Oryzomys couesi, also known as Coues's rice rat, is a semiaquatic rodent in the family Cricetidae occurring from southernmost Texas through Mexico and Central America into northwestern Colombia.

After studies of the contact zone in Texas, where O. couesi and the marsh rice rat meet, were published in 1979 and underscored the distinctness of the two, they were again regarded as separate.

Generally, Oryzomys couesi is common and of no conservation concern, and it is even considered a plague species in places, but some populations are threatened.

[3] As then recognized, the ranges of the marsh rice rat, a United States species, and Oryzomys couesi meet in southern Texas.

In 1960, Raymond Hall reviewed specimens from this contact zone and found no grounds on which to separate the two species; thus, he reduced O. couesi to a subspecies of the marsh rice rat.

[3] Afterward, some of the other forms synonymized under O. couesi or O. palustris were resurrected as separate species—Oryzomys nelsoni from the Marías Islands, western Mexico, and Oryzomys antillarum from Jamaica.

[34] These animals differ by 4.4% from Oryzomys couesi in the strict sense,[35] which occurs to the north and east, are separated by mountain ranges from the latter, harbor different species of hantavirus, and according to Merriam (1901) have more robust skulls, with larger molars, stronger zygomatic arches (cheekbones), and better developed ridges along the margins of the interorbital region of the skull (between the eyes).

[38] This distributional pattern is similar to that of other western Mexican rodents such as Sigmodon mascotensis, Hodomys alleni, Peromyscus perfulvus, and Osgoodomys banderanus and has been recognized as a distinct biogeographic zone in some reviews.

[41] Another subspecies, Oryzomys couesi lambi, was described by Burt in 1934 from central coastal Sonora,[30] which extended the range of the species by 400 mi (640 km) at the time.

[43] Oryzomys zygomaticus was first described by Merriam in 1901 as a separate species[33] similar to mexicanus, but with the zygomatic arches broadly spreading and curved downward.

[62] Goldman wrote that it was similar to crinitus, but more intensely colored, and differed in the form of the interorbital region; he retained it as a separate species pending further investigations.

[63] Carleton and Arroyo-Cabrales noted that archival research may yet uncover the precise origin of O. fulgens, which could establish it as an older name for one of the other central Mexican Oryzomys.

[78] Here the range of O. couesi meets that of the marsh rice rat;[79] in parts of Kenedy, Willacy and Cameron counties, Texas, and in far northeastern Tamaulipas, the two are sympatric (occur in the same places).

[80] In experimental conditions, the two fail to interbreed[81] and genetic analysis yields no evidence of gene flow or hybridization in the wild.

[85] The form peragrus is known from further south in Mexico, in the Río Verde basin of San Luis Potosi, the state of Hidalgo, and far northern Veracruz.

[98] In reviewing Nicaraguan Oryzomys in 1986, Jones and Engstrom did not keep richmondi as separate, because they thought the difference in color too small for the recognition of subspecies.

[100] According to Jones and Engstrom, rice rats from the island of Ometepe in Lake Nicaragua are distinctive in their large skull and small external measurements, with an especially short tail, soft fur that is orange-brown above and buffish below, and lack of sphenopalatine vacuities (openings in the roof of the mesopterygoid fossa, the gap behind the end of the bony palate).

[103] It is a brown form, lacking the reddish tones of nearby populations, and has a broad skull with a short rostrum (front part) and ridges on the interorbital region like those of gatunensis.

[3] Oryzomys couesi is a medium-sized to large rat[125] with coarse fur that is buff to reddish above, becoming paler towards the sides and cheeks and darker on the rump and face.

[132] 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.

[138] The papilla (nipple-like projection) on the dorsal (upper) side of the penis is covered with small spines, a character Oryzomys couesi shares only with Oligoryzomys and the marsh rice rat among oryzomyines examined.

[155] The squamosal lacks a suspensory process that contacts the tegmen tympani, the roof of the tympanic cavity, a defining character of oryzomyines.

[164] The flexi and flexids (valleys between the cusps and crests) at the labial (outer) side of the molars are closed by cingula (ridges).

[107] In Texas, it occurs in marsh vegetation along resacas (oxbow lakes)[176] and in Veracruz, it has even been found on the dry coastal plain among shrubs.

[179] Oryzomys couesi lives on the ground and is semiaquatic, spending much time in the water,[107] as Alston in his original description already recognized,[180] but is also a good climber.

It is probably able to forage underwater, which may help differentiate its niche from that of the ecologically similar cotton rat Sigmodon hirsutus, which also swims well, but does not dive.

[189] It probably breeds around the year and after a pregnancy of 21 to 28 days,[125] the female produces litters of two to seven young, with an average of 3.8, according to Reid's Mammals of Central America & Southeast Mexico.

[191] The mites Eubrachylaelaps circularis and Gigantolaelaps boneti have been found on Oryzomys couesi in Oaxaca,[192] the sucking louse Hoplopleura oryzomydis in Nicaragua,[193] the mites Laelaps oryzomydis, Echinonyssus microchelae, Ornithonyssus bacoti, Prolistrophorus frontalis, and Prolistrophorus bakeri in Colima,[194] and the apicomplexan Eimeria couesii in Mexico.

[197] The IUCN lists Oryzomys couesi as "Least Concern", because it is a widely distributed, common species with broad habitat tolerance that occurs in many protected areas.

[200] A 2001 study predicted that climate change would drive the Texas population to extinction, because no suitable habitats would continue to exist.