Tables 1 and 2 and Fig. 1, based on 11,109 records representing each month over a four-year period, show the changing trends from month to month. The perforate condition recorded in Table 1 may represent any of several stages in oestrus or pregnancy, but is regarded as a crude index of rate of breeding, since voles in the anoestrus stage lack the vaginal orifice. Highest percentages of perforate females occurred in the months of February, March, April, May, and June, while by far the lowest percentages were recorded in the drought summers of 1952 and 1953. Even in mid-winter a substantial proportion of the females trapped were perforate. [Illustration: FIG. 1. Average catch per day in a three-acre field, in a grid of 100 live-traps, over a four-year period. For each year, solid line represents total and dashed line represents number of young up to 30 grams in weight. Numbers caught are roughly indicative of population density, but many variables distort this relationship. Young are never represented in the catch in their true ratio to adults, since on the average they are less vagile and less attracted to traps.] Table 1. Percentages of Adult Females Recorded as Perforate in the Monthly Samples From 1950 Through 1953. =======+======+======+======+====== | 1950 | 1951 | 1952 | 1953 -------+------+------+------+------ Jan. | .... | 27.3 | 41.7 | 33.3 Feb. | .... | 47.7 | 53.1 | 72.9 Mar. | 40.6 | 38.5 | 77 | 50 Apr. | 76 | 41.9 | 51.9 | 73 May | 84 | 40 | 52 | 58.2 June | 67.7 | 41.5 | 19.3 | 16.6 July | 57.3 | 45.5 | 12.7 | 15.4 Aug. | 43.1 | 52.2 | 5.4 | 31.3 Sept. | 47 | 56.5 | 51.6 | 56.2 Oct. | 44.8 | 48.9 | 43.4 | 60 Nov. | 24.4 | 45 | 24.1 | 61.5 Dec. | 31.1 | 45 | 37.5 | 41.6 Table 2. Percentages of Adult Females Recorded to Be in Late Pregnancy in the Monthly Samples From 1950 Through 1953. =======+======+======+======+====== | 1950 | 1951 | 1952 | 1953 -------+------+------+------+------ Jan. | .... | 2.3 | 0 | 0 Feb. | .... | 0 | 10.4 | 9.1 Mar. | 5.8 | 0 | 22.6 | 13.3 Apr. | 8 | 19.4 | 22.6 | 27.5 May | 21 | 37.1 | 29.5 | 39.4 June | 13.3 | 14.9 | 16.5 | 5.5 July | 57.3 | 6.7 | 7.9 | 3.8 Aug. | 43.8 | 15.2 | 10.8 | 12.5 Sept. | 40.4 | 15 | 20.3 | 6.2 Oct. | 45.2 | 21.9 | 18.9 | 10 Nov. | 7 | 8.9 | 3.3 | 23 Dec. | 0 | 0 | 0 | 8.3