In other species of _Microtus_, so far as known, a 21-day gestation
period seems to be the rule (Bailey, 1924:528; Hamilton, 1941:13;
Hatfield, 1935:264). _M. ochrogaster_ seems to conform to this
pattern, but the data obtained were meager, because breeding activity
was usually inhibited in voles kept in confinement.
A female live-trapped on July 23, 1951, appeared to be in breeding
condition. When trapped two days later, she had a copulatory plug, and
21 days after this she was found with a newborn litter in a trap. A
female thought to have given birth to a litter between successive
captures on July 20, and July 21, 1951 (on the basis of appearance of
genitalia, and reduction in weight from 53 to 46 grams), appeared to
have just completed parturition when she was examined on August 10. A
female that gave birth to a litter in confinement on May 18, 1954,
bred and was released the same day. She was recorded as pregnant in
the first week of June, but on June 7 was no longer pregnant. If this
pregnancy terminated normally, a gestation of 20 days or less is
indicated.
Greenwald (1956:221) suggested that in _M. californicus_, oestrus
might occur in the period of lactation, because he found recently
formed corpora lutea in lactating females. In the course of my field
work on _M. ochrogaster_, I obtained precise or approximate dates of
successive litters born at intervals of somewhat more than 21 days
apart. In different females, intervals of 23, 23, 24, 26, and
approximately 27 (between 26 and 28) days were recorded between
successive litters. In four other females intervals between litters
were known only approximately because one of two records was based on
a capture in late pregnancy judged to be within two or three days of
parturition. For these females, intervals of 23, 24, 24, and 26 days
were recorded. From the trend of these records, it seems that females
often became pregnant within a few days after birth of a litter.
Pregnancy from post-partum oestrus would seem to be less frequent than
pregnancies beginning a few days after birth of the previous litter,
and within the period of lactation.