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Temporal range: Rancholabrean (300,000 years before present) – present
Oryzomys palustris in vegetation.jpg
Marsh rice rat (Oryzomys palustris)
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Rodentia
Family: Cricetidae
Subfamily: Sigmodontinae
Tribe: Oryzomyini
Baird, 1857
Type species
Oryzomys palustris
Harlan, 1837
Species [1]

and see text.

Oryzomys distribution.png
Distribution of Oryzomys: dark blue, marsh rice rat (O. palustris); light blue, former distribution of the marsh rice rat; red, O. couesi ; pink, O. albiventer ; dark green, O. peninsulae ; orange, O. nelsoni ; light green, O. antillarum ; yellow, O. dimidiatus and O. couesi; brown, O. gorgasi .
Synonyms [1]

Oryzomys is a genus of semiaquatic rodents in the tribe Oryzomyini living in southern North America and far northern South America. It includes eight species, two of which—the marsh rice rat (O. palustris) of the United States and O. couesi of Mexico and Central America—are widespread; the six others have more restricted distributions. The species have had eventful taxonomic histories, and most species were at one time included in the marsh rice rat; additional species may be recognized in the future. The name Oryzomys was established in 1857 by Spencer Fullerton Baird for the marsh rice rat and was soon applied to over a hundred species of American rodents. Subsequently, the genus gradually became more narrowly defined until its current contents were established in 2006, when ten new genera were established for species previously placed in Oryzomys.

A genus is a taxonomic rank used in the biological classification of living and fossil organisms, as well as viruses, in biology. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclature, the genus name forms the first part of the binomial species name for each species within the genus.

Oryzomyini tribe of mammals

Oryzomyini, or rice rat, is a tribe of rodents in the subfamily Sigmodontinae of family Cricetidae. It includes about 120 species in about thirty genera, distributed from the eastern United States to the southernmost parts of South America, including many offshore islands. It is part of the clade Oryzomyalia, which includes most of the South American Sigmodontinae.

Marsh rice rat A semiaquatic North American rodent in the family Cricetidae

The marsh rice rat is a semiaquatic North American rodent in the family Cricetidae. It usually occurs in wetland habitats, such as swamps and salt marshes. It is found mostly in the eastern and southern United States, from New Jersey and Kansas south to Florida and northeasternmost Tamaulipas, Mexico; its range previously extended further west and north, where it may have been a commensal in corn-cultivating communities. Weighing about 40 to 80 g, the marsh rice rat is a medium-sized rodent that resembles the common black and brown rat. The upperparts are generally gray-brown, but are reddish in many Florida populations. The feet show several specializations for life in the water. The skull is large and flattened, and is short at the front.


Species of Oryzomys are medium-sized rats with long, coarse fur. The upperparts are gray to reddish and the underparts white to buff. The animals have broad feet with reduced or absent ungual tufts of hair around the claws and, in at least some species, with webbing between the toes. The rostrum (front part of the skull) is broad and the braincase is high. Both the marsh rice rat and O. couesi have 56  chromosomes, lack a gall bladder, and have a complex penis (as is characteristic of the Sigmodontinae) with some traits that are rare among oryzomyines; these characteristics are unknown in the other species of this genus.

Buff (colour)

Buff is the light brownish yellow colour of buff leather. Buff colour is synonymous with ochreous colour.

Interdigital webbing is the presence of membranes of skin between the digits. Normally in mammals, webbing is present in the embryo but resorbed later in development, but in various mammal species it occasionally persists in adulthood. In humans, it can be found in those suffering from LEOPARD syndrome and from Aarskog-Scott syndrome.

Chromosome DNA molecule containing genetic material of a cell

A chromosome is a deoxyribonucleic acid (DNA) molecule with part or all of the genetic material (genome) of an organism. Most eukaryotic chromosomes include packaging proteins which, aided by chaperone proteins, bind to and condense the DNA molecule to prevent it from becoming an unmanageable tangle.

The habitat includes various kinds of wetlands, such as lakes, marshes, and rivers. Oryzomys species swim well, are active during the night, and eat both plant and animal food. They build woven nests of vegetation. After a gestation period of 21 to 28 days, about four young are born. Species of Oryzomys are infected by numerous parasites and carry at least three hantaviruses, one of which (Bayou virus) also infects humans. Two, maybe three, species have gone extinct over the last two centuries and at least one other is endangered, but the widespread marsh rice rat and O. couesi are not threatened.


