Greenwald (1956: 220) found that in females of _Microtus
californicus_ some individuals are extremely precocious sexually,
and might, at an age of as little as two weeks, produce corpora
lutea and have sperm in the uterus. Greenwald mentioned one
perforate female which weighed only 10 grams, but most reached a
weight of at least 30 grams before their first pregnancies. The
sterile cycles passed through earlier seemed to represent a
"tuning-up" stage before establishment of the pituitary-gonad
relationship.
[Illustration: FIG. 6. Weight plotted against age in young
voles, from birth up to 25 days. The range is wide at the start
and increases as development proceeds.]
Although females of _M. ochrogaster_ are much less precocious in
their manifestations of puberty, they may become perforate well
before impregnation can occur, and seem to pass through sterile
cycles before becoming pregnant. The 18 smallest females recognized
as being pregnant were of the following over-all lengths, in mm.:
149, 149, 149, 148, 148, 148, 147, 146, 145, 145, 144, 144, 143,
143, 143, 142, 135, and 134. As pregnancy is ordinarily recognized
only in the last four days the females must have been impregnated
from 20 to 17 days earlier--when they were in most instances 7 to 11
weeks old and 135 to 145 mm. in length. The two smallest
individuals, recorded as pregnant at 135 and 134 mm., must, if they
were of typical size for their age, have become pregnant at an age
of approximately one month, when they were only 119 and 122 mm. in
length. The smallest lactating females (some of them pregnant also)
were recorded at lengths of 149, 148, 148, 147, 147, 146, 144, 144,
143, 143, and 142 mm. Occasionally females of less than 120 mm. were
found to be perforate, and seemingly had begun oestral cycles.
Records of a female of definitely known age, typical of many of the
same size in her development, are cited below:
March 19, 1956 Born in captivity.
April 7, 1956 (19 days old) Released on study area at site of mother's
capture; length 102 mm., weight 11.1 gms.
April 15, 1956 (27 days old) Recaptured; perforate with a copulatory
plug; length 113 mm., weight 13.4 gms.
April 27, 1956 (39 days old) Recaptured; imperforate; length 131 mm.,
weight 24.3 gms.
May 12, 1956 (54 days old) Recaptured; perforate and in late pregnancy;
length 146 mm.
May 25, 1956 (67 days old) Recaptured; imperforate, in an advanced
state of lactation; length 150 mm., weight 33 gms.
[Illustration: FIG. 7. Growth curve in the prairie vole;
dots are based on means of series of definitely known age (born
in captivity); circles are based on mean lengths of recaptured
marked young whose ages were not precisely known.]
[Illustration: FIG. 8. Over-all length in young prairie
voles of definitely known ages, up to 40 days. All were born
in captivity. Some were released with the female and developed
under natural conditions, but their growth rate did not differ
discernibly from that of those kept in the laboratory. Dots
indicate individual records; circles are means for ages at
which four or more records were obtained.]
When captured on May 12, at an age of 54 days, this female appeared
to be within two or three days of parturition, and hence must have
become pregnant at an age of approximately 35 or 36 days. Pregnancy
in the more precocious females probably occurs at a length of
approximately 130 mm. and an age of a little less than 40 days. Such
females are still growing so rapidly that by the time their litters
are born, they have grown to more than 140 mm.