Short description of Ondatra zibethicus, Muskrat

Muskrats are semi-aquatic rodents, related to voles and lemmings but considerably larger.  They are intermediate in size between brown rat and coypu, generally around 50 cm in length including the tail.  Their thick fur is darkish brown, slightly paler on the underside.  The tail is slender and scaly, and flattened from side to side.  The ears are small.  The lips can close behind the incisor teeth, facilitating underwater chewing.

Impact summary: Ondatra zibethicus, Muskrat

Burrowing by muskrats caused a great deal of damage to waterways and flood defences and their grazing can destroy large areas of aquatic vegetation.  The animals can be vectors of the parasite that causes Weil's disease.  They compete with and can displace a wide range of native fauna.

Habitat summary: Ondatra zibethicus, Muskrat

Muskrats occur in lakes, ponds, rivers and associated marshland.

Overview table

Environment	Terrestrial and Freshwater
Species status	Non-Native
Native range	Northern America, Alberta, British Columbia, Labrador, Manitoba, New Brunswick, Nova Scotia, Ontario, Saskatchewan, Yukon
Functional type	Herbivore
Status in England	Non-Native
Status in Scotland	Non-Native
Status in Wales	Non-Native
Location of first record	?
Date of first record	1929

Origin

Muskrats are native to North America and occur from Alaska and Canada south to northern Mexico.

First Record

Muskrats were first brought to GB for fur farming in the 1920s, and escapes into the wild were being documented by 1929.

Pathway and Method

Muskrats were imported to be kept for their fur. By 1931, however, escapes or releases were known to have occurred from 14 of at least 87 fur farms set up in England during the 1920s, with similar escapes in Scotland.

Species Status

Fur farms across Eurasia also gave rise to introduced populations, which today extend from France to northern China and Kamchatka.  Further non-native populations occur in South America.  The species is widely regarded as a pest.  An incipient Irish population was successfully extirpated by 1937.

Dispersal Mechanisms

Muskrats introduced from Ontario to Shropshire escaped in 1930 and by late the following year had spread along 30 km of the River Severn and subsequently along 60 km.

Reproduction

Nest chambers are built in 'lodges', tall heaps made of marsh vegetation at or near water level, or in burrows with underwater entrances, according to local conditions.  In England, breeding occurred during March to early August, with two or possibly three litters a year.

Known Predators/Herbivores

None known in GB.  Elsewhere, they sometimes fall prey to foxes, mink and other carnivores.

Resistant Stages

Muskrats become rather inactive in icy conditions.

Habitat Occupied in GB

Introduced populations in GB inhabited river systems and associated marshland.

Prior to eradication there were major population centres in the Forth Basin in Scotland, on the Severn in Shropshire, and in Surrey and Sussex.

Environmental Impact

Grazing in waterways and on marshland can severely damage or destroy areas of emergent or marsh vegetation, removing important habitats for native animals.

Health and Social Impact

Muskrats can be vectors of leptospirosis, which can be transferred to humans as Weil's disease.

Economic Impact

Burrowing by muskrats undermines river banks and flood defences, damages irrigation systems, and can cause serious flooding.  In the few years that they were resident, muskrats caused extensive damage to the banks of the River Severn.

Identification

Biology, ecology, spread, vectors

Parker, G.R. & Maxwell, J.W. (1980) Characteristics of a population of muskrats (Ondatra zibethicus) in New Brunswick. Canadian Field-Naturalist, 94, 1–8.

Proulx, G. & Gilbert, F.F. (1983) The ecology of the muskrat (Ondatra zibethicus) at Luther Marsh, Ontario. Canadian Field-Naturalist, 97, 377–390.

Management and impact

Gosling, L.M. & Baker, S.J. (1989) The eradication of muskrats and coypus from Britain. Biological Journal of the Linnean Society, 38, 39–51.

General

Sheail, J. (1988) The extermination of the muskrat (Ondatra zibethicus) in inter-war Britain. Archives of Natural History, 15, 155–170.

Harris, S. & Yalden, D.W. (2008) Mammals of the British Isles: Handbook. Fourth edition. The Mammal Society, Southampton.

Lever, C. (2009) The Naturalized Animals of Britain and Ireland. New Holland Publishers, London.

Hinterland Who's Who

https://www.cabi.org/isc/datasheet/71816