48 Robert K. Rose and Michael H. Mitchell 
Prenatal Mortality 
Preimplantation mortality, which occurs before the embryo has 
implanted in the uterine wall, can be estimated by comparing the 
number of ovulation sites with the number of embryos. After ovulation, 
the remnant of each ovarian follicle is retained. It quickly enlarges into 
a corpus luteum, a structure 2. 0-2. 5 mm in diameter, which can easily be 
seen and counted. If all ova are fertilized and the resulting embryos are 
successfully implanted, the number of corpora lutea corresponds exactly 
to the number of embryos. However, if there are, for example, seven 
corpora lutea but only six embryos, then one ovum has been lost to 
preimplantation mortality. 
There are two potential obstacles to making accurate estimates of 
preimplantation mortality. The corpus luteum enlarges quickly as it 
produces progesterone to maintain the thick wall of the pregnant uterus. 
However, embryos do not appear as bulges in the uterus until day 10 in 
the 27-day gestation period (Meyer and Meyer 1944). Thus, for a few 
days the enlarged corpora lutea indicate pregnancy but no embryos are 
evident. A second problem is twinning, the production of two embryos 
from the same ovum. In this study, at least four females were judged to 
be pregnant (i.e. had enlarged corpora lutea) though no embryos were 
seen, and there was one case of probable twinning. When these females 
were eliminated from the analysis, preimplantation mortality averaged 
5.7% of 371 ova. 
Males 
Reproductive potential (fertility) in males is most reliably indicated 
by the presence of convolutions in the cauda epididymides, which 
Jameson (1950) found to be highly correlated with the presence of 
sperm in the tubules. Relative testicular mass (the ratio of weight of 
testes to weight of animal) is a fair predictor of maturity, because the 
testes grow rapidly in late winter prior to the onset of the breeding 
season. We used both of these indicators of male breeding capability. 
Using convoluted cauda epididymides as a criterion, we found that 
males were fertile longer than females (Fig. 2), from February (73% 
fertile) to November (33% fertile); from March through July, all males 
were fertile. According to this criterion, the breeding season of males 
begins about one month earlier and ends about one month later than 
that of females. 
As is typical of males of many temperate-zone mammals, testes 
undergo a dramatic regression in late autumn. In S. hispidus, the mass 
of the paired testes of a 120-g male might be 2,000 mg at the height of 
the breeding season, compared with only 80 mg after testicular regression. 
With regression, the cauda epididymides lose their convolutions and 
become looped. Such males are no longer fertile. 
