Table 3. Number of Young per Litter Correlated
with Age or Size of Female.
| Age or Size Group of Female | Number of females in sample | Average number of young per litter |
| More than one year old | 4 | 4.25 |
| 6 to 12 months old | 16 | 3.50 |
| Large (age indeterminate) | 9 | 3.44 |
| 2 to 5 months old | 9 | 2.90 |
| Small and medium (age indeterminate) | 10 | 2.80 |
It seems that the exceptionally high average numbers of young per litter in March and April result from the breeding females in those months being nearly all fully mature survivors of the previous year. In summer, when many females that are only a few weeks old become pregnant, the average litter declines to less than three young. The small average litter of 3.0 young for 1950 probably resulted from the fact that the population on the Reservation was then expanding rapidly in the newly favorable habitat created by one year's crop of vegetation after discontinuance of grazing, and had an unusually high percentage of breeding females that were not fully adult.
In four newborn young, total lengths, in mm., were 47, 45, 45, and 42. From the length-weight relationships shown in [Fig. 2], it seems that a length of approximately 47 mm. is typical of newborn young of average weight. Martin (1956:388) found a mean weight of 2.8 ± 0.36 grams in sixteen newborn prairie voles from the Reservation. For a series of 67 other newborn voles representing 27 different litters in seven different years, I found an average of 2.9 ± .05 grams. Young ranged in weight from 3.8 to 2.0 grams. Weights of the newborn voles could not be correlated with season, size, age of females, or food conditions. However, a distinct trend toward larger size in those litters that contained fewer young was evident, as shown in [Table 4].
Table 4. Weight of Newborn Young, Correlated with
Number of Young per Litter.
| Known Young Per Litter | Mean weight in grams | Number of litters in sample | Number of young in sample |
| 2 | 3.1 ± .09 | 7 | 13 |
| 3 | 3.0 ± .17 | 11 | 28 |
| 4 | 2.7 ± .22 | 6 | 17 |
| 5 | 2.6 ± .42 | 3 | 9 |
Voles less than 100 mm. in total length were seldom captured, because those less than this size are dependent on the female, and rarely venture far enough from the nest to be caught in a trap. A further difficulty in obtaining growth records on the smallest young is that of making accurate measurements. During their first few days they partially retain the fetal posture, usually lying on one side, with the head, body and tail flexed in an arc almost completed by the tail approximating the muzzle. Straightening the animal by stretching it and holding it with sufficient firmness to obtain a measurement might have involved injury to it. Therefore, in most instances the newborn voles examined were merely weighed or an approximate measurement was estimated without stretching the young to its full length.