Hispid Cotton Rat 
53 
becoming sexually mature at an earlier age, increasing litter size, or 
increasing the number of litters per year (extending the breeding season 
or decreasing the interval between litters). Decreasing the time between 
litters is achieved by a short gestation period and rapid postnatal 
development, so that the interval between conception and weaning is 
minimal. In the most rapidly breeding individuals, mating often occurs 
within 24 to 48 hours after parturition; consequently, a lactating female 
frequently is pregnant with the next litter. In general, small mammals in 
the tropics have small litters and long breeding seasons (Sadleir 1969). 
However, in temperate locations, small mammals tend to compensate 
for shorter breeding seasons and increased mortality by producing 
larger litters (e.g. in Peromyscus\ Smith and McGinnis 1968). Of the 
seven species of New World Sigmodon, only S. hispidus has a widespread 
and expanding distribution in temperate North America, making it a 
candidate species to examine for evidence of adjustments in its breeding 
biology in response to the harsher winter conditions endured by 
populations colonizing northern locations. 
Pregnancy Rate 
Maximum rates of pregnancy were achieved early and sustained 
throughout the breeding season in Virginia. The observed pregnancy 
rate often exceeded the theoretically observable maximum pregnancy 
rate, such as in April when 95% of 21 females were pregnant (Fig. 1). 
Because bulges in the uterus cannot be detected during the first 9 days 
of pregnancy, embryos can be counted only for 18 days of the 27-day 
gestation period. During the peak breeding season, mating usually 
occurs within 24 hours of parturition, resulting in a 28-day interval 
between litters. Because embryos can be seen only for 18 days of these 
28 days, the theoretical maximum pregnancy rate that can be observed 
is 18/28, or 64.3%, which is the detectable pregnancy rate if all females 
are pregnant all of the time. The higher rate in April likely is a result of 
synchronous breeding at the start of the reproductive season. Breeding 
synchrony in small mammals diminishes progressively from the start of 
the breeding season and disappears after the second litters are born, in 
part because of increasing variation in litter interval among overwintered 
females but mostly as a result of spring-born females entering the 
breeding population (at 45-60 days of age for cotton rats). 
Nothing is known of the actual litter intervals of cotton rats in 
natural populations, but longer post-partum mating intervals would 
lower the maximum observable pregnancy rate below the 64.3% value. 
The observed pregnancy rate during the breeding season (68.7%) slightly 
exceeded the theoretical value; that can be explained by sampling error 
or, more likely, by changes in the behavior of pregnant females. 
