''Oryzomys'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

semiaquatic In biology, semiaquatic can refer to various types of animals that spend part of their time in water, or plants that naturally grow partially submerged in water. Examples are given below. Semiaquatic animals Semi aquatic animals include: * Ve ...

rodents in the tribe

Oryzomyini Oryzomyini is a tribe of rodents in the subfamily Sigmodontinae of the family Cricetidae. It includes about 120 species in about thirty genera,Weksler et al., 2006, table 1 distributed from the eastern United States to the southernmost parts of ...

living in southern North America and far northern South America. It includes eight

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

, 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''. Species of ''Oryzomys'' are medium-sized rats with long, coarse fur. The upperparts are gray to reddish and the underparts white to

buff Buff or BUFF may refer to: People * Buff (surname), a list of people * Buff (nickname), a list of people * Johnny Buff, ring name of American world champion boxer John Lisky (1888–1955) * Buff Bagwell, a ring name of American professional ...

. 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

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

s, lack a

gall bladder In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow organ where bile is stored and concentrated before it is released into the small intestine. In humans, the pear-shaped gallbladder lies beneath the liver, although ...

, 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. The

habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...

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

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

s and carry at least three

hantavirus ''Orthohantavirus'' is a genus of single-stranded, enveloped, negative-sense RNA viruses in the family '' Hantaviridae'' within the order ''Bunyavirales''. Members of this genus may be called orthohantaviruses or simply hantaviruses. Orthohantav ...

es, 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.

Taxonomy

''Oryzomys'' is one of about thirty genera within the tribe

, a diverse group of well over a hundred species, many of which were formerly also included in ''Oryzomys''. Oryzomyini is one of several tribes within the subfamily

Sigmodontinae 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 specie ...

of the family

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

, which includes hundreds of other species of mainly small rodents, distributed mainly in the Americas and Eurasia. Within Oryzomyini, a 2006

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

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 ''Holochilus'' is a genus of semiaquatic rodents in the tribe Oryzomyini of family Cricetidae,* sometimes called marsh rats. It contains five living species, '' H. brasiliensis'', '' H. chacarius'', '' H. nanus'', '' H. oxe'', and '' H. sciureus' ...

'', '' Lundomys'', and ''

Pseudoryzomys ''Pseudoryzomys simplex'', also known as the Brazilian false rice rat or false oryzomys, is a species of rodent in the family Cricetidae from south-central South America. It is found in lowland palm savanna and thorn scrub habitats. It is a me ...

''. 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 ''Nectomys'' is a genus of rodent in the tribe Oryzomyini of family Cricetidae. Musser and Carleton, 2005. It is closely related to '' Amphinectomys'' and was formerly considered congeneric with ''Sigmodontomys''. It consists of five species, whic ...

'', '' Sigmodontomys'', and a few other genera. In all analyses, ''Oryzomys'' appeared within clade D of Oryzomyini. 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 In rodent anatomy, the zygomatic plate is a bony plate derived from the flattened front part of the zygomatic arch (cheekbone). At the back, it connects to the front (maxillary) root of the zygomatic arch, and at the top it is connected to the ...

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

lifestyle that ''Oryzomys'' and the members of the ''Holochilus'' group share, and may thus be examples of

convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...

Circumscription

The name ''Oryzomys'' was introduced in 1857 by Spencer Fullerton Baird for the marsh rice rat (now ''Oryzomys palustris'') of the eastern United States,Baird, 1857, p. 482 which had been first described twenty years earlier by

Richard Harlan Richard Harlan (September 19, 1796 – September 30, 1843) was an American paleontologist, anatomist, and physician. He was the first American to devote significant time and attention to vertebrate paleontology and was one of the most importan ...

.Musser and Carleton, 2005, p. 1144 The name combines the Greek ''oryza'' "rice" and ''mys'' "mouse" and refers to the feeding habits of the marsh rice rat.Schwartz and Schwartz, 2001, p. 192 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 Elliott Ladd Coues (; September 9, 1842 – December 25, 1899) was an American army surgeon, historian, ornithologist, and author. He led surveys of the Arizona Territory, and later as secretary of the United States Geological and Geographic ...

in 1874 and 1877. 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. In the 1898 ''Catalogus Mammalium'',

Édouard Louis Trouessart Édouard Louis Trouessart (25 August 1842 – 30 June 1927) was a French zoologist born in Angers. He studied military medicine in Strasbourg, but was forced to leave school due to serious health problems. In 1864 he started work as ''préparateur ...

listed 67 species of ''Oryzomys'', including some that are now placed in '' Calomys'', ''

Necromys ''Necromys'' is a genus of South American sigmodontine rodents allied to ''Akodon''. This genus has also been known as ''Cabreramys'' or more recently ''Bolomys'', and the northern grass mouse (''N. urichi'') has recently been transferred from ' ...

