Marsh Rice Rat - Description

Description

The marsh rice rat is a medium-sized rodent that looks much like the common black and brown rats, but has greater differences in color between the upper- and underparts. The fur is thick and short. The upperparts are generally gray to grayish brown, with the head a bit lighter, and are sharply delimited from the underparts, which are off-white, as are the feet. There are small cheek pouches. The ears are about the same color as the upperparts, but there is a patch of light hairs in front of them. The tail is dark brown above and may be paler below. The guard hairs are long and have unpigmented, silvery tips. When rice rats swim, air is trapped in the fur, which increases buoyancy and reduces heat loss. As in most other oryzomyines, females have eight mammae.

The forefeet have four and the hindfeet five digits. On the forefeet, the ungual tufts (tufts of hair on the digits) are absent. The hindfeet are broad and have a short fifth digit. Many of the pads are reduced, as are the ungual tufts, but there are small interdigital webs. The Florida Keys form, argentatus, has even more reduced ungual tufts. Many of these traits are common adaptations to life in the water in oryzomyines.

There is some geographic variation in fur color: western populations (texensis) are lighter than those from the east (nominate palustris), and Florida populations are generally more tawny or reddish than either, with those from southern Florida (coloratus) being brighter than those from the center of the state (natator). The Florida Keys form (argentatus) is silvery, and the two other Florida forms—planirostris and sanibeli—lack the reddish tones of mainland Florida populations and are instead grayish, resembling nominate palustris (planirostris), or brownish (sanibeli). In 1989, Humphrey and Setzer reviewed variation in color among Florida populations. They found argentatus to be substantially lighter and planirostris and sanibeli to be somewhat darker than mainland populations, and argentatus to have a less yellow fur, but found no significant differences in redness. There was also substantial variation within populations.

Total length is 226 to 305 mm (8.9 to 12.0 in), tail length 108 to 156 mm (4.3 to 6.1 in), hindfoot length 28 to 37 mm (1.1 to 1.5 in), and body mass 40 to 80 g (1.4 to 2.8 oz), with males slightly larger than females. The largest individuals occur in Florida and along the Gulf Coast east of the Mississippi River delta.

The stomach has the characteristic pattern of sigmodontines (unilocular-hemiglandular): it is not split in two chambers by an incisura angularis and the front part (antrum) is covered by a glandular epithelium. The gall bladder is absent, a synapomorphy (shared-derived character) of Oryzomyini. The karyotype includes 56 chromosomes and a fundamental number of 60 chromosomal arms (2n = 56, FN = 60). The form of the sex chromosomes has been used to distinguish the marsh rice rat from Oryzomys couesi, but may be too variable among Oryzomys to be useful in differentiating species. X chromosome inactivation occurs in the marsh rice rat, even though the animal lacks LINE-1 retrotransposons, which have been suggested as components of the inactivation process. Mutants with fused or additional molars and with light fur have been recorded in laboratory colonies; the abnormal molars are apparently the result of a single autosomal recessive mutation. At about 50%, hematocrit (the proportion of red blood cells in the blood) is high in the marsh rice rat compared to other rodents; this may be an adaptation that enables the rice rat to increase oxygen capacity while swimming underwater.