Oryzomys is one of about thirty genera within the tribe Oryzomyini, a diverse group of well over a hundred species, many of which were formerly also included in Oryzomys. [9] Oryzomyini is one of several tribes within the subfamily Sigmodontinae of the family Cricetidae, which includes hundreds of other species of mainly small rodents, distributed mainly in the Americas and Eurasia. [10]

Sigmodontinae subfamily of mammals

The rodent subfamily Sigmodontinae includes New World rats and mice, with at least 376 species. Many authorities include the Neotominae and Tylomyinae as part of a larger definition of Sigmodontinae. When those genera are included, the species count numbers at least 508. Their distribution includes much of the New World, but the genera are predominantly South American, such as brucies. They invaded South America from Central America as part of the Great American Interchange near the end of the Miocene, about 5 million years ago. Sigmodontines proceeded to diversify explosively in the formerly isolated continent. They inhabit many of the same ecological niches that the Murinae occupy in the Old World.

Cricetidae family of mammals

The Cricetidae are a family of rodents in the large and complex superfamily Muroidea. It includes true hamsters, voles, lemmings, and New World rats and mice. At almost 608 species, it is the second-largest family of mammals, and has members throughout the Americas, Europe and Asia.

Within Oryzomyini, a 2006 phylogenetic analysis by Marcelo Weksler which used both morphological and DNA sequence data found some evidence that Oryzomys is most closely related to a group including Holochilus , Lundomys , and Pseudoryzomys . Although analyses based on morphological and combined data supported this relationship, sequences of the Rbp3 gene alone instead placed Oryzomys among a group that included Nectomys , Sigmodontomys , and a few other genera. In all analyses, Oryzomys appeared within clade D of Oryzomyini. [11] The relationship between Oryzomys and the Holochilus group was supported by five synapomorphies (shared derived characters)—absence or reduction of both the hypothenar and interdigital pads; reduction of ungual tufts of hairs surrounding the claws; having the back margin of the zygomatic plate of the skull at the same level as the front of the first upper molar; and the anterocone (front cusp) of the first upper molar divided by an anteromedian fossette. The first three are adaptations to the semiaquatic lifestyle that Oryzomys and the members of the Holochilus group share, and may thus be examples of convergent evolution. [12]

Phylogenetics Study of the evolutionary history and relationships among individuals or groups of organisms

In biology, phylogenetics is the study of the evolutionary history and relationships among individuals or groups of organisms. These relationships are discovered through phylogenetic inference methods that evaluate observed heritable traits, such as DNA sequences or morphology under a model of evolution of these traits. The result of these analyses is a phylogeny – a diagrammatic hypothesis about the history of the evolutionary relationships of a group of organisms. The tips of a phylogenetic tree can be living organisms or fossils, and represent the "end", or the present, in an evolutionary lineage. A phylogenetic tree can be rooted or unrooted. A rooted tree indicates the common ancestor, or ancestral lineage, of the tree. An unrooted tree makes no assumption about the ancestral line, and does not show the origin or "root" of the gene or organism in question. Phylogenetic analyses have become central to understanding biodiversity, evolution, ecology, and genomes.

Holochilus is a genus of semiaquatic rodents in the tribe Oryzomyini of family Cricetidae, sometimes called marsh rats. It contains three living species, H. brasiliensis, H. chacarius, and H. sciureus, which are widely distributed in South America east of the Andes. A fourth species from the Pleistocene of Bolivia was formerly classified as H. primigenus, but is now placed in the genus Reigomys.

<i>Lundomys</i> A semiaquatic rat species from southeastern South America.

Lundomys molitor, also known as Lund's amphibious rat or the greater marsh rat, is a semiaquatic rat species from southeastern South America.


The name Oryzomys was introduced in 1857 by Spencer Fullerton Baird for the marsh rice rat (now Oryzomys palustris) of the eastern United States, [13] which had been first described twenty years earlier by Richard Harlan. [8] The name combines the Greek oryza "rice" and mys "mouse" and refers to the feeding habits of the marsh rice rat. [14] Baird placed Oryzomys as a subgenus of the now-defunct genus Hesperomys and included only the marsh rice rat in it, a classification which was followed by Elliott Coues in 1874 and 1877. [15] In 1890, Oryzomys was raised to generic rank, and in subsequent years numerous additional species were ascribed to it, many of which were soon moved to separate genera. [16] In the 1898 Catalogus Mammalium, Édouard Louis Trouessart listed 67 species of Oryzomys, [17] including some that are now placed in Calomys , Necromys , Thomasomys , and other genera unrelated to Oryzomys. [18] Some of the new genera proposed were soon subsumed in Oryzomys again, [19] and in The Families and Genera of Living Rodents (1941), John Ellerman listed Microryzomys , Oligoryzomys , Melanomys , Nesoryzomys , and Oecomys as synonyms of Oryzomys [20] and included about 127 species in it. [21] In 1948, Philip Hershkovitz suggested that other oryzomyines like Nectomys and Megalomys could as well be included in Oryzomys, [22] and Clayton Ray followed this suggestion in 1962. [23]

Spencer Fullerton Baird American naturalist, ornithologist, ichthyologist and museum curator

Spencer Fullerton Baird was an American naturalist, ornithologist, ichthyologist, herpetologist, and museum curator. Baird was the first curator to be named at the Smithsonian Institution. He would eventually serve as assistant Secretary of the Smithsonian from 1850 to 1878, and as Secretary from 1878 until 1887. He was dedicated to expanding the natural history collections of the Smithsonian which he increased from 6,000 specimens in 1850 to over 2 million by the time of his death. He published over 1,000 works during his lifetime.

