Marsh Rice Rat
Order Rodentia : Family Muridae : Oryzomys palustris (Harlan)

Description. Ratlike, with long, nearly naked, scaly tail; ears short and hairy; upperparts grizzled grayish brown, heavily lined with black, especially in winter pelage; underparts whitish. External measurements average: total length, 245 mm; tail, 116 mm; hind foot, 29 mm. Weight, 40-68 g, averaging 51 g.

Distribution in Texas. Found in eastern Texas west to Brazos County and south to Cameron County.

Habits. These rats typically inhabit marshy areas but they may be found in almost any situation where grasses and sedges offer an adequate food supply and protective cover. They are semiaquatic and do not hesitate to swim or dive to escape capture. Near Copano Bay their runways are so situated in the salt grasses and sedges that the rats have to travel in shallow water most of the time. In southeastern Texas, the rats are common on the dikes and levees thrown up in the coastal marshes. In inland areas they prefer marshes and moist meadows; occasionally they live in forested areas.

Their surface runways resemble those made by cotton rats. They are 5-8 cm in width and lead from the shallow burrows or surface nests to the feeding areas. The globular nest is composed of grasses, sedges, or weeds and frequently is placed under debris above high water in the emergent vegetation. They occasionally take over and remodel for their own use the nests of blackbirds.

The marsh rice rat is omnivorous, with about equal amounts of plant and animal matter making up the diet, although the types of food eaten varies with season and availability. Plant foods include green vegetation, fungus, and the seeds of sedges, marsh grasses, and rice. Animal foods include insects, fiddler crabs, snails, fishes, and the carcasses of small rodents and birds. It is because of their fondness for cultivated rice that they were named rice rats. In most places these rats do no damage, but in rice fields they may become economically important by consuming large quantities of rice.

They are prolific. The breeding season is nearly yearlong, during which time breeding females may bear several litters. A single female may bear five to six litters per year, although suboptimal conditions may restrict reproductive output. Litter size ranges from two to seven (average, four) but may be affected by population density as crowded conditions appear to restrict the number of young produced.

The gestation period is about 25 days. A captive female produced six litters, totaling 20 young, in 1 year — an average of 3.3 young per litter. At birth the young are blind, helpless, nearly naked, and weigh about 3 g each. They grow rather rapidly. Their eyes open on the fifth or sixth day; they are weaned on the 11th day; and sexual maturity is reached between 40 and 45 days of age. These rats appear to grow continually throughout their lifetime.

Photo credit: John L. Tveten, courtesy of Texas A&M University Press.