488.

Texas Marsh Rice Rat

Oryzomys texensis

French: Oryzomys du Texas / German: Texas-Sumpfreisratte / Spanish: Rata arrocera de marisma de Texas

Other common names: Texan Rice Rat

Taxonomy. Oryzomys palustris texensis J. A. Allen, 1894, Rockport, Aransas County, Texas.

Oryzomys texensis was described as, and long regarded to be, a subspecies of the polytypic and paraphyletic O. palustris until sharp and deep molecular-based clade structure that separated western populations (texensis) from eastern ones ( palustris) was documented. Monotypic.

Distribution. SE Kansas, SW & SE Missouri, E Oklahoma, Arkansas, E Texas, Louisiana, and SW Mississippi, USA, and extreme NE Tamaulipas, Mexico.

Descriptive notes. Head-body 120-146 mm, tail 100-140 mm, ear 14-18 mm, hindfoot 24-33 mm; weight 40-74 g. The Texas Marsh Rice Rat is medium-sized, similar to but typically smaller than the parapatric Common Marsh Rice Rat ( O. palustris) and especially the sympatric Coues’s Marsh Rice Rat ( O. couesi). The Texas Marsh Rice Rat is notable forits sparsely haired and scaly tail about equal to head-body length; dark grayish upperparts are heavily lined with black, especially over head and at midback; underparts are grayish white; tail is bicolored, brown above and white below; and upper surfaces of forefeet and hindfeet are white. Sexes are monomorphic in size, and immature individuals are decidedly more brownish above and grayer, less whitish, below. Skull is generally similar to that of the Common Marsh Rice Rat, but detailed comparisons identifying diagnostic characteristic differences between the Texas Marsh Rice Rat, the Common Marsh Rice Rat, or the sympatric Coues’s Marsh Rice Rat are unavailable. Chromosomal complement is 2n = 56, FNa = 58. X-chromosome is acrocentric, and Y-chromosome is a small acrocentric, which contrasts sharply with large subtelocentric Y-chromosome of Coues’s Marsh Rice Rat in southern Texas.

Habitat. Aquatic habitats, including marshes and swamps, grassy ditches, edges of streams and lakes, and old fields where soil remains damp. Texas March Rice Rats are seldom found in dry fields or well-drained woodlands far from water. In coastal regions, they occur in tall grass that lines banks of bayous and tidal marshes where ground vegetation is not flooded by high tides. In eastern Texas where they have been most intensively studied, they occur in pine ( Pinus, Pinaceae)savanna wetlands, bottomland forests, baygall (boggy) habitats, and briar thickets along edges of agricultural fields.

Food and Feeding. Texas Marsh Rice Rats eat seeds and succulent parts of various marsh grasses and sedges and substantial amounts of animal matter (including insects, crustaceans, and even bird eggs) throughout most of the year. They have been recorded to eat fungi in winter.

Breeding. The Texas Marsh Rice Rat is highly fecund. Females can produce up to 7 litters/year but 4-5 are typical. Gestation is 23 days; number of young in a litter is densitydependent, with litters averaging 3-7 young (range 2-5) at high densities and 6 young (range 4-7) at low densities. Postpartum estrus is typical, and females usually mate within ten hours of parturition. Young are altricial at birth; well-developed vibrissae at birth are posited to help neonates find mothers’ nipples. Eyes open at c.6 days of age and weaning occurs at 11-13 days old. Sexual maturity is reached in ¢.50 days old. Reproduction typically extends form late winter to late autumn but can occur year-round.

Activity patterns. The Texas Marsh Rice Rat is primarily nocturnal; activity (at least as measured by trapability) is greater on cloudy and rainy nights.

Movements, Home range and Social organization. Male Texas Marsh Rice Rats have slightly larger home ranges (0-32 ha) than females (0-20 ha), with considerable overlap. Along coastal marshlands, home ranges tend to be oriented with their long axes roughly parallel to shorelines. When disturbed, individuals dart quickly for cover, including diving into water without hesitation to swim out of potential danger; they exhibit great ability for rapid and strong swimming underwater for considerable distances. Densities are 0-49-17 ind/ha and typically peak in summer and autumn and decline sharply in late winter. Reproduction and mortality appear to be closely related to local climatic factors and changing food supply. Unsheltered nature of habitats of Texas Marsh Rice Rats, particularly along coasts prone to impacts from seasonal hurricane activity, likely negatively influences reproduction and increases mortality. Nests are globular to elongate, constructed of loosely woven balls of shredded leaves and typically placed at ends of shallow burrows or in shallow depressions among tangled vegetation. Texas Marsh Rice Rats might be communal during non-breeding winter months, with several individuals sharing a single nest.

Status and Conservation. Not assessed on The IUCN Red List as a distinct species. The Texas Marsh Rice Rat was included in the most recent assessment of the Common Marsh Rice Rat, which was listed as Least Concern.

Bibliography. Allen (1894a), Benson & Gehlbach (1979), Goldman (1918a), Hall (1960, 1981), Hanson et al. (2010), Lowery (1974), Negus et al. (1961), Schmidly (1983), Svihla (1931).