Richard Harlan

Richard Harlan was an American naturalist, zoologist, herpetologist, physicist, and paleontologist. He was the author of Fauna Americana, published in 1825, and American Herpetology.

Subgenus taxonomic rank

In biology, a subgenus is a taxonomic rank directly below genus.

Hershkovitz and Ray's classification was never widely followed, and from 1976 on authors started to reinstate some of the other groups lumped in Oryzomys as separate genera. [24] The genus was reduced to 43 species (out of 110 in Oryzomyini) in the third edition (2005) of Mammal Species of the World, [25] but it was still not a natural, monophyletic group; [26] rather, it mostly united those oryzomyines that lacked the conspicuous specializations of other genera. [27] In 2006, Marcelo Weksler's comprehensive phylogenetic analysis produced further evidence that the genus was polyphyletic, as species of Oryzomys were dispersed all over the oryzomyine tree. He proposed that eleven new genera should be created to accommodate those species that were not closely related to the type species of Oryzomys, the marsh rice rat; [28] he considered other options that would require fewer new genera, but argued that that would result in less meaningful genus-level groups in Oryzomyini. [29] Later in the same year, Weksler, Percequillo, and Voss created ten new genera— Aegialomys , Cerradomys , Eremoryzomys , Euryoryzomys , Hylaeamys , Mindomys , Nephelomys , Oreoryzomys , Sooretamys , and Transandinomys —for species formerly placed in Oryzomys and placed six more species related to "Oryzomys" alfaroi in Handleyomys pending the description of more new genera for them. [30] They left only five species in Oryzomys, which was now finally a natural, monophyletic group. Because of subsequent taxonomic work, the number of species has since increased to at least eight. [31]

Some problems remain: ?Oryzomys pliocaenicus, a Miocene fossil from Kansas, is of uncertain identity but may belong in Bensonomys , [32] and fossils from the Miocene of Oregon and Pliocene of New Mexico have also been ascribed to Oryzomys, but probably incorrectly. [32] A possible Oryzomys has been recorded from the Irvingtonian (Pleistocene) of Saskatchewan. [33]


Drawing of Oryzomys molestus, now a synonym of Oryzomys albiventer Oryzomys molestus Eliot.jpg
Drawing of Oryzomys molestus, now a synonym of Oryzomys albiventer

The current concept of Oryzomys derives from the palustris-mexicanus group recognized within a much larger genus Oryzomys by Merriam (1901) and the palustris group proposed by Goldman (1918). [1] Merriam recognized 21 species within his group, but Goldman consolidated them into eight—the marsh rice rat in the United States, O. couesi in far southern Texas, Mexico, and Central America, and six others with small distributions. [36] In 1960, Raymond Hall united O. couesi and the marsh rice rat into a single species, Oryzomys palustris, and thereafter, other localized forms were also included in O. palustris. [37] Hershkovitz described another species in the group, O. gorgasi from Colombia, in 1970 [38] and the next year he noted that O. dimidiatus , previously classified as a Nectomys, was similar to O. palustris. [39] After 1979, the marsh rice rat and O. couesi were again regarded as separate as a result of further work in Texas, where their ranges meet. [37] While reviewing O. gorgasi in 2001, J. Sánchez H. and colleagues redefined and characterized the O. palustris group and listed O. couesi, O. dimidiatus, O. gorgasi, and the marsh rice rat as its members; [40] Guy Musser and Michael Carleton in the 2005 third edition of Mammal Species of the World additionally listed O. nelsoni from María Madre Island in western Mexico. [8]

In 2006, Weksler and colleagues followed the 2001 definition by Sánchez and others for the restricted genus Oryzomys, but added O. antillarum from Jamaica as a species. [41] Carleton and Joaquin Arroyo-Cabrales reviewed Oryzomys from western Mexico in 2009 and in this context provided an extended diagnosis of Oryzomys. They recognized eight species: the six previously mentioned plus O. albiventer and O. peninsulae . [1] Also in 2009, Robert Voss and Weksler identified the subfossil Oryzomys curasoae from Curaçao as an island population of O. gorgasi. [42] The next year, Delton Hanson and colleagues published a study using DNA sequence data from the cytochrome b, interphotoreceptor retinoid-binding protein, and alcohol dehydrogenase 1 genes to assess relationships within Oryzomys. They recommended that the marsh rice rat be split into two species and that O. couesi be split into four species on the basis of the observed sequence divergence and other data. [43]

