[F] These data are from material collected at Lawrence, Douglas County, Kansas.
REPRODUCTION
Age Classes
Each prairie vole was assigned to one of three age classes (juvenile, subadult, or adult) principally on the basis of weight, but partly on the quality and color of the pelage. The three age classes are characterized in table 8.
Table 8. Characters of Juvenile, Subadult, and Adult Prairie Voles
| Juvenile | Subadult | Adult |
|---|---|---|
| Less than 21 grams | 21-38 grams | 38 grams or more |
| Weight usually less than 20 grams | Average weight 30-32 grams | Average weight 40-45 grams |
| Entire pelage dull | Pelage of rump dull; rest of pelage glossy | Pelage usually entirely glossy (rump sometimes dull) |
| Dorsal color black | Dorsal color grizzled except on rump | Entire dorsal color grizzled except sometimes on rump |
Fecundity
Hamilton (1941:4) found for Microtus pennsylvanicus that macroscopic tubules of the cauda epididymis were an indication of fecundity. By noting the size of the tubules (whether macroscopic or not) and by making smears from them in approximately every 25th male caught, I found that the presence of sperm was positively correlated with large-sized tubules of the cauda epididymis in Microtus ochrogaster. Inferentially, males with sperm were fecund.
There is a relationship almost positive between the size of the tubules of the cauda epididymis and the length of the testes. Testes longer than 7 mm. have macroscopic tubules in the cauda, and in testes shorter than 7 mm. these tubules cannot be seen with the naked eye, Hamilton (1937b) found that in M. pennsylvanicus testes smaller than 8 × 4 mm. did not contain sperm. The testes of the prairie vole descend into the scrotum in the breeding season. In the two winter months, when the voles did not bring forth young, the testes decreased in size (see [figure 3]) and were withdrawn into the body cavity. The presence of the testes in the body cavity does not mean that a vole is not in breeding condition, for many specimens with abdominal testes were fecund.