'', ''

Thomasomys ''Thomasomys'' is a genus of rodent in the family Cricetidae, named after British zoologist Oldfield Thomas. Nuclear DNA sequence analysis has indicated that it is a sister taxon to '' Rhagomys''. It contains the following species: * Anderson' ...

'', and other genera unrelated to ''Oryzomys''. Some of the new genera proposed were soon subsumed in ''Oryzomys'' again, and in ''The Families and Genera of Living Rodents'' (1941), John Ellerman listed '' Microryzomys'', '' Oligoryzomys'', '' Melanomys'', '' Nesoryzomys'', and ''

Oecomys ''Oecomys'' is a genus of rodent within the tribe Oryzomyini of family Cricetidae. It contains about 17 species, which live in trees and are distributed across forested parts of South America, extending into Panama and Trinidad Trinidad i ...

'' as synonyms of ''Oryzomys''Ellerman, 1941, p. 340 and included about 127 species in it. In 1948, Philip Hershkovitz suggested that other oryzomyines like ''Nectomys'' and ''

Megalomys ''Megalomys'' is a genus of rodent in the family Cricetidae, part of the tribe Oryzomyini. The genus contains five large rodents from various Caribbean islands, of which two are known to have survived into modern times, but all of which are now ex ...

'' could as well be included in ''Oryzomys'', and Clayton Ray followed this suggestion in 1962. 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. The genus was reduced to 43 species (out of 110 in Oryzomyini) in the third edition (2005) of ''Mammal Species of the World'',Musser and Carleton, 2005, p. 900 but it was still not a natural, monophyletic group; rather, it mostly united those oryzomyines that lacked the conspicuous specializations of other genera.Weksler, 2006, p. 82 In 2006, Marcelo Weksler's comprehensive phylogenetic analysis produced further evidence that the genus was

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

, 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 In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...

of ''Oryzomys'', the marsh rice rat;Weksler, 2006, p. 75 he considered other options that would require fewer new genera, but argued that that would result in less meaningful genus-level groups in Oryzomyini. 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. 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. Some problems remain: ?''Oryzomys pliocaenicus'', a

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

fossil from Kansas, is of uncertain identity but may belong in '' Bensonomys'',Weksler, 2006, p. 87 and fossils from the Miocene of Oregon and

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Irvingtonian (Pleistocene) of

Saskatchewan Saskatchewan ( ; ) is a province in western Canada, bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, to the northeast by Nunavut, and on the south by the U.S. states of Montana and North Dak ...

Species

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).Carleton and Arroyo-Cabrales, 2009, p. 116 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. 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''.Musser and Carleton, 2005, p. 1147 Hershkovitz described another species in the group, '' O. gorgasi'' from Colombia, in 1970 and the next year he noted that '' O. dimidiatus'', previously classified as a ''Nectomys'', was similar to ''O. palustris''. 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. 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;

Guy Musser Guy Graham Musser (August 10, 1936 – October 2019) was an American zoologist. His main research was in the field of the rodent subfamily Murinae, in which he has described many new species. Musser was born in Salt Lake City, Utah. He attended ...

and

Michael Carleton Michael may refer to: People * Michael (given name), a given name * Michael (surname), including a list of people with the surname Michael Given name "Michael" * Michael (archangel), ''first'' of God's archangels in the Jewish, Christian an ...

in the 2005 third edition of ''Mammal Species of the World'' additionally listed '' O. nelsoni'' from María Madre Island in western Mexico. In 2006, Weksler and colleagues followed the 2001 definition by Sánchez and others for the restricted genus ''Oryzomys'', but added '' O. antillarum'' from

Jamaica Jamaica (; ) is an island country situated in the Caribbean Sea. Spanning in area, it is the third-largest island of the Greater Antilles and the Caribbean (after Cuba and Hispaniola). Jamaica lies about south of Cuba, and west of His ...

as a species. 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''. Also in 2009, Robert Voss and Weksler identified the subfossil ''Oryzomys curasoae'' from Curaçao as an island population of ''O. gorgasi''.Voss and Weksler, 2009, p. 73 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 Alcohol dehydrogenases (ADH) () are a group of dehydrogenase enzymes that occur in many organisms and facilitate the interconversion between alcohols and aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide (NAD+) to NA ...

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. Merriam divided his ''palustris-mexicanus'' group in two "series" according to the color of the underparts (white or

fulvous Fulvous is a colour, sometimes described as dull orange, brownish-yellow or tawny; it can also be likened to a variation of buff, beige or butterscotch. As an adjective it is used in the names of many species of birds, and occasionally other an ...

). 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 The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly separ ...