Merriam divided his palustris-mexicanus group in two "series" according to the color of the underparts (white or fulvous). [44] Goldman divided his palustris group in two "sections"—a couesi section with O. couesi and six related species, and a palustris section with O. palustris only. He noted that the latter differed from the former in the generally darker, more brownish, longer fur, and larger sphenopalatine vacuities (openings in the mesopterygoid fossa, the gap behind the end of the palate). [45] As Weksler's 2006 analysis included only O. couesi and the marsh rice rat among species of Oryzomys in the strict sense, he could not test those groups. [46] Carleton and Arroyo-Cabrales concurred with Goldman's division, listing additional characters, and noted that the palustris group may be more semiaquatically adapted than the members of the couesi group are. In the latter, the fur is usually reddish-brown, as opposed to grayish-brown in the palustris group. Members of the couesi group have smaller sphenopalatine vacuities and a smaller sphenopalatine foramen, a foramen (opening) in the side of the skull above the molars, and a more highly developed anterolabial cingulum on the third lower molar (a crest at the front of the tooth). The hypothenar pad of the hindfoot, located on the sole far from the fingers, is present in the couesi group, but absent in the palustris group. Interdigital webbing may be more highly developed in the palustris group. [35] Using morphological data, Voss and Weksler found a closer relationship between O. couesi and O. gorgasi to the exclusion of O. palustris, but with low confidence. [47] The DNA sequence data of Hanson and colleagues supported a deep separation between the palustris and couesi groups, but a Costa Rican sample (assigned to O. couesi) was about as distant from the two groups as they were from each other. [48]

The genus currently includes the following species: [1]

NameDistributionCharacteristicsTaxonomic comments
Oryzomys albiventer Michoacán, Guanajuato, Jalisco (inland western Mexico) [49] Large, long tail, robust skull [49] Described in 1901 as a species; subspecies of O. couesi in 1918; reinstated as a species in 2009 [50]
Oryzomys antillarum Jamaica (extinct) [51] Long nasal bones, short incisive foramina (perforations in the palate), robust zygomatic arches (cheekbones) [52] Described in 1898 as a species; subspecies of O. couesi in 1966; reinstated as a species in 1993/2006 [53]
Oryzomys couesi Southern Texas and coastal Sonora (northwestern Mexico) to northwestern Colombia [54] Upperparts buff to reddish, underparts white to buff, sphenopalatine vacuities small to absent [55] Described in 1877 as a species; subspecies of marsh rice rat in 1960; reinstated as a species in 1979; various other species split from and lumped into it from time to time. [56] Genetic data suggest four species may be present: one along the Pacific coast from Sonora to El Salvador; one from Texas to Nicaragua; one in Costa Rica; and one in Panama. [43]
Oryzomys dimidiatus Southeastern Nicaragua [57] Gray underparts, brown feet, tail nearly the same color above as below [58] Described in 1905 as a species of Nectomys ; reclassified in Oryzomys in 1948 [59]
Oryzomys gorgasi Northwestern Colombia, northwestern Venezuela; extinct on Curaçao [60] Robust rostrum (front part of skull); incisive foramina tapering at the back; sphenopalatine vacuities absent; subsquamosal fenestra (opening at the back of the skull) small [61] Described in 1970; Oryzomys curasoae described in 2001; [57] both synonymized in 2009 [42]
Oryzomys nelsoni María Madre Island, Nayarit, western Mexico (extinct) [62] Large, long tail, rostrum heavy and bent downward, incisors large and wide [63] Described in 1898 as a species; subspecies of O. palustris in 1971; otherwise regarded as a distinct species [62]
Marsh rice rat (Oryzomys palustris)In the eastern United States from New Jersey and Kansas south to Florida and Texas, and into Tamaulipas, Mexico; previously further north to Iowa and southwestern Pennsylvania [64] Compared to sympatric O. couesi where the two meet: short tail, white underparts, sphenopalatine foramen large [65] Described in 1827; [62] specific status of Florida Keys form (argentatus; first described in 1978) disputed; [66] genetic data suggest populations west of Alabama may be a separate species [67]
Oryzomys peninsulae Southern tip of Baja California Sur (perhaps extinct) [68] Moderately large, gray on head and forequarters, broad, squared zygomatic arches, long, broad incisive foramina, upper incisor nearly orthodont [69] Described in 1899 as a species; subspecies of O. palustris in 1971 and of O. couesi in 1994; reinstated as a species in 2009 [69]


