56 
Robert K. Rose and Michael H. Mitchell 
rates were less than 60%. Haines (1961) measured spermatogenesis and 
found that the production of sperm remained high from February 
through October (which was the breeding season for males in our 
study). Dunaway and Kaye (1964), who assessed male reproduction 
based on live-caught animals, observed that just under 100% of males 
had descended testes (i.e. were mature) during the June-September 
period. They noted a decline in the percentages of mature males from 
October to December, but by January, the proportion of males with 
descended testes again began to increase. 
Testicular mass is closely related to reproductive condition in males 
(Haines 1961). Our results agree with those of McClenaghan and Gaines 
(1978), who found testicular mass to be highest from June through 
September, also the peak breeding months for males as determined by 
convolutions of the cauda epididymides in the present study. In Kansas, 
smallest testicular masses were recorded for December, but we found 
the smallest testicular masses in the October to December period, with 
dramatic monthly increases from December through April. Testicular 
mass remained high from April through July and then declined sharply 
(Fig. 3), probably at first because of the recruitment of young males in 
the trappable population and later also because of testicular regression 
of adult males. Goertz (1965) reported large testicular masses from 
February through September and low values in the remaining months. 
In Texas, Haines (1961) reported spermatogenesis in males with the 
largest testicular masses; he provided perhaps the best available 
information on the relationship between these two variables. He found 
the largest average testicular mass per 10 g of body mass during the 
period from February through August, after which testicular mass 
declined until November. McClenaghan and Gaines (1978) and Haines 
(1961) found that testicular regression resulted in a reduction to about 
1 / 30th of the maximum testicular mass, compared with a value of about 
1 /26th in our study. 
Breeding Season 
During the breeding season, 68.7% of females were pregnant, and 
in most of the same months all males were judged to be fertile. The 
breeding season in males started one month earlier and ended one 
month later than the breeding season in females. That pattern is 
common in mammals (Sadleir 1969), and it is interpreted as adaptive in 
that the energy costs for breeding in females are greater than those in 
males. As a result of the earlier onset of fertility in males, mature males 
are ready to copulate and produce fertile matings when females undergo 
the first estrous cycle of the spring. 
The breeding season in Virginia closely paralleled that found by 
McClenaghan and Gaines (1978) in Kansas. Both locations are at or 
