominantly willow coppice) with channels and pools. Flooding peripheral woodland allows easy access to float trees back to lodge (homestead). In the US, beavers have readily adapted to suburban environments, but this has been less apparent in Europe.

Longevity: Live for 7-8 years in wild and commonly into teens in captivity. One Russian record of 25 years. Highest mortality appears to be during first 6 months and in dispersal periods when prone to vehicle collisions.

Sexing: No external sexual dimorphism – sexes appear identical. Lack of external genitalia means sex verification requires obtaining sample of anal fluid via cloaca; male fluid is brown, females’ white or clear.

Activity: Primarily nocturnal, commencing activity shortly before dusk and returning to lodge before dawn. Occasionally diurnal in undisturbed locations and may be active within lodge during day. Do not hibernate, but confine activity to lodge or under ice during winter. Cannot remain in water indefinitely owing to loss of body heat and must return to land to warm up. Slow and ungainly on land but well adapted to swimming and can dive well, typically remaining underwater for 5-6 mins.

Territory/Home Range: Territory size varies according to habitat (specifically food quality and population density) with range 0.5-13km (0.3-8 mi.) river length; average ~3km (2 mi.). Territories larger in spring/summer cf. winter and may be patrolled daily. Boundaries marked with “castoreum” (musky secretion produced by castor glands of both sexes sprayed onto objects) and anal gland secretion (dragging anus along ground); scent typically applied to piles of soil, rocks and fallen trees. Seven scents identified. Territories may be aggressively defended, particularly in spring.

Usually single lodge within territory, constructed from wood and soil, although may use natural bank holes or burrow into banks in poorer habitats. Entrance below water level.

Diet & Feeding: Entirely herbivorous – contrary to some reports, they do not take fish. Herbaceous vegetation makes up bulk of spring and summer forage; bark main component during winter. Hardwoods (e.g. birch, aspen, willow, cherry, oak, alder and poplar) preferred and conifers apparently ignored. Trees are typically felled; those with trunk diameters up to 10cm (4 in.) preferred, although some reports of much larger trees (up to 1m/3 ft. diameter) hewed. Most felling occurs during late summer and autumn for inclusion in a larder, stored underwater near the lodge.

Most foraging occurs within 60m (200 ft.) of water’s edge; create flooded channels to help move trees/branches. Rely on microbiota in gut to digest lignocellulose-rich plants, but results in poor cellulose digestion and requires large amounts of food.

Reproduction: Apparently monogamous and monoestrous with breeding season running December-April. Oestrous lasts 7-12 days and gestation for ~3.5 months. Typically produce 2-3 young (kits), upto 6, May-June, weighing 300-700g (10-25 oz.). Kits swim within hours of birth, leave nest at ~1 month old, weaned by late August. Disperse at 2yrs when sexually mature. Unlikely to breed until third year.

Behaviour and Sociality: Typically live in small social groups (“colonies”) consisting of related animals. May fight with rival colonies; lunging, biting, wrestling, growling, hissing and screaming. Sometimes slap water surface with paddle-like tail before submerging to warn family members of danger or in bid to scare off intruder. Group size average four members, increasing with population density.

Predators: Few, if any, in UK. Foxes may take kits as could larger raptors, such as eagles. More widely, wolves, lynx and wolverines will take individuals of varying sizes.

Threats: Disliked by many involved with country sports and farming over concerns for flooding farmland and misapprehension they detrimentally impact fish stocks. Science suggests opposite: dams reduce overall landscape flooding and increase fish abundance. Hunted as game in many parts of Europe, including Norway, Finland, Sweden and Russia; also trapped for fur and castoreum. No specific UK protection. Currently listed in Appendix III of Bern Convention, regulating exploitation, and further protection provided under Annex II & IV of EC Directive on Conservation of Natural Habitats & of Wild Fauna & Flora.

Eurasian beaver in detail


Bibliography

Atlas of the Mammals of Great Britain & Northern Ireland - by Multiple Contributors
Pelagic Publishing -- 2020 -- ISBN: 978-1784272043

Britain’s Mammals 2018: The Mammal Society’s Guide to their Population & Conservation Status - by Multiple Contributors
The Mammal Society -- 2020 -- ISBN: 978-0993567339

Collins Field Guide: Mammals of Britain and Europe - by David MacDonald and Priscilla Barrett
HarperCollins Publishers -- 1993 -- ISBN: 978-0002197793

Mammals of the British Isles: Handbook, 4th Edition - by Stephen Harris and Derek Yalden (eds)
The Mammal Society -- 2008 -- ISBN: 978-0906282656
The natural history "bible" covering all British mammals with detailed coverage of their biology, behaviour, ecology and taxonomy written by experts in the field and referenced to the primary literature.

The Encyclopaedia of Mammals - by David MacDonald (ed.)
Brown Reference Group -- 2006 -- ISBN: 978-0199206087

UK Mammals: Species Status & Population Trends - by The Tracking Mammals Partnership
JNCC/TMP -- 2005 -- ISBN: 978-1861075680