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 x 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and were withdrawn into the body cavity.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.
The females were considered to be fecund if they were gravid, or if
there were placental scars in the horns of the uteri.
[Illustration: FIGURE 2. Fecundity of Prairie Voles by Months. Adults
and Subadults are Considered Together.]
[Illustration: FIGURE 3. Seasonal Changes in the Length of Testes.]