Nicaraguan Rice Rat
Encyclopedia
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.
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
. 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, spending some time in the water. Its conservation status
is currently assessed as "Least Concern
".
of the British Museum of Natural History described this animal as the holotype
of a new species
he named Nectomys dimidiatus. He placed it in the genus Nectomys
, 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
overviews in the next decades, including a 1944 review of the genus by Philip Hershkovitz
.
After examining the holotype in London, Hershkovitz instead placed the species in the genus Oryzomys
in 1948. He remarked that it was an especially distinctive member of that genus, and hence classified it in its own subgenus
Micronectomys. J. Hernández-Camacho described a second species of Micronectomys, Oryzomys (Micronectomys) borreroi, from Colombia in 1957. In 1970, Hershkovitz treated O. dimidiatus in another publication and conceded that his name Micronectomys was a nomen nudum
("naked name") because he had not explicitly mentioned characters differentiating it from other taxa in his 1948 publication. Nevertheless, he did not do anything to rectify the situation, and Micronectomys remains a nomen nudum. Hershkovitz also noted that while O. dimidiatus resembles a juvenile Nectomys in external anatomy, it is otherwise similar to the marsh rice rat
(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 that O. borreroi was a Zygodontomys
, and in his 1991 review of that genus Robert Voss confirmed that it is the same as Zygodontomys brunneus.
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. 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. 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
O. palustris, but kept O. dimidiatus as an Oryzomys.
Oryzomys dimidiatus is now recognized as one of eight species in the genus Oryzomys. O. dimidiatus is further part of the O. couesi section, which is centered around the widespread Central American O. couesi and also includes six other species with more limited and peripheral distributions. O. couesi occurs with O. dimidiatus in southeastern Nicaragua. Many aspects of the systematics
of the O. couesi section remain unclear and it is likely that the current classification underestimates the true diversity of the group. Oryzomys is classified in the tribe Oryzomyini
("rice rats"), a diverse assemblage of American rodents of over a hundred species, and on higher taxonomic levels in the subfamily Sigmodontinae
of family Cricetidae
, along with hundreds of other species of mainly small rodents.
, but they lack long tufts of hair
on the digits and some of the pads 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 without conspicuous ridges on it. The zygomatic plate
is broad. The incisive foramina
(perforations of the front part of the palate
) extend between the first molars
and are broadest in their back halves. The broad mesopterygoid fossa, the gap behind the end of the palate, is perforated by sphenopalatine vacuities
. The molar crowns are not as simplified as in Nectomys species, but the front cusps of the upper (anterocone) and lower first molar (anteroconid) 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
, 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. 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).
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 near El Rama
. 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, and Oligoryzomys fulvescens), the cotton rat Sigmodon hirsutus, and the cottontail rabbit Sylvilagus brasiliensis. The third was caught at a stream near Bluefields
. Reid suggested that the species is semiaquatic, spending some time in the water, like other Oryzomys.
assessed the conservation status of Oryzomys dimidiatus as "Least Concern
", noting that its distribution may be larger than currently known. Its population is presumed to be large and no threats to its habitat are known.
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
in the genus Oryzomys
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 of the United States and O. couesi of Mexico and Central America—are widespread; the six others have...
of the family Cricetidae
Cricetidae
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...
. It is known from only three specimens, all collected in southeastern Nicaragua since 1904. Placed in Nectomys
Nectomys
Nectomys is a genus of rodent in the tribe Oryzomyini of family Cricetidae. It is closely related to Amphinectomys and was formerly considered congeneric with Sigmodontomys...
upon its discovery, it was later classified in its own subgenus
Subgenus
In biology, a subgenus is a taxonomic rank directly below genus.In zoology, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the generic name and the specific epithet: e.g. the Tiger Cowry of the Indo-Pacific, Cypraea tigris Linnaeus, which...
of Oryzomys
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 of the United States and O. couesi of Mexico and Central America—are widespread; the six others have...
and finally recognized as closely related to other species now placed in Oryzomys, including the marsh rice rat
Marsh Rice Rat
The marsh rice rat is a semiaquatic North American rodent in the family Cricetidae. 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...
and Oryzomys couesi, 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
Oryzomyini 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...
. 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, spending some time in the water. Its conservation status
Conservation status
The conservation status of a group of organisms indicates whether the group is still extant and how likely the group is to become extinct in the near future...
is currently assessed as "Least Concern
Least Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
".
Taxonomy
The first known specimen was obtained by W.G. Palmer in 1904 and the next year, Oldfield ThomasOldfield Thomas
Oldfield Thomas FRS was a British zoologist.Thomas worked at the Natural History Museum on mammals, describing about 2,000 new species and sub-species for the first time. He was appointed to the Museum Secretary's office in 1876, transferring to the Zoological Department in 1878...
of the British Museum of Natural History described this animal as the holotype
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
of a new species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
he named Nectomys dimidiatus. He placed it in the genus Nectomys
Nectomys
Nectomys is a genus of rodent in the tribe Oryzomyini of family Cricetidae. It is closely related to Amphinectomys and was formerly considered congeneric with Sigmodontomys...
