Thomas's Rice Rat
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''Oryzomys dimidiatus'', also known as the Nicaraguan oryzomys, Thomas's rice rat, or the Nicaraguan rice rat, is a
rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and Mandible, lower jaws. About 40% of all mammal specie ...
in 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 over 870 species, it is either the largest or second-largest family ...
. It is known from only three specimens, all collected in southeastern Nicaragua since 1904. Placed in ''
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, wh ...
'' upon its discovery, it was later classified in its own
subgenus In biology, a subgenus ( subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the ge ...
of ''
Oryzomys ''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 ...
'' and finally recognized as closely related to other species now placed in ''Oryzomys'', including the
marsh rice rat The marsh rice rat (''Oryzomys palustris'') 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, f ...
and '' Coues' rice rat'', which occurs in the same region. With a head and body length of 110 to 128 mm (4.3 to 5.0 in), ''Oryzomys dimidiatus'' is a medium-sized
rice rat 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 S ...
. The upperparts are gray-brown and the underparts are grayish, not buffy as in ''O. couesi''. The tail is only slightly darker above than below. All three specimens were caught near water and the species may be
semiaquatic In biology, being semi-aquatic refers to various macroorganisms that live regularly in both aquatic and terrestrial environments. When referring to animals, the term describes those that actively spend part of their daily time in water (in ...
, spending some time in the water. Its
conservation status The conservation status of a group of organisms (for instance, a species) indicates whether the group still exists and how likely the group is to become extinct in the near future. Many factors are taken into account when assessing conservation ...
is currently assessed as "
Data Deficient A data deficient (DD) species is one which has been categorized by the International Union for Conservation of Nature (IUCN) as offering insufficient information for a proper assessment of conservation status to be made. This does not necessaril ...
".


Taxonomy

The first known specimen was obtained by W.G. Palmer in 1904Thomas, 1905, p. 587 and the next year,
Oldfield Thomas Michael Rogers Oldfield Thomas (21 February 1858 – 16 June 1929) was a British zoologist. Career Thomas worked at the Natural History Museum, London, Natural History Museum on mammals, describing about 2,000 new species and subspecies for ...
of the
British Museum of Natural History The Natural History Museum in London is a museum that exhibits a vast range of specimens from various segments of natural history. It is one of three major museums on Exhibition Road in South Kensington, the others being the Science Museum (Lo ...
described this animal as the
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
of a new
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
he named ''Nectomys dimidiatus''. He placed it in the genus ''
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, wh ...
'', commenting that it was much smaller than, but otherwise similar to previously known members of that genus. The species was listed as a ''Nectomys'' in
taxonomic 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme of classes (a taxonomy) and the allocation ...
overviews in the next decades, including a 1944 review of the genus by
Philip Hershkovitz Philip Hershkovitz (12 October 1909 – 15 February 1997) was an American mammalogy, mammalogist. Born in Pittsburgh, he attended the Universities of Pittsburgh and Michigan and lived in South America collecting mammals. In 1947, he was appointed ...
.Hershkovitz, 1944, p. 80 After examining the holotype in London, Hershkovitz instead placed the species in the genus ''
Oryzomys ''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 ...
'' in 1948. He remarked that it was an especially distinctive member of that genus, and hence classified it in its own
subgenus In biology, a subgenus ( subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the ge ...
''Micronectomys''.Hershkovitz, 1948, pp. 54–55 J. Hernández-Camacho described a second species of ''Micronectomys'', ''Oryzomys (Micronectomys) borreroi'', from Colombia in 1957.Hershkovitz, 1970, p. 792 In 1970, Hershkovitz treated ''O. dimidiatus'' in another publication and conceded that his name ''Micronectomys'' was a ''
nomen nudum In Taxonomy (biology), taxonomy, a ''nomen nudum'' ('naked name'; plural ''nomina nuda'') is a designation which looks exactly like a scientific name of an organism, and may have originally been intended to be one, but it has not been published ...
'' ("naked name") because he had not explicitly mentioned characters differentiating it from other taxa in his 1948 publication.Hershkovitz, 1970, p. 791 Nevertheless, he did not do anything to rectify the situation, and ''Micronectomys'' remains a ''nomen nudum''.Musser and Carleton, 2005, p. 1144 Hershkovitz also noted that while ''O. dimidiatus'' resembles a juvenile ''Nectomys'' in external anatomy, it is otherwise similar to the
marsh rice rat The marsh rice rat (''Oryzomys palustris'') 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, f ...
(''Oryzomys palustris''). He accepted ''O. borreroi'' as an ''Oryzomys'', but did not think it closely related to ''O. dimidiatus''. Six years later, Alfred Gardner and James Patton instead suggested ''O. borreroi'' was a ''
Zygodontomys ''Zygodontomys'' is a genus of rodent in the tribe Oryzomyini of the family Cricetidae. Its closest relative may be '' Scolomys''. It ranges from Central America east to the Guianas The Guianas, also spelled Guyanas or Guayanas, are a geog ...
'', and in his 1991 review of that genus Robert Voss confirmed that it is the same as ''
Zygodontomys brunneus ''Zygodontomys brunneus'', also known as the brown zygodont, brown cane mouse, or Colombian cane mouse,Duff and Lawson, 2004 is a rodent species in the genus ''Zygodontomys'' of tribe Oryzomyini. It is found only in Colombia Colombia, o ...
''.Voss, 1991, p. 46 A second specimen was obtained in 1966 and the find was published in 1971 by Hugh Genoways and Knox Jones, who noted that the species is closely similar to ''O. palustris''.Genoways and Jones, 1971, p. 433 Later workers affirmed the relationship between ''O. dimidiatus'', ''O. palustris'' and associated species like '' O. couesi''. Fiona Reid reported in 1997 that a third specimen had been found.Reid, 1997, p. 204 In 2006, Marcelo Weksler and coworkers removed most of the species formerly placed in ''Oryzomys'' from the genus, because they are not closely related to the
type species In International_Code_of_Zoological_Nomenclature, 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 spe ...
''O. palustris'', but kept ''O. dimidiatus'' as an ''Oryzomys''.Weksler et al., 2006, table 1, footnote e ''Oryzomys dimidiatus'' is now recognized as one of eight species in the genus ''Oryzomys''.Carleton and Arroyo-Cabrales, 2009, p. 116 ''O. dimidiatus'' is further part of the ''O. couesi'' section, which is centered on the widespread Central American ''O. couesi'' and also includes six other species with more limited and peripheral distributions.Carleton and Arroyo-Cabrales, 2009, p. 117 ''O. couesi'' occurs with ''O. dimidiatus'' in southeastern Nicaragua.Jones and Engstrom, 1986, p. 12 Many aspects of the
systematics Systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: phylogenetic trees, phylogenies). Phy ...
of the ''O. couesi'' section remain unclear and it is likely that the current classification underestimates the true diversity of the group.Carleton and Arroyo-Cabrales, 2009, p. 107 ''Oryzomys'' is classified 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 S ...
("rice rats"), a diverse assemblage of American rodents of over a hundred species,Weksler, 2006, p. 3 and on higher taxonomic levels in 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 species ...
of 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 over 870 species, it is either the largest or second-largest family ...
, along with hundreds of other species of mainly small rodents.Musser and Carleton, 2005 ''Oryzomys dimidiatus'' or "divided in the middle rice rat" derives respectively from the
Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
''oryza'' "rice", ''mys'' "mouse, rat", and the
Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
''dimidiatus'' "divided in the middle", from ''dimidio'' "halve, divide in two equal parts".