Measurements of species of Oryzomys
Speciesn [Note 4] Total lengthTailHindfoot
Oryzomys albiventer [70] 12285.4 (245–314)155.4 (129–173)36.1 (33–40)
Oryzomys antillarum [71] 3247 (228–260)119.7 (108–132)29.3 (28–30)
Oryzomys couesi from Nayarit [70] 62244.8 (210–288)125.1 (105–150)30.5 (27–33)
Oryzomys dimidiatus [72] 3249 (228–278)129 (110–150)28.3 (27–31)
Oryzomys gorgasi [73] 6–10 [Note 5] 259 (220–290)130 (116–138)31 (30–32)
Oryzomys nelsoni [70] 4322 (288–344)181.5 (160–191)37.3 (35–39)
Marsh rice rat [74] 226–305108–15628–37
Oryzomys peninsulae [70] 14265.6 (227–305)136.8 (114–156)32.0 (29–34)
Measurements are in millimeters and in the form "average (minimum–maximum)".

Oryzomys contains medium-sized, semiaquatically specialized oryzomyine rodents. They have long, coarse fur that is grayish to reddish on the upperparts and white to buff on the underparts. [75] The marsh rice rat superficially resembles the introduced species black rat and brown rat, but has larger differences in color between the upper- and underparts. [74] The vibrissae (whiskers) are short and the ears are small and well-haired. The tail is usually as long as or longer than the head and body and is sparsely haired, but the hairs on the lower side are longer than those above. Females have eight mammae, as in most oryzomyines. The hindfeet are broad and have the first and fifth digits notably shorter than the middle three. The upper surface is hairy, but the underside is naked and covered with small irregularities (squamae). The pads are generally poorly developed, as are the ungual tufts. [76] Interdigital webbing may be present, but its development is variable within the genus. [35]

The karyotype has been recorded in various populations of the marsh rice rat and O. couesi and is apparently stable within the genus at 56  chromosomes, with the fundamental number of chromosomal arms ranging from 56 to 60 (2n = 56, FN = 56–60). [43] In both species, 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. [77] Furthermore, the gall bladder is absent, a synapomorphy of Oryzomyini. [78]

Holotype skull of Oryzomys antillarum, seen from above (A), below (B) and the left (C) Oryzomys antillarum Ray.png
Holotype skull of Oryzomys antillarum , seen from above (A), below (B) and the left (C)

Oryzomys species have a large skull with a short rostrum and high braincase. The interorbital region, located between the eyes, is narrowest to the front and is flanked by well-developed beads at its margins. The zygomatic plate is broad and has a well-developed zygomatic notch at its front. The zygomatic arch is robust and contains a small but distinct jugal bone. The interparietal bone, part of the roof of the braincase, is narrow and short; [80] its narrowness is a synapomorphy for O. couesi plus the marsh rice rat according to Weksler's analysis. [12] The incisive foramina are long, with their back margin at the front of the first molars or further back. The palate is also long, extending beyond the back margin of the maxillary bone, and is perforated near the third molars by well-developed posterolateral palatal pits. There is no alisphenoid strut, an extension of the alisphenoid bone that in some other oryzomyines separates two foramina in the skull. The auditory bullae are large. The condition of the arteries in the head is highly derived. [80] In the mandible (lower jaw), the coronoid process, a process at the back, is well developed [81] and the capsular process, a raising of the mandibular bone housing the root of the lower incisor, is conspicuous. [45]

As usual in oryzomyines, the molars are pentalophodont (have the mesolophs and mesolophids, accessory crests, well developed) and bunodont, with the cusps higher than the connecting crests. [1] The cusps on the upper molars are arranged in two longitudinal series, not three as in the black and brown rats. [82] The front cusps of the first upper and lower molar (anterocone and anteroconid, respectively) are broad and not divided completely by an anteromedian flexus or flexid. Behind the anterocone, the anteroloph (a smaller crest) is complete and separated from the anterocone. [1] On both the second and third lower molars, the anterolophid (a crest on the inner front corner) is present, a putative synapomorphy of the genus. [12] The first molars have additional small roots in addition to the main ones, so that the upper first molar has four and the lower has three or four roots. [1]

As is characteristic of Sigmodontinae, the marsh rice rat and O. couesi have a complex penis, with the baculum (penis bone) displaying large protuberances at the sides. [83] The outer surface of the penis is mostly covered by small spines, but there is a broad band of nonspinous tissue. [84] The papilla (nipple-like projection) on the dorsal (upper) side of the penis is covered with small spines, a character these two species share only with Oligoryzomys among oryzomyines examined. [85] On the urethral process, located in the crater at the end of the penis, [86] a fleshy process (the subapical lobule) is present; it is absent in all other oryzomyines with studied penes except Holochilus brasiliensis . [87] Both traits are recovered as synapomorphies of O. couesi plus the marsh rice rat in Weksler's analysis. [12]

