. The Canadian field-naturalist. 1974 Stewart and Bider: Ditch-Dwelling Muskrats in Southern Quebec 433 o 200 < a: I- o < (/) hi Q. U) (D Z CO CO o a: o o CD < ID O 60- 20- 80- 40-. 241261281 3 I 6 i 12 I 16 1 18 I 28 I I I 6 I 8 I 12 i 15 I 19 121 126 I 31 I 2 I 6 25 27 29 5 7 13 17 19 29 5 7 11 14 16 20 24 30 1 5 MAY I JUNE I JULY | AUGUST |SEPT. Figure 3. Cumulative frequency diagram of muskrat activity in 1972. lack of data prevented the precise dating of the second increase in activity of period C in 1971 and 1972. But the interval between periods B and C in both years contains t
. The Canadian field-naturalist. 1974 Stewart and Bider: Ditch-Dwelling Muskrats in Southern Quebec 433 o 200 < a: I- o < (/) hi Q. U) (D Z CO CO o a: o o CD < ID O 60- 20- 80- 40-. 241261281 3 I 6 i 12 I 16 1 18 I 28 I I I 6 I 8 I 12 i 15 I 19 121 126 I 31 I 2 I 6 25 27 29 5 7 13 17 19 29 5 7 11 14 16 20 24 30 1 5 MAY I JUNE I JULY | AUGUST |SEPT. Figure 3. Cumulative frequency diagram of muskrat activity in 1972. lack of data prevented the precise dating of the second increase in activity of period C in 1971 and 1972. But the interval between periods B and C in both years contains the period where the increase in muskrat activity from second litters would be expected if it followed the first by approximately a month. To estimate the dates of the first peak of litter production in 1973 an adaptation of Dolbeer's (1973) method was used. We adjusted the fetal length growth curve of laboratory rats {Rattus norvegicus) by the ratio of 30 millimeters for rat birth-lengths (Altman and Dittmer 1962) to 100 millimeters for muskrat birth-lengths (Errington 1939). The mean fetus length of each muskrat litter was applied to the simulated growth curve to obtain its estimated embryonic age. The number of days required to complete the gestation period of each litter was added to the capture date of the female, yielding an estimated birth date. The predicted birth dates were estimated to the nearest 5-day period of the month to allow for the difference between the 22-day gestation period for rats (Altman and Dittmer 1962) and an as- sumed 25-day gestation period for muskrats (Beer 1950; McLeod and Bondar 1952). Sixteen of 32 female muskrats trapped in the spring of 1973 were visibly pregnant and one female trapped on 30 April had a greatly dis- tended uterus, indicating it had just given Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may
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