Description

''Oryzomys dimidiatus'' is a medium-sized rice rat, smaller than ''O. couesi'', with thick, lustrous fur and velvety underfur. The hairs on the back are about 6 mm in length. The upperparts are gray-brown with some dark hairs, appearing darker overall than in ''O. couesi''; the color becomes more yellowish towards the sides. According to Thomas, a faint
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 wr ...
line extends from the sides to the inner sides of the hindlegs. The underparts are grayish, contrasting with the buffy underparts of ''O. couesi''. The snout is short and the well-haired ears are partly hidden by the fur. The hands and feet are off-white or brownish above, not white as in ''O. couesi''. The hindfeet show small interdigital webs, but they lack long tufts of hair on the digits and some of the
pads Pads (also called leg guards) are a type of protective equipment used in a number of sports and serve to protect the legs from the impact of a hard ball, puck, or other object of play travelling at high speed which could otherwise cause injuries t ...
are reduced or absent. The tail is about as long as the head and body and contains about 15 rings per centimeter. It is slightly darker (grayish) above than below (whitish), but the difference in color is much less pronounced than in ''O. couesi''. Compared to that of ''Nectomys'', the skull is lightly built and has narrow nasals and a broad, round
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
without conspicuous ridges on it. 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 ...
is broad. The
incisive foramina In the human mouth, the incisive foramen (also known as: "''anterior palatine foramen''", or "''nasopalatine foramen''") is the opening of the incisive canals on the hard palate immediately behind the incisor teeth. It gives passage to blood vesse ...
(perforations of the front part 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 sep ...
) extend between the first
molars The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...
and are broadest in their back halves. The broad mesopterygoid fossa, the gap behind the end of the palate, is perforated by
sphenopalatine vacuities In rodents, sphenopalatine vacuities are perforations of the roof of the mesopterygoid fossa, the open space behind the palate, in between the parapterygoid fossae. They may perforate the presphenoid or basisphenoid bone. Their development and ...
. The molar crowns are not as simplified as in ''Nectomys'' species, but the front cusps of the upper ( anterocone) and lower first molar (
anteroconid Many different terms have been proposed for features of the tooth crown in mammals. The structures within the molars receive different names according to their position and morphology. This nomenclature was developed by Henry Fairfield Osborn i ...
) are not divided in two. In addition to the main roots, the upper and lower first molars have smaller additional roots. Measurements for the first two known examples are as follows (in each case, the first measurement given is from the
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
, taken in 1904, the second, from the specimen taken in 1966): head and body length 125 and 118 mm (4.9 and 4.6 in), tail length 115 and 110 mm (4.5 and 4.3 in), hindfoot length 27 and 28 mm (1.1 and 1.1 in), ear length 13 and 15 mm (0.51 and 0.59 in), skull length 29.8 and 29.0 mm (1.17 and 1.14 in). The 1966 specimen weighed 46.0 g (1.62 oz) and had testes 11 mm (0.43 in) long; these measurements were not recorded in the 1904 specimen.Jones and Engstrom, 1986, p. 13 Reid, who mentioned the third specimen, reported a maximum head and body length of 128 mm (5.0 in), tail length of 150 mm (5.9 in), hindfoot length of 31 mm (1.2 in), and ear length of 19 mm (0.75 in).Reid, 2009, p. 207