Distribution, ecology, and behavior

Oryzomys couesi (above) and Tylomys panamensis (below) Oryzomys couesi, Tylomys panamensis.jpg
Oryzomys couesi (above) and Tylomys panamensis (below)

The range of Oryzomys extends from New Jersey in the eastern United States through Mexico and Central America south to northwestern Colombia and east to northwestern Venezuela and Curaçao. [89] Species of Oryzomys usually live in wet habitats such as marshes, streams, and mangroves, [1] but both the marsh rice rat and O. couesi are also occasionally encountered in drier habitats. [90] They occur or occurred on many continental-shelf islands and one oceanic island, Jamaica; their adeptness at colonizing islands may be caused by their close association with water and frequent occurrence in coastal wetlands. [68] The oldest fossils date to the Rancholabrean of the United States, about 300,000 years ago; although there have been some earlier North American records, those are not in fact referable to Oryzomys or even Oryzomyini. [91] Oryzomyines likely evolved in South America east of the Andes; the presence of Oryzomys in Central America and other trans-Andean regions is thought to be the result of one of several independent invasions of this region by oryzomyines. [92] Alternatively, Oryzomys may have evolved from the Pliocene North American Jacobsomys . [93] O. antillarum may have reached Jamaica during the last glacial period while sea levels were low. [51]

Behavior is known mainly from the marsh rice rat and O. couesi, with some scattered data from the other species. Oryzomys are semiaquatic, spending much time in the water, and otherwise mainly live on the ground; [94] both the marsh rice rat and O. couesi are known to be excellent swimmers and will flee into the water when disturbed. [95] Both are also active during the night and build nests of interwoven vegetation, which may be suspended above the water. [96] Breeding may occur throughout the year in both species, but is known to be seasonally variable in the marsh rice rat. [97] In both, gestation takes about 21 to 28 days and litter size is usually one to seven, averaging three to five. [98] Young marsh rice rats and O. couesi become reproductively active when about 50 days old. [99]

The marsh rice rat, O. couesi, and O. gorgasi are known to be omnivores, eating both plant and animal material. They eat both seeds and green plant parts and consume a variety of animals, including insects, crustaceans, and many others. [100] The barn owl (Tyto alba) is a major predator on the marsh rice rat [101] and remains of O. antillarum, O. couesi, and O. gorgasi have been found in owl pellet deposits. [102] Several other animals are known to prey on Oryzomys. [103] A variety of parasites are known from O. couesi [104] and the marsh rice rat [105] and two parasitic nematodes have been found in O. gorgasi. [106]

Human interactions

Two species of Oryzomys, O. antillarum and O. nelsoni, have gone extinct since the 19th century, [107] and a third, O. peninsulae, is unlikely to be still extant. [68] Their extinction may have been caused by habitat destruction and by introduced species such as the small Asian mongoose and the brown and black rat. [108] These same causes may threaten O. gorgasi, which the IUCN Red List assesses as "Endangered". [109] O. albiventer has been affected by human alteration of its habitat, but likely still survives. [110] In contrast, the widespread species, the marsh rice rat and O. couesi, are common and of no conservation concern—indeed, both have been considered a pest—but some populations are threatened. [111] Like these two species, O. dimidiatus is assessed as "Least Concern" by the Red List. [112]

The marsh rice rat is the natural reservoir of the Bayou virus, the second most common cause of hantavirus pulmonary syndrome in the United States. [113] Two other hantaviruses, Catacamas virus and Playa de Oro virus, occur in O. couesi in Honduras and western Mexico, respectively, but are not known to infect humans. [114]


  1. As a subgenus of Hesperomys .
  2. First usage as genus. [1] Goldman [4] and Tate [5] attribute the first usage of Oryzomys as a genus to Coues, also in 1890. [6]
  3. Nomen nudum ("naked name"). [8]
  4. Number of specimens measured.
  5. Six for total length, eight for tail length, and ten for hindfoot length.

Related Research Articles

Oryzomys nelsoni extinct rodent of María Madre Island, Nayarit, Mexico

Oryzomys nelsoni is an extinct rodent of María Madre Island, Nayarit, Mexico. Within the genus Oryzomys of the family Cricetidae, it may have been most closely related to the mainland species O. albiventer. Since its first description in 1898, most authors have regarded it as a distinct species, but it has also been classified as a mere subspecies of the marsh rice rat (O. palustris).

