In the region of my study the prairie vole breeds the year round, but
the rate of breeding changes continually. There is no regularity in
the trend of the breeding season from year to year. It is obvious that
the species is responsive to environmental changes and is so well
attuned that its breeding is speedily initiated or inhibited by
changes to favorable or unfavorable weather. The incidence of breeding
is highest when temperature is moderate and both water and foods of
preferred sorts are plentiful.
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
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.
In Fig. 1 the catch per day of voles, varying from month to month,
reflects chiefly the changing population density. However, other
factors also have important effects on the catch. For example, bait
acceptance is better in the winter when natural foods, especially
greens, are scarce, with the result that a higher catch can be made
with the same population density. Interference with the trap line by
other animals also affected the catch of voles. In warm weather the
traps were checked in both morning and evening, and the catch was
correspondingly greater than it was in cool weather when the traps
were checked only once daily. The ratios obtained of young to adult
voles cannot be accepted at face value as the true ratios in the
population, either. For the first several days of each trapping
period, the voles caught were mostly adults previously marked and,
presumably, conditioned to the grain bait. Later, young voles not
previously recorded, came to the traps in increasing numbers. The
young, being at first not conditioned to the bait, and also having
relatively small home ranges, would generally be less well represented
in the catch than would the adults.