A Contribution to the Ecology of the Musk‐rat (Ondatra zibethica) in the British Isles

Abstract

Summary.An investigation has been made of Musk‐rats Ondatra zibethica zibethica L. in Britain from 1933‐35. British specimens are similar in size and weight to those from North America and Europe. The main infestations were in the Severn Valley around Shrewsbury in Shropshire and in the valleys of the Rivers Earn and Allan in Stirlingshire and Perthshire. In Shropshire, a favourable locality, Musk‐rats were exterminated from pools and ditches earlier than in the main river; generally speaking the reverse has been true in Scotland. The relatively small infestations 111 West Sussex, Surrey, and Ireland, were soon brought within control. Musk‐rat activity is at a peak during the mating season in March, when males may wander long distances, and again less abruptly in November. During the summer activity is a t its minimum, especially amongst the females. Adult Musk‐rats weigh between lo00 and 1400 gm. The autumn migration is caused by activity amongst Musk‐rats born the same year, weighing on the average 800–1000 gm. They do not breed the year of birth. The first mating is in mid‐February and March, and the first litter is born in April. There are two or rarely three litters of (on the average) seven in the year. The young weigh 21 gm. at birth and are soon independent of their parents, they are more terrestrial in habits than the adults. The sex ratio of 1330 Musk‐rats is 56.9±0.92 males per cent. The greater activity of the males in March is reflected in a rise in the sex ratio of those caught at this time. Burrows usually open under the water and those left exposed by a fall in water‐level are plugged with vegetation as are also air holes. Canals are made as in North America. Houses are built from early September and onwards through the autumn: they are built of any vegetation near at hand. “Eating houses” are also built. Stomach examinations show that the Musk‐rat is largely vegetarian, though some Gastropods and Anodonta were found. Ranunculus fluitans is the main food in Shropshire. Defsecation usually takes place in the water. Most Musk‐rats examined were healthy, but one in Shropshire was found dead from lung disease in August 1934, and another caught soon after had an early stage of apparently the same epidemic disease, which was caused by a hemolytic streptococcus. In Scotland in November 1934 a Musk‐rat caught had a disease of the spleen, probably a form of tuberculosis. In severe floods Musk‐rats may use rabbit burrows; their unused holes may be used by brown rats and water voles. Large numbers of Musk‐rats were caught in Shropshire and Scotland, and the graph of monthly catches in the former place resembles similar graphs of Central European data. After 1933 the worst of the infestation in Britain had been dealt with. Catches at Farnham, Pulborough, and Ireland show an almost continuous drop from the commencement. Foxes and stoats are enemies of Musk‐rats in Britain. Bitten‐off vegetation, houses, bubbles under the ice, and turbid water help in detection, while the probing bar is used to find burrows. Steel traps of various types with plain jaws are used for catching Musk‐rats in Britain. Floating trap “sets” and cage traps are also used. Traps are mainly set in burrows. Damage has been done to river banks, mill dams, and drainage ditches in Shropshire and to an artificial pond embankment in Scotland. All the British Musk‐rat campaigns have been discontinued and the Musk‐rat is now (1939) considered extinct in the British Isles. No modification of their North American habits seem to have taken place in Britain.