52 
Robert K. Rose and Michael H. Mitchell 
Because not all males are of the same size and because testicular 
mass is approximately proportional to body mass (Keller and Krebs 
1970), we computed the testicular mass per 10 g of body mass (Fig. 3). 
This assessment of male reproduction closely parallels the breeding 
season of males based on cauda epididymides (Fig. 2). The testes grew 
rapidly in late winter (February) so that overwintered males were fully 
mature by March. The somewhat lower fertility rates of males in late 
summer (September) probably were a result of an increasing proportion 
of young males included in the samples. However, later decreases in 
fertility (November and December in Fig. 2 and 3) were primarily a 
result of testicular regression in adult males. 
Dynamics of Body Size 
In some parts of the United States, populations of cotton rats have 
substantial winter mortality (e.g. Dunaway and Kaye 1964, Sauer 1985). 
Slade et al. (1984) showed that cotton rats surviving the winter in 
eastern Kansas tend to weigh nearly the same regardless of age; large 
animals lose mass and young animals entering the winter grow slowly, 
so that by spring most animals are approximately the same mass. Severe 
mortality and weight loss in the winter make an evaluation of body 
weight dynamics particularly important in S. hispidus at the northern 
limit of its distribution, such as in southeastern Virginia. 
Of course, chance plays a role in determining the average mass of a 
sample of field-caught cotton rats, particularly during periods when 
young animals are entering the trappable population. However, in this 
study, those effects are minimized because juveniles and small subadults 
(<50 g) were not collected for necropsy. Overall, males (x = 101.48 =t 
2.027 g) were significantly heavier than females (jt = 94.26 ± 1.872 g). 
Body mass differences were smallest (Fig. 4) at the end of the breeding 
season (October and November). Males were much larger (20-30 g) than 
females throughout the winter in this study. 
Body length (Fig. 5) showed similar trends, with males averaging 
141.11 ± 1.557 mm and females 137.63 ± 1.441mm. Males had roughly 
linear growth in body length throughout the late autumn and winter, 
and the decline in mean length was probably a result of the recruitment 
of spring-born animals into the trappable populations. Body lengths of 
males and females were most similar in October and November, a 
pattern also seen with body mass (Fig. 4). 
DISCUSSION 
Mammals seem to adjust the breeding rate to the mortality rate at a 
given location (Sadleir 1969). Mammals, particularly small mammals, 
can increase reproduction by one or more of the following means: 
