SIZE AND AGE AT SEXUAL MATURITY
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.
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.
[Pg 158] 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.