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.
| Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. | |
| 1950 | .... | .... | 40.6 | 76.0 | 84.0 | 67.7 | 57.3 | 43.1 | 47.0 | 44.8 | 24.4 | 31.1 |
| 1951 | 27.3 | 47.7 | 38.5 | 41.9 | 40.0 | 41.5 | 45.5 | 52.2 | 56.5 | 48.9 | 45.0 | 45.0 |
| 1952 | 41.7 | 53.1 | 77.0 | 51.9 | 52.0 | 19.3 | 12.7 | 5.4 | 51.6 | 43.4 | 24.1 | 37.5 |
| 1953 | 33.3 | 72.9 | 50.0 | 73.0 | 58.2 | 16.6 | 15.4 | 31.3 | 56.2 | 60.0 | 61.5 | 41.6 |
Table 2. Percentages of Adult Females Recorded to Be in Late Pregnancy
in the Monthly Samples From 1950 Through 1953.
| Jan. | Feb. | Mar. | Apr. | May | June | July | Aug. | Sept. | Oct. | Nov. | Dec. | |
| 1950 | .... | .... | 5.8 | 8.0 | 21.0 | 13.3 | 57.3 | 43.8 | 40.4 | 45.2 | 7.0 | 0 |
| 1951 | 2.3 | 0 | 0 | 19.4 | 37.1 | 14.9 | 6.7 | 15.2 | 15.0 | 21.9 | 8.9 | 0 |
| 1952 | 0 | 10.4 | 22.6 | 22.6 | 29.5 | 16.5 | 7.9 | 10.8 | 20.3 | 18.9 | 3.3 | 0 |
| 1953 | 0 | 9.1 | 13.3 | 27.5 | 39.4 | 5.5 | 3.8 | 12.5 | 6.2 | 10.0 | 23.0 | 8.3 |
Usually pregnancy can be recognized only in the last week before birth of the litter, when the female's abdomen is noticeably distended by the enlarged fetuses. Palpating to detect embryos was not attempted because of the danger of injuring them or the female. Because gestation is of approximately three weeks duration, the figures in [Table 2] represent roughly perhaps one-third, or a little less, of the adult females actually pregnant. At most times of year a substantial proportion of adult females (sometimes nearly all) are pregnant. Only in the winter (including March in 1951) were samples taken in which no recognizably pregnant females were found. Incidence of pregnancy was notably high in July, August, September, and October of 1950, May, 1951, May, 1952, and April and May, 1953. A high rate of breeding was not necessarily followed by an increase in the population. A relatively low rate of breeding was adequate to maintain the population level, provided that environmental factors remained favorable. [Fig. 1] shows the average catch per day (with approximately 100 live-traps) over the four-year period, 1950 through 1953. The young (including all those weighing 30 grams or less, and corresponding roughly with the part of the population less than two months old) are shown separately. It is noteworthy that throughout the entire period the ratio of young to adults tended to be fairly stable—usually fluctuating between ten and thirty per cent of the total catch. Ratios of young to adults were notably high in March and May, 1950; April, June and July, 1952; and April, May and June, 1953. Ratios of young were notably low in June and December, 1950; January, February, March, and June through October, 1951; January, February, and March, 1952; and November, 1953.