487.

Common Marsh Rice Rat

Oryzomys palustris

French: Oryzomys des marais / German: Gemeine Sumpfreisratte / Spanish: Rata arrocera de marisma

Other common names: Marsh Oryzomys, Marsh Rice Rat, Rice Meadow Mouse, Swamp Rice Rat

Taxonomy. Mus palustris Harlan, 1837, “Fast Land,” near Salem, Salem County, New Jersey, USA .

Oryzomys palustris is the type species of the genus. Proposed subspecies argentatus, named by N. C. Spitzer and J. D. Lazell in 1978, is here included in coloratus. Five subspecies recognized.

Subspecies and Distribution.

O.p.palustrisHarlan,1837—widelydistributedinSIllinois,SKentucky,Tennessee,SNewJersey,SEPennsylvania,Delaware,EMaryland,EVirginia,ENorthCarolina,SouthCarolina,Georgia,Alabama,Mississippi,andNW&NFlorida,E&SEUSA.

O.p.coloratusBangs,1898—SpeninsularandtheLowerKeysofFlorida,USA.

O.p.natatorF.M.Chapman,1893—N&CpeninsularFlorida,USA.

O.p.planirostrisHamilton,1955—limitedtoasmallareaonWCcoastofpeninsularFlorida,USA.

O. p. sanibeli Hamilton, 1955 — Sanibel I on the SW coast of peninsular Florida, USA.

Descriptive notes. Head-body 131 mm, tail 121 mm, hindfoot 30 mm (Virginia, USA); head-body 134 mm, tail 143, hindfoot 34 mm; weight 45-80 g (Florida). Fur of the Common Marsh Rice Rat is soft and woolly in appearance. Upperparts are grayish, mixed with black and some brown hair, slightly darker on mid-back; face is paler; underparts are grayish white; forefeet and hindfeet are white above; and tail is dark brown or blackish above, slightly paler below, but not distinctly bicolored. Individuals from peninsular Florida are generally more tawny, grizzled grayish brown to tawny olive above, darkest on rump, and with white underparts, sometimes suffused with buff. Detailed comparisons of skull and teeth are required to document diagnostic differences between the Common Marsh Rice Rat and its sister-species, the Texas Marsh Rice Rat (O. texensis). Chromosomal complement is 2n = 56, FNa = 58. X-chromosome is large acrocentric, and Y-chromosome is small acrocentric.

Habitat. Wetlands of all types, including salt marshes of the Atlantic and Gulf coasts and various other local habitats, especially hydric hammocks, swamps, and freshwater marshes and meadows from sea level to elevations of nearly 1000 m. The Common Marsh Rice Rat occurs along stream edges and clearings in wooded areas on lower slopes of the Appalachian Mountains and can be found in wet agricultural lands, such as rice and sugar cane fields.

Food and Feeding. The Common Marsh Rice Rat eats seeds and green plant parts of various water-tolerant grasses and sedges and animal foods, including insects, fiddler crabs (Uca) and other crustaceans, snails, fishes, young turtles, bird eggs, and even carrion of other rodents (particularly muskrats and deermice, Peromyscus). One study reported that the Common Marsh Rice Rat was a major predator on eggs and young of marsh wrens (Cistothorus palustris).

Breeding. Reproduction occurs year-round, but cessation during winter months is typical in extreme northern populations. Bi-modal and unimodal seasonal peaks have been reported, with latter occurring in April-May. Gestation lasts 21-28 days; litters in different populations average 3-6-5 young. Females have postpartum estrus, with mean duration of 7-7 days (range 6-9). Young are born altricial, sparsely covered with fine hair; eyes open at 8-11 days of age when solid food is ingested and pelage becomes sleek and shiny. Weaning occurs at 11-20 days old. Sex ratio at birth is 1:1. Both sexes reach sexual maturity at 50-60 days old.

Activity patterns. Trapping data and presence of remains in hawk and owl pellets confirm that the Common Marsh Rice Rat is primarily nocturnal; moreover, captive individuals exhibited only nocturnal activity. Not surprisingly giving their obvious tie to water, marsh rice rats enter water and swim readily, covering under water distances greater than 10 m with apparent ease. Intensive self-grooming of fur is important to maintain water-repellent quality of pelage.

Movements, Home range and Social organization. In one study, male Common Marsh Rice Rats had smaller home ranges (average 0-25 ha) than females (0-33 ha)—an unusual pattern for a presumably promiscuous rodent. Individuals moved an average distances of 74-7 m in Maryland and 67-7-82-3 m in northern Florida. Local densities vary extensively seasonally and over consecutive years but reach can reach 50 ind/ha, especially in hammocks and prairie habitats. Individuals make grapefruit-sized nests of woven grasses and sedges, usually place on high ground under debris but also in short tunnels, either self-dug or those of turtles and other animals in dirt banks. Individual rice rats may also nest within the matrix of vegetation in houses constructed by muskrats—both Round-tailed Muskrats ( Neofiber alleni) and Common Muskrats ( Ondatra zibethicus). In salt marshes, these rice rats may make extensive and well-defined runways, and they may tunnel into banks for short distances.

Status and Conservation. Classified as Least Concern on The IUCN Red Lust.

Bibliography. Bangs (1898b), Benson & Gehlbach (1979), Cassola (2016aa), Chapman (1893), Goldman (1918a), Hall (1960, 1981), Hamilton (1946, 1955), Harlan (1837), Humphrey & Setzer (1989), Kale (1965), Spitzer & Lazell (1978), Wolfe (1982).