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Adjustments of the Northwestern muskrat (ondatra zibethicus spatulatus) to a Northern environment Stevens, Ward Earl

Abstract

The northwestern muskrat (Ondatra zibethicus spatulatus) is nowhere more abundant than in the delta of the Mackenzie River in northern Canada. An investigation was undertaken to assess the adjustments resident animals have made in order to inhabit a region more than one hundred miles inside the Arctic Circle. The rigours of the physical environment demand that these animals live in burrows rather than in lodges of vegetation, as in more southern areas. During summer the breeding pairs may occupy shallow temporary habitat which is not suitable for tenure during winter. These summer sites must be vacated before ice seals the lakes and prevents escape from a rapidly deteriorating environment. That all muskrats do not desert this temporary summer habitat was indicated by the fact that only half as many marked animals from such sites were encountered subsequently. It may be assumed that a depressed survival was the rule in such locations. Only deeper lakes with adequate submerged food plants constituted satisfactory wintering environment. Normal movement of muskrats in the Mackenzie delta is an average distance of about 300 yards in summer and 100 yards in winter. Winter activity is supported by an extensive system of feeding stations or "push-ups" constructed on the lake ice. These structures are a necessary part of the dally life of the individual muskrat because the dispersed nature of the food plants demands a relatively great radius of activity. The number of muskrats using each push-up varies from three to thirteen with an average of six. The relatively short period of open water in this latitude so shortens the breeding season that primiparous females probably produce only one litter of young their first year of life. However, they can, by maturing sexually at an earlier date their second year, produce two litters. Inasmuch as the late winter population is comprised of four yearlings to each adult female, the delay in breeding induced by the late removal of ice on the lakes and channels is significant In reducing the rate of population Increase. The restrictive effects of climate on breeding activity are compensated for by the birth of larger litters (8.3 young) and by a very satisfactory survival of these young to yearling status. Intolerance between adults is noted during the early part of the breeding season but does not persist during the rearing of the young. As a consequence there are few losses from depredations of adults upon young animals as has been reported in other areas. Densities of animals per unit area are low when compared with races of muskrats from other regions. In addition the size of the individual animal is small, and the majority do not survive long past their second year of life. These observations support the view that the Mackenzie delta provides marginal habitat for muskrats. It is suggested that physical factors induced by the severity of the climate represent the major influence limiting population growth. The fur industry is another significant drain on animal numbers but other factors appear to be less important. All mortality factors taken together, however, have suppressed or eliminated any tendency for muskrat numbers to fluctuate in a cyclic manner as has been reported by several authors for other parts of North America. Reference is made throughout the text to races of muskrats inhabiting more southern latitudes.

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