_Natural history._--habitat and numbers.--Along the International Boundary east of the Turtle Mountains, Soper (1946:136) found this species present only in timbered areas and absent from many untimbered areas. Of the same species to the westward he comments "so far as I know at present, there is no evidence to show that any short-tailed weasels inhabit a broad strip of treeless territory immediately north of the International Boundary in Canada from southwestern Alberta to southeastern Saskatchewan." The same author (1942) reports that in the general area of Wood Buffalo Park, Northwest Territory, south of Great Slave Lake, the ermine is uncommon on pine-grown sand ridge and rolling upland and common in lower spruce-aspen parklands, stream-side coniferous belts, and grassy, semi-wooded swamplands.
Nine ermines per square mile is the number that Soper (1919:46-47) estimated at Edmonton on the basis of the numbers that he trapped there in the winters of 1912-13 and 1913-14 and on the basis of the tracks of remaining ermines. From corresponding data he estimated the population in the winter of 1913 on the Hay River, north of Jasper Park, to be nine per square mile. In each of these instances he estimated ten weasels per square mile but he inclined to the view that one-tenth of the animals involved in his counts were long-tailed weasels (_Mustela frenata_). Osgood (1909B:30) and his field companion in the period July 31 to September 3, 1903, took a series of 42 specimens within a radius of 500 yards of their camp at the head of Seward Creek, Alaska, all caught in four traps, in one month. Of the 42 specimens, 28 are males and 14 are females.
Fluctuations of a multiannual nature are marked in this species. Bailey (1929:156) observes that in Sherburne County, Minnesota, when meadow mice are abundant for two or three years these weasels become abundant but that when the mice are scarce the weasels also become scarce. Manning (1943:56), on Southampton Island, noted "that the maximum and minimum points of the weasel cycle are much more sharply marked than those of the fox cycle and the increase and decrease are more rapid."
How far an ermine will travel in a given length of time has seldom been recorded but Hamilton (1933:293), on March 20, 1932, "followed the track of a small weasel, presumably a male _cicognanii_, for four miles in the fresh snow," and Ingles (1942) observed a diminutive ermine of the subspecies _M. e. muricus_, at Woods Lake, California, 286 yards from its den.