<i>Oryzomys couesi</i> A semiaquatic rodent in the family Cricetidae found from southernmost Texas through Mexico and Central America into northwestern Colombia

Oryzomys couesi, also known as Coues' rice rat, is a semiaquatic rodent in the family Cricetidae occurring from southernmost Texas through Mexico and Central America into northwestern Colombia. It is usually found in wet habitats, such as marshes, but also lives in drier forests and shrublands. Weighing about 43 to 82 g, O. couesi is a medium-sized to large rat. The coarse fur is buff to reddish above and white to buff below. The hindfeet show some specializations for life in the water, such as reduced ungual tufts of hair around the digits. It has 56 chromosomes. There is much geographic variation in size, proportions, color, and skull features. Oryzomys couesi is active during the night and builds nests of vegetation that are suspended among reeds about 1 m (3.3 ft) above the ground. It is an excellent swimmer and dives well, but can also climb in vegetation. An omnivore, it eats both plant and animal food, including seeds and insects. It breeds throughout the year; females give birth to about four young after a pregnancy of 21 to 28 days. The species may be infected by several different parasites and by two hantaviruses.

<i>Euryoryzomys emmonsae</i> A rodent of the family Cricetidae from the Amazon rainforest of Brazil

Euryoryzomys emmonsae, also known as Emmons' rice rat or Emmons' oryzomys, is a rodent from the Amazon rainforest of Brazil in the genus Euryoryzomys of the family Cricetidae. Initially misidentified as E. macconnelli or E. nitidus, it was formally described in 1998. A rainforest species, it may be scansorial, climbing but also spending time on the ground. It lives only in a limited area south of the Amazon River in the state of Pará, a distribution that is apparently unique among the muroid rodents of the region.

Oreoryzomys balneator, also known as the Peruvian rice rat or Ecuadoran oryzomys, is a species of rodent in the tribe Oryzomyini of family Cricetidae. It is found in Ecuador and northern Peru in cloud forest at elevations from 1500 to 1800 m. It is the only species in the genus Oreoryzomys, which was included in Oryzomys until 2006. The genus name Oreoryzomys is a combination of ορος the Greek word for "mountain" with the old genus name Oryzomys and refers to the mountainous habitat of O. balneator. Recent research suggests that O. balneator is not closely related to Oryzomys, but instead is probably related to Microryzomys within a clade also including Neacomys and Oligoryzomys.

<i>Transandinomys bolivaris</i> A small rodent found from northeastern Honduras to western Ecuador

Transandinomys bolivaris, also known as the long-whiskered rice rat, is a rodent in the genus Transandinomys. It is found in humid forest from northeastern Honduras to western Ecuador, up to 1,800 m (5,900 ft) above sea level. Since it was first described in 1901 from Ecuador, six scientific names have been introduced for it, but their common identity was not documented until 1998 and the species has long been known under the name Oryzomys bombycinus, described from Panama in 1912. The name Oryzomys bolivaris was used before it was moved to the new genus Transandinomys with Transandinomys talamancae in 2006.

<i>Oryzomys dimidiatus</i> A rodent of the family Cricetidae known from southeastern Nicaragua

Oryzomys dimidiatus, also known as the Nicaraguan oryzomys, Thomas's rice rat, or Nicaraguan rice rat, is a rodent in the genus Oryzomys of the family Cricetidae. It is known from only three specimens, all collected in southeastern Nicaragua since 1904. Placed in Nectomys upon its discovery, it was later classified in its own subgenus of Oryzomys and finally recognized as closely related to other species now placed in Oryzomys, including the marsh rice rat and Oryzomys couesi, which occurs in the same region.

<i>Aegialomys galapagoensis</i> species of mammal

Aegialomys galapagoensis, also known as the Galápagos rice rat or Galápagos oryzomys, is a species of rodent in the family Cricetidae from the Galápagos Islands.

<i>Oryzomys gorgasi</i> A rodent in the family Cricetidae from northwestern Columbia and Venezuela

Oryzomys gorgasi, also known as Gorgas's oryzomys or Gorgas's rice rat, is a rodent in the genus Oryzomys of family Cricetidae. First collected as a living animal in 1967, it is known from only a few localities, including a freshwater swamp in the lowlands of northwestern Colombia and a mangrove islet in northwestern Venezuela. It formerly occurred on the island of Curaçao off northwestern Venezuela; this extinct population has been described as a separate species, Oryzomys curasoae, but does not differ morphologically from mainland populations.

<i>Mindomys</i> A species of rodent in the family Cricetidae from Ecuador

Mindomys hammondi, also known as Hammond's rice rat or Hammond's oryzomys, is a species of rodent in the tribe Oryzomyini of family Cricetidae. Formerly considered to be related with Nectomys, Sigmodontomys, Megalomys, or Oryzomys, it is now placed in its own genus, Mindomys, but its relationships remain obscure; some evidence supports a placement near Oecomys or as a basal member of Oryzomyini.