).Goldman, 1918, p. 20 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. 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 The sphenopalatine foramen is a Foramina of the skull, foramen in the skull that connects the nasal cavity with the pterygopalatine fossa. Structure The processes of the superior border of the palatine bone are separated by the ''sphenopalatine no ...

, a

foramen In anatomy and osteology, a foramen (;Entry "foramen"
in

(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.Carleton and Arroyo-Cabrales, 2009 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. 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. The genus currently includes the following species:

Description

''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. The marsh rice rat superficially resembles the

introduced species An introduced species, alien species, exotic species, adventive species, immigrant species, foreign species, non-indigenous species, or non-native species is a species living outside its native distributional range, but which has arrived ther ...

black rat and

brown rat The brown rat (''Rattus norvegicus''), also known as the common rat, street rat, sewer rat, wharf rat, Hanover rat, Norway rat, Norwegian rat and Parisian rat, is a widespread species of common rat. One of the largest muroids, it is a brown o ...

, but has larger differences in color between the upper- and underparts. The

vibrissa Vibrissae (; singular: vibrissa; ), more generally called Whiskers, are a type of stiff, functional hair used by mammals to sense their environment. These hairs are finely specialised for this purpose, whereas other types of hair are coarser ...

e (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 A mammary gland is an exocrine gland in humans and other mammals that produces milk to feed young offspring. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in primat ...

, 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. Interdigital webbing may be present, but its development is variable within the genus. 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

s, with the

fundamental number A karyotype is the general appearance of the complete set of metaphase chromosomes in the cells of a species or in an individual organism, mainly including their sizes, numbers, and shapes. Karyotyping is the process by which a karyotype is disce ...

of chromosomal arms ranging from 56 to 60 (2n = 56, FN = 56–60). In both species, the

stomach The stomach is a muscular, hollow organ in the gastrointestinal tract of humans and many other animals, including several invertebrates. The stomach has a dilated structure and functions as a vital organ in the digestive system. The stomach i ...

has the characteristic pattern of sigmodontines ( unilocular-hemiglandular): it is not split in two chambers by an incisura angularis and the front part (

antrum ''This is a disambiguation page for the biological term. For the 2018 horror movie, see Antrum (film)'' In biology, antrum is a general term for a cavity or chamber, which may have specific meaning in reference to certain organs or sites in the bod ...

) is covered by a

glandular epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

. Furthermore, the

is absent, a synapomorphy of Oryzomyini. Oryzomys antillarum Ray

''Oryzomys'' species have a large skull with a short rostrum and high braincase. The

interorbital region The interorbital region of the skull is located between the eyes, anterior to the braincase. The form of the interorbital region may exhibit significant variation between taxonomic groups. In oryzomyine rodents, for example, the width, form, an ...

, 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 jugal is a skull bone found in most reptiles, amphibians and birds. In mammals, the jugal is often called the malar or zygomatic. It is connected to the quadratojugal and maxilla, as well as other bones, which may vary by species. Anato ...

. The

interparietal bone An interparietal bone (os interparietale or Inca bone or ''os inca var.'') is a dermal bone situated between the parietal and supraoccipital. It is homologous to the postparietal bones of other animals. In humans, it corresponds to the upper p ...

, part of the roof of the braincase, is narrow and short;Goldman, 1918, p. 19; Carleton and Arroyo-Cabrales, 2009, p. 116 its narrowness is a synapomorphy for ''O. couesi'' plus the marsh rice rat according to Weksler's analysis.Weksler, 2006, p. 131 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

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...

ry 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 bulla The tympanic part of the temporal bone is a curved plate of bone lying below the squamous part of the temporal bone, in front of the mastoid process, and surrounding the external part of the ear canal. It originates as a separate bone (tympanic ...

e are large. The condition of the

arteries An artery (plural arteries) () is a blood vessel in humans and most animals that takes blood away from the heart to one or more parts of the body (tissues, lungs, brain etc.). Most arteries carry oxygenated blood; the two exceptions are the pu ...

in the head is highly derived. In the

mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower teeth in place. The mandible sits beneath the maxilla. It is the only movable bone ...

(lower jaw), the coronoid process, a

process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management *Business process, activities that produce a specific se ...

at the back, is well developedGoldman, 1918, p. 19 and the capsular process, a raising of the mandibular bone housing the root of the lower incisor, is conspicuous. As usual in oryzomyines, the molars are pentalophodont (have the mesolophs and mesolophids, accessory crests, well developed) and

bunodont The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...

, with the cusps higher than the connecting crests. The cusps on the upper molars are arranged in two longitudinal series, not three as in the black and brown rats. 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. 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. 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. 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. The outer surface of the penis is mostly covered by small spines, but there is a broad band of nonspinous tissue. 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. On the

urethral process The urethra (from Greek οὐρήθρα – ''ourḗthrā'') is a tube that connects the urinary bladder to the urinary meatus for the removal of urine from the body of both females and males. In human females and other primates, the urethra con ...

