Marsh Rice Rat - Distribution and Habitat

Distribution and Habitat

The marsh rice rat currently occurs in much of the eastern and southern United States, northeast to southern New Jersey, and south to southeastern Texas and far northeastern Tamaulipas, Mexico. The northernmost records in the interior United States are in eastern Oklahoma, southeastern Kansas, southern Missouri and Illinois, and the southern half of Kentucky, but the species is absent in much of the Appalachians. Fossils of the marsh rice rat are known from Rancholabrean (late Pleistocene, less than 300,000 years ago) deposits in Florida and Georgia and remains referred to the extinct subspecies O. p. fossilis are from the Wisconsinan and Sangamonian of Texas and Illinoian and Sangamonian of Kansas. In the Florida Keys, rice rats occur on most of the Lower Keys, but are absent from the Upper Keys, which are of a different geological origin and were probably never connected to the mainland. The western and eastern Cytb clades within the marsh rice rat may represent expansions from different glacial refugia which the species was restricted to during a glacial period.

Cave and archeological remains indicate that the range of the marsh rice rat has extended substantially further north and west earlier in the Holocene, into central Texas, eastern Nebraska, southwestern Iowa, central Illinois, southern Indiana, southern Ohio, West Virginia, and southwestern Pennsylvania. Most northern archeological sites date from about 1000 CE and are associated with corn cultivation, but in some older cave sites the rice rat is found with the extinct giant armadillo Dasypus bellus, suggesting warm climatic conditions. Perhaps a warm period during the Quaternary enabled the rice rat to disperse northward and when the climate cooled, relict populations were able to survive in the north as commensals in corn-cultivating Native American communities. Some subfossil animals are slightly larger than living marsh rice rats, possibly because environmental constraints were relaxed in commensal populations.

In Tamaulipas and southern Texas, the ranges of the marsh rice rat and the related Oryzomys couesi meet; in parts of Kenedy, Willacy and Cameron counties, Texas, and in far northeastern Tamaulipas, the two are sympatric (occur in the same places). In experimental conditions, they fail to interbreed and genetic analysis yields no evidence of gene flow or hybridization in the wild. Compared to O. couesi, the marsh rice rat shows less genetic variability within but more between populations in the contact zone, probably because the species is restricted to isolated populations near the coast.

The marsh rice rat occurs in several habitats, ranging from coastal salt marshes to mountain streams and clearings. It is semiaquatic, spending much time in the water, and usually occurs in wetland habitats. It prefers areas where the ground is covered with grasses and sedges, which protect it from predators. In southern Illinois, marsh rice rats are more likely to occur in wetlands with more herbaceous cover, visual obstruction, and nearby grasslands. The species also occurs in drier uplands, which serve as sinks for young, dispersing animals and as refuges during high tide. Rice rats are adept overwater dispersers; studies on islands off Virginia's Delmarva Peninsula show that they readily cross 300-m (1000 ft) channels between islands.