The marsh rice rat is thought to breed year-round (1) (2), although in the more northerly areas of its range, breeding may be seasonal (3). A prolific breeder (5), the female is able to produce up to eight litters per year (5). After a gestation period of 21 to 28 days (1) (2) (3), the female will usually give birth to between 4 and 6 young (1), although litters containing as many as 7 have been reported (1) (5). The nest of the marsh rice rat is a spherical, grapefruit-sized structure (2) (4) (5), which is made of loosely woven dried grass or sedge and is usually placed at the base of a shrub (2) (4). The young are weaned 2 weeks after birth (1), and become sexually mature after 50 to 60 days (2) (3) (5). The female is able to become pregnant again immediately after giving birth (4) (5).
The primarily nocturnal marsh rice rat (1) (2) (4) is active year-round and does not hibernate (4). Both the male and female establish a home range, although the male will usually occupy a larger area (2). A rather cleanly species, the marsh rice rat creates ‘latrine sites’ outside of its burrow where it goes to defecate, and spends a large amount of time grooming itself, which is thought to help keep its pelage water repellent (4). A semi-aquatic species (2), it is a skilled swimmer (5), and it is known to regularly swim underwater for distances of over ten metres (2), using its feet to propel it through the water (4).
A generalist and opportunistic feeder, the marsh rice rat eats a wide range of prey items (4), with seeds and succulent vegetation taken for the majority of the year, which are supplemented with insects, crabs, snails (2) (4), young turtles (3) (4), fish and carrion (2) (3), as well as the eggs and young of birds (4) (5). The diet of an individual is seasonally variable and will generally depend on what is available nearby (4).