44 
Robert K. Rose and Michael H. Mitchell 
some features of the expected pattern have been reported in Kansas 
(McClenaghan and Gaines 1978), Oklahoma (Goertz 1965), and Tennessee 
(Dunaway and Kay 1964), although the details vary somewhat from 
location to location. Kilgore (1970) examined the possibility that northern 
populations might have larger litters than central populations as a way 
of compensating for increased winter mortality; he found significantly 
larger litter sizes in Kansas than in Texas. 
The primary objective of our study was to examine details of 
reproduction and patterns of body growth in a population of cotton rats 
at the northern limit of the species distribution on the East Coast. Using 
monthly samples of live-caught cotton rats that were necropsied and 
examined for evidence of reproduction, we learned that cotton rats in 
Virginia suspended breeding from early November through late March, 
had litters no larger than those in central populations (Texas), and had 
a larger weight gain in males than in females during the winter. 
MATERIALS AND METHODS 
From October 1983 to November 1984, cotton rats were obtained 
using Fitch and Sherman live traps baited with chicken scratch feed (a 
mixture of wheat, millet, and cracked corn). Although not always 
attained, the goal was a sample of 30 animals per month. No animals 
were taken in January or August. The 250-ha study area, an old field in 
Portsmouth, Va., was dominated by grasses, Panicum spp. and 
Andropogon spp.; a spikerush, Juncus effusus; and, at the margins, 
young sweet gum trees, Liquidambar styraciflua. Other common species 
of plants found in the study area were trumpet creeper, Campsis 
radicans; cane, Arudinaria gigantea ; saltbush, Iva frutescens\ goldenrods, 
Solidago spp.; and giant ragweed, Ambrosia artemisiifolia. Less common 
were black oak, Quercus nigra\ grape, Vitis rotundifolia; loblolly pine, 
Pinus taeda\ smooth sumac, Rhus copallina\ dogfennel, Eupatorium 
capillifolium; blackberries, Rubus spp.; briers, Smilax spp.; willows, 
Salix spp.; and cattails, Typha angustif olia . Traps were moved from 
place to place to prevent excessive depletion of the cotton rat at a local 
site. 
All animals > 50 g (lower limit of potential breeders) were killed 
with chloroform in the laboratory and frozen until necropsy, when the 
following information was recorded for each: (1) body mass (g), (2) 
overall body length (mm), and (3) length of tail (mm). 
Additional data were recorded for females: (1) number of placental 
scars, (2) number of embryos, (3) uterine mass (uterus + embryos), (4) 
number of corpora lutea, and (5) parity class. The parity classes were 
defined as nulliparous females without embryos or placental scars (also 