, located in the crater at the end of the penis, a fleshy process (the subapical lobule) is present; it is absent in all other oryzomyines with studied penes except ''

Holochilus brasiliensis ''Holochilus brasiliensis'', also known as the Brazilian marsh ratMusser, G.G. and Carleton, M.D. 2005. Superfamily Muroidea. Pp. 894–1531 in Wilson, D.E. and Reeder, D.M. (eds.)Mammal Species of the World: a taxonomic and geographic refer ...

''. Both traits are recovered as synapomorphies of ''O. couesi'' plus the marsh rice rat in Weksler's analysis.

Distribution, ecology, and behavior

The range of ''Oryzomys'' extends from

New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...

in the eastern United States through Mexico and Central America south to northwestern Colombia and east to northwestern Venezuela and Curaçao. Species of ''Oryzomys'' usually live in wet habitats such as marshes, streams, and

mangrove A mangrove is a shrub or tree that grows in coastal saline or brackish water. The term is also used for tropical coastal vegetation consisting of such species. Mangroves are taxonomically diverse, as a result of convergent evolution in severa ...

s, but both the marsh rice rat and ''O. couesi'' are also occasionally encountered in drier habitats. They occur or occurred on many continental-shelf islands and one

oceanic island An island (or isle) is an isolated piece of habitat that is surrounded by a dramatically different habitat, such as water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys. An island ...

, Jamaica; their adeptness at colonizing islands may be caused by their close association with water and frequent occurrence in coastal wetlands. The oldest fossils date to the

Rancholabrean The Rancholabrean North American Land Mammal Age on the geologic timescale is the North American faunal stage according to the North American Land Mammal Ages chronology (NALMA), typically set from less than 240,000 years to 11,000 years BP, a pe ...

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. 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.Weksler, 2006, p. 88 Alternatively, ''Oryzomys'' may have evolved from the Pliocene North American '' Jacobsomys''. ''O. antillarum'' may have reached Jamaica during the last glacial period while sea levels were low. 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;Reid, 2009, p. 205 both the marsh rice rat and ''O. couesi'' are known to be excellent swimmers and will flee into the water when disturbed. Both are also active during the night and build nests of interwoven vegetation, which may be suspended above the water. Breeding may occur throughout the year in both species, but is known to be seasonally variable in the marsh rice rat. In both, gestation takes about 21 to 28 days and litter size is usually one to seven, averaging three to five. Young marsh rice rats and ''O. couesi'' become reproductively active when about 50 days old. 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

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...

s, and many others. The barn owl (''Tyto alba'') is a major predator on the marsh rice rat and remains of ''O. antillarum'', ''O. couesi'', and ''O. gorgasi'' have been found in owl pellet deposits. Several other animals are known to prey on ''Oryzomys''. A variety of

s are known from ''O. couesi'' and the marsh rice rat and two parasitic nematodes have been found in ''O. gorgasi''.

Human interactions

Two species of ''Oryzomys'', ''O. antillarum'' and ''O. nelsoni'', have gone extinct since the 19th century, and a third, ''O. peninsulae'', is unlikely to be still extant. Their extinction may have been caused by habitat destruction and by introduced species such as the

small Asian mongoose Small Asian mongoose is a common name applied to two mammals which were formerly considered to be a single species: * Javan mongoose *Small Indian mongoose The small Indian mongoose (''Urva auropunctata'') is a mongoose species native to Iraq an ...

and the brown and black rat. These same causes may threaten ''O. gorgasi'', which the

IUCN Red List The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is the world's most comprehensive inventory of the global conservation status of biol ...

assesses as "

Endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and in ...

". ''O. albiventer'' has been affected by human alteration of its habitat, but likely still survives.Carleton and Arroyo-Cabrales, 2009, p. 115 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. Like these two species, ''O. dimidiatus'' is assessed as "

Least Concern A least-concern species is a species that has been categorized by the International Union for Conservation of Nature (IUCN) as evaluated as not being a focus of species conservation because the specific species is still plentiful in the wild. T ...

" by the Red List. The marsh rice rat is the

natural reservoir In infectious disease ecology and epidemiology, a natural reservoir, also known as a disease reservoir or a reservoir of infection, is the population of organisms or the specific environment in which an infectious pathogen naturally lives and r ...

of the Bayou virus, the second most common cause of

hantavirus pulmonary syndrome Hantavirus pulmonary syndrome (HPS) is one of two potentially fatal syndromes of zoonotic origin caused by species of hantavirus. These include Black Creek Canal virus (BCCV), New York orthohantavirus (NYV), Monongahela virus (MGLV), ''Sin Nomb ...

in the United States. 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.Milazzo et al., 2006; Chu et al., 2008

Notes

References

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