The objectives of our study were to determine the age of maturity, litter size, and the timing of the breeding season of marsh rice rats (Oryzomys palustris) of coastal Virginia. From May 1995 to May 1996, monthly samples of rice rats were live-trapped in two coastal tidal marshes of eastern Virginia, and then necropsied. Sexual maturity was attained at 30-40 g for both sexes. Mean litter size of 4.63 (N= 16) did not differ among months or in mass or parity classes. Data from two other studies conducted in the same county, one of them contemporaneous, also were examined. Based on necropsy, rice rats bred from March to October; breeding did not occur in December-February. By contrast, rice rats observed during monthly trapping on nearby live-trap grids were judged, using external indicators, to be breeding year-round except January. Compared to internal examinations, external indicators of reproductive condition were not reliable for either sex in predicting breeding status in the marsh rice rat.
Rose, R. K. and E. A. Dreelin. 2011. Breeding Biology of Oryzomys Palustris, the Marsh Rice Rat, in Eastern Virginia. Virginia Journal of Science 62 (3): 113-122.