Distribution, ecology, and behavior

''Oryzomys dimidiatus'' is known from three specimens collected in the lowlands of the
South Caribbean Coast Autonomous Region The South Caribbean Coast Autonomous Region is one of two autonomous regions in Nicaragua. It was created along with the North Caribbean Coast Autonomous Region by the Autonomy Statute of 7 September 1987 through a division of the former Zelay ...
in southeastern Nicaragua. The first, an old male, was collected on November 5, 1904, in a banana plantation with very moist red clay on the
Río Escondido Rio or Río is the Portuguese and Spanish word for "river". The word also exists in Italian, but is largely obsolete and used in a poetical or literary context to mean "stream". Rio, RIO or Río may also refer to: Places United States * Rio, Fl ...
near
El Rama Rama is a municipality and a city in the South Caribbean Coast Autonomous Region of Nicaragua.
Inifom ...
. The second, a young adult male, was caught on July 26, 1966, in dense cane on the south bank of the Río Mico at El Recreo, 15 km (9 mi) west from the location of the first specimen, along with three other rice rats (''O. couesi'', ''
Melanomys caliginosus ''Melanomys caliginosus'', also known as the dusky melanomysMusser and Carleton, 2005 or dusky rice rat, is a species of rodent in the genus '' Melanomys'' of family Cricetidae. It is found from Central America, in Honduras, Nicaragua, Costa Rica ...
'', and ''
Oligoryzomys fulvescens ''Oligoryzomys'' is a genus of rodents in the tribe Oryzomyini of family Cricetidae. Many species are known as pygmy rice rats or colilargos.Musser and Carleton, 2005 The genus is found from Mexico to Tierra del Fuego and includes approximately 1 ...
''), the cotton rat '' Sigmodon hirsutus'', and the cottontail rabbit ''
Sylvilagus brasiliensis The common tapeti (''Sylvilagus brasiliensis''), also known as the Brazilian cottontail, forest cottontail, or (formerly) simply tapeti is a species of cottontail rabbit. It is small to medium-sized with a small, dark tail, short hind feet, and s ...
''.Genoways and Jones, 1971, p. 833; Musser and Carleton, 2005, pp. 1125, 1141, 1147, 1175 The third was caught at a stream near
Bluefields Bluefields is the capital of the South Caribbean Autonomous Region in Nicaragua. It was also the capital of the former Kingdom of Mosquitia, and later the Zelaya Department, which was divided into North and South Caribbean Coast Autonomous Reg ...
. Reid suggested that the species is
semiaquatic In biology, being semi-aquatic refers to various macroorganisms that live regularly in both aquatic and terrestrial environments. When referring to animals, the term describes those that actively spend part of their daily time in water (in ...
, spending some time in the water, like other ''Oryzomys''.Carleton and Arroyo-Cabrales, 2009, p. 114 The 2019
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 an inventory of the global conservation status and extinction risk of biological ...
assessed the conservation status of ''Oryzomys dimidiatus'' as "
Data Deficient A data deficient (DD) species is one which has been categorized by the International Union for Conservation of Nature (IUCN) as offering insufficient information for a proper assessment of conservation status to be made. This does not necessaril ...
", noting that very little is currently known about the species.


References


Literature cited

* * * * * * * * * * * * * * * {{Taxonbar, from=Q2890164 Endemic fauna of Nicaragua Mammals described in 1905 Oryzomys Rodents of Central America Taxa named by Oldfield Thomas Taxonomy articles created by Polbot