Breeding Biology of Oryzomys Palustris, the Marsh Rice Rat, in Eastern Virginia

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. INTRODUCTION Oryzomys palustris (Harlan), the marsh rice rat, has the northernmost range in the genus Oryzomys, extending from southern Texas and Florida as far north as southern New Jersey. Apart from O. couesi Alston, found in two counties in southern Texas, other Oryzomys species are distributed from Mexico to South America (Hall 1981). (NOTE: A recent paper [Hanson et al. 2010], which explores molecular divergence in Oryzomys, suggests the possibility that Oryzomys in parts of Mississippi and Louisiana should be assigned to O. texensis. But even after a possible revision of the taxonomy, O. palustris lives in the most seasonal environments for this tropical genus.) Studies of the reproductive biology of the marsh rice rat have not provided a consensus on the timing and duration of the breeding season. In Mississippi, using assessments from live-trapped animals only, Wolfe (1985) observed breeding year round with the major breeding period from late spring to late autumn. By contrast, in Louisiana, breeding was not observed during two of three winters in which rice rats were trapped (Negus et al. 1961). Edmonds and Stetson (1993) report the breeding season in Delaware usually extends from March to September, but can last into late autumn when environmental conditions are favorable. Svihla (1931) and Worth (1950) report February-October breeding seasons in eastern Texas and Florida, respectively.