Etat de nos connaissances sur l’écologie du Rat musqué

Abstract

The muskrat (Ondatra zibethica zibethica Linn.) is a rodent adapted for semi-aquatic life. Its original geographical distribution ranges over North-America, but due to fur-breeding, it has settled in the Northern parts of Europe and Asia. Still water areas such as lakes, ponds, marshes are the preferred habitats, but the muskrat may also be found in running waters (streams, rivers, creeks, etc.). Its normal dwelling-places are bank-burrows and shallow-water houses. The muskrat feeds principally on aquatic plants, but, at times it may consume terrestrial plants or supplement its diet with animal food. A clear territorial behavior allows one to define a territory occupied by a single family group : in defending this territory, the muskrat sometimes engages in vicious intraspecific strife, specially during spring and autumn. Many authors believe that the species in monogamous. Its reproductive season extends from March/April to September in temperate and cold regions, but usually continues throughout the year in warmer areas. The œstrous cycle takes on average 6.1 days. The reproductive cycle including the pregnancy period and a postpartum oestrus, occupies 27 to 33 days. Depending upon living conditions, a female gives birth 1 to 4 times a year ; each time 6 young are produced on average. This number varies according to the age of the female, season, social stress and is believed to follow a 10-year cycle. Two month old young establish themselves in auxiliary quarters within the limits of the parental territory : at four months, the young start leaving their parental territory. The last litter stays with the parents during its first winter. In general, males slightly outnumber females ; the adult male/female ratio is at a maximum during spring and at a minimum in autumn. Males predominate in young age groups : the survival rate of very young females seems to be less than that of males of the same age. The extent and frequency of movements have led to such generalisations as the «migratory muskrat». In fact, the significance and importance of movements have yet to be thoroughly studied. Spring and autumn are seasons of increased activity in muskrats. The constitution of breeding pairs or the search for adequate breeding territories could explain the spring movements, while a partial explanation of the summer and autumn season movements could be the growing up of young to become a part of the active population. Erratic individual movements occur throughout the year. The principal predators of the muskrat are : mink, otter, owls and occasionally fox, wolf and raptorial birds. The main competitors are the brown rat (Rattus norvegicus) the water vole (Arvicola sp.) and bank-dwelling rodents. These are generally quickly eliminated by muskrats ; it seems that they desert the territory of muskrats. Examination of the effects of abiotic factors such as soil type, stability of water-levels and climatic factors (particularly rainfall) illustrates the adaptability of muskrats to extreme living conditions. There is an astonishing diversity in individual responses to various environmental changes ; this is seen in the fidelity of some muskrats to completely dry quarters, even in the proximity of more suitable habitats, or in the conversion to an erratic way of life of muskrats living in apparently favourable habitats. At the population level, these «external» factors seem to be responsible for local trangressions of «optimal density limits», resulting in destruction or overcrowding of local populations. Overcrowding results in starvation and disease, which in turn reduce population density. Under normal conditions, predation and the auto-regulation of such intrinsic population growth characteristics as reproduction, territoriality and movement tend to maintain a muskrat population within an «optimum» density range.