<i>Eremoryzomys</i> A rodent species in the family Cricetidae from central Peru

Eremoryzomys polius, also known as the gray rice rat or the Marañon oryzomys, is a rodent species in the tribe Oryzomyini of the family Cricetidae. Discovered in 1912 and first described in 1913 by Wilfred Osgood, it was originally placed in Oryzomys and named Oryzomys polius. In 2006, a cladistic analysis found that it was not closely related to Oryzomys in the strict sense or to any other oryzomyine then known, so that it is now placed in its own genus, Eremoryzomys. The Brazilian genus Drymoreomys, named in 2011, is probably the closest relative of Eremoryzomys. Eremoryzomys has a limited distribution in the dry upper valley of the Marañón River in central Peru, but may yet contain more than one species.

<i>Transandinomys talamancae</i> A small rodent found from Costa Rica to northern South America

Transandinomys talamancae is a rodent in the genus Transandinomys that occurs from Costa Rica to southwestern Ecuador and northern Venezuela. Its habitat consists of lowland forests up to 1,525 m (5,003 ft) above sea level. With a body mass of 38 to 74 g, it is a medium-sized rice rat. The fur is soft and is reddish to brownish on the upperparts and white to buff on the underparts. The tail is dark brown above and lighter below and the ears and feet are long. The vibrissae (whiskers) are very long. In the skull, the rostrum is long and the braincase is low. The number of chromosomes varies from 34 to 54.

Aegialomys xanthaeolus, also known as the yellowish oryzomys or yellowish rice rat, is a species of rodent in the family Cricetidae. It belongs to the genus Aegialomys in tribe Oryzomyini, which was not recognized as distinct from Oryzomys until 2006. It is found in coastal Ecuador and Peru. Though it is currently the only formally recognized mainland species of Aegialomys, at least one other exists. The specific name is sometimes incorrectly spelled "xantheolus", without the second "a".

<i>Oryzomys antillarum</i> An extinct rodent from Jamaica

Oryzomys antillarum, also known as the Jamaican rice rat, is an extinct rodent of Jamaica. A member of the genus Oryzomys within the family Cricetidae, it is similar to O. couesi of mainland Central America, from where it may have dispersed to its island during the last glacial period. O. antillarum is common in subfossil cave faunas and is also known from three specimens collected live in the 19th century. Some historical records of Jamaican rats may pertain to it. The species probably became extinct late in the 19th century, perhaps due to the introduction of the small Asian mongoose, competition with introduced rodents such as the brown rat, and habitat destruction.

<i>Transandinomys</i> A genus of small rodents from Central and South America

Transandinomys is a genus of rodents in the tribe Oryzomyini of family Cricetidae. It includes two species—T. bolivaris and T. talamancae—found in forests from Honduras in Central America south and east to southwestern Ecuador and northwestern Venezuela in northern South America. Until 2006, its members were included in the genus Oryzomys, but phylogenetic analysis showed that they are not closely related to the type species of that genus, and they have therefore been placed in a new genus. They may be most closely related to genera like Hylaeamys and Euryoryzomys, which contain very similar species. Both species of Transandinomys have had eventful taxonomic histories.

<i>Oryzomys peninsulae</i> A species of rodent from western Mexico

Oryzomys peninsulae, also known as the Lower California rice rat, is a species of rodent from western Mexico. Restricted to the southern tip of the Baja California peninsula, it is a member of the genus Oryzomys of family Cricetidae. Only about twenty individuals, collected around 1900, are known, and subsequent destruction of its riverine habitat may have driven the species to extinction.

In anatomy, posterolateral palatal pits are gaps at the sides of the back of the bony palate, near the last molars. Posterolateral palatal pits are present, in various degrees of development, in several members of the rodent family Cricetidae. Many members of the family lack them or have only simple pits, but Arvicolinae and Oryzomyini have more highly developed posterolateral palatal pits. Posterolateral palatal pits are also present in some other rodents, including Glis, Jaculus, Hystrix, Abrocoma, Ctenomys, Chinchilla, and Lagidium.

<i>Oryzomys albiventer</i> species of mammal

Oryzomys albiventer is a rodent in the genus Oryzomys of family Cricetidae from interior western Mexico, in the states of Jalisco, Guanajuato, and Michoacán. First described in 1901 as a separate species, it was later lumped under O. couesi and the marsh rice rat (O. palustris) until it was reinstated as a species in 2009. It differs from neighboring Oryzomys populations in size and measurements and is a large, brightly colored species with a long tail and robust skull and molars. Its range has been much impacted by agricultural development, but isolated populations are thought to persist.

In mammals, ungual tufts are tufts of hairs at the base of claws of the fore- and hindfeet. Their presence has been used as a character in cladistic studies of Cricetidae.


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Literature cited