Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
0 2.8 3.9 3.2 3.4 3.1 2.8 3.0 .. 3.2 2.6 0
.. (4) (10) (6) (8) (9) (5) (2) .. (5) (5) ..
[Footnote G: These months are from October, 1945, until August, 1946.
The numbers in parentheses indicate the number of gravid females
collected each month.]
Table 9 shows that the prairie vole produced the largest litters in
March. A comparison of table 9 with figure 2 shows that the largest
litters were produced at the height of the breeding season. Baker and
Ransom (1933), studying _Microtus agrestis_, also found that larger
litters were characteristic of the height of the breeding season; and
that at the beginning and at the end of the breeding season the litters
averaged smaller.
The size of litters varied also with the age of the female. To place a
gravid female in its proper age class, the weight of the embryos was
subtracted from the total weight, and the remaining weight was used as
the body weight. The average size of the litters of 14 subadults was
2.9, and in 35 adults it was 3.4. Hatfield (_op. cit._) found that the
younger females of _M. californicus_ gave birth to smaller litters than
did the adults.
Not included in either of the above analyses are nine gravid females
collected in November in a pasture watered by an artesian spring in
Atchison County, Kansas. In this pasture there was a high concentration
of prairie voles, and the percentage of fecundity was much higher than
in Douglas County at the same time. In November only 29 per cent of the
female prairie voles in Douglas County were fecund, as against 59 per
cent in Atchison County. The average number of embryos of these nine
voles was 4.1. Data from Atchison County are not included in table 9.