, 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
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
overviews in the next decades, including a 1944 review of the genus by Philip Hershkovitz
Philip Hershkovitz
Philip Hershkovitz was an American mammalogist. Born in Pittsburgh, he attended the Universities of Pittsburgh and Michigan and lived in South America collecting mammals. In 1947, he was appointed a curator at the Field Museum of Natural History in Chicago and he continued to work there until his...
.
After examining the holotype in London, Hershkovitz instead placed the species in the genus Oryzomys
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 of the United States and O. couesi of Mexico and Central America—are widespread; the six others have...
in 1948. He remarked that it was an especially distinctive member of that genus, and hence classified it in its own subgenus
Subgenus
In biology, a subgenus is a taxonomic rank directly below genus.In zoology, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the generic name and the specific epithet: e.g. the Tiger Cowry of the Indo-Pacific, Cypraea tigris Linnaeus, which...
Micronectomys. J. Hernández-Camacho described a second species of Micronectomys, Oryzomys (Micronectomys) borreroi, from Colombia in 1957. In 1970, Hershkovitz treated O. dimidiatus in another publication and conceded that his name Micronectomys was a nomen nudum
Nomen nudum
The phrase nomen nudum is a Latin term, meaning "naked name", used in taxonomy...
("naked name") because he had not explicitly mentioned characters differentiating it from other taxa in his 1948 publication. Nevertheless, he did not do anything to rectify the situation, and Micronectomys remains a nomen nudum. Hershkovitz also noted that while O. dimidiatus resembles a juvenile Nectomys in external anatomy, it is otherwise similar to the marsh rice rat
Marsh Rice Rat
The marsh rice rat is a semiaquatic North American rodent in the family Cricetidae. 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...
(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 that O. borreroi was a Zygodontomys
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. It contains two species: Zygodontomys brunneus and Zygodontomys brevicauda....
, and in his 1991 review of that genus Robert Voss confirmed that it is the same as Zygodontomys brunneus.
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. 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. 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
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
O. palustris, but kept O. dimidiatus as an Oryzomys.
Oryzomys dimidiatus is now recognized as one of eight species in the genus Oryzomys. O. dimidiatus is further part of the O. couesi section, which is centered around the widespread Central American O. couesi and also includes six other species with more limited and peripheral distributions. O. couesi occurs with O. dimidiatus in southeastern Nicaragua. Many aspects of the systematics
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...
of the O. couesi section remain unclear and it is likely that the current classification underestimates the true diversity of the group. Oryzomys is classified in the tribe Oryzomyini
Oryzomyini
Oryzomyini 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...
("rice rats"), a diverse assemblage of American rodents of over a hundred species, and on higher taxonomic levels in the subfamily Sigmodontinae
Sigmodontinae
The subfamily Sigmodontinae is one of the most diverse groups of mammals. It 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...
of family Cricetidae
Cricetidae
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...
, along with hundreds of other species of mainly small rodents.
Description
Oryzomys dimidiatus is a medium-sized rice rat, smaller than O. couesi, with thick, glossy 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 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 websInterdigital webbing
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...
, but they lack long tufts of hair
Ungual tuft
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....
on the digits and some of the pads 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 without conspicuous ridges on it. The zygomatic plate
Zygomatic plate
In rodent anatomy, the zygomatic plate is a bony plate derived from the flattened front part of the zygomatic arch . At the back, it connects to the front root of the zygomatic arch, and at the top it is connected to the rest of the skull via the antorbital bridge. It is part of the maxillary...
is broad. The incisive foramina
Incisive foramen
The fossa incisiva is an opening in the bone of the oral hard palate where blood vessels and nerves may pass. There are four of these openings in the incisive fossa.-Formation:...
(perforations of the front part of the palate
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 separate. The palate is divided into two parts, the anterior...
) extend between the first molars
Molar (tooth)
Molars are the rearmost and most complicated kind of tooth in most mammals. In many mammals they grind food; hence the Latin name mola, "millstone"....
and are broadest in their back halves. The broad mesopterygoid fossa, the gap behind the end of the palate, is perforated by sphenopalatine vacuities
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...
. The molar crowns are not as simplified as in Nectomys species, but the front cusps of the upper (anterocone) and lower first molar (anteroconid) 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
Holotype
A holotype is a single physical example of an organism, known to have been used when the species was formally described. It is either the single such physical example or one of several such, but explicitly designated as the holotype...
, 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. 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).
Distribution, ecology, and behavior
Oryzomys dimidiatus is known from three specimens collected in the lowlands of the Región Autónoma del Atlántico SurRegión Autónoma del Atlántico Sur
Región Autónoma del Atlántico Sur , sometimes shortened to RAAS, is one of two autonomous regions in Nicaragua. It covers an area of 27,407 km² and has a population of 382,100...
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 near El Rama
El Rama
El Rama is a municipality in the Región Autónoma del Atlántico Sur department of Nicaragua. It is along the Escondido River which branches off into three other rivers: the Sumi, Rama, and Escondido. It is home to an important Atlantic/Caribbean port. Its population is approximately 50,000...
. 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, and Oligoryzomys fulvescens), the cotton rat Sigmodon hirsutus, and the cottontail rabbit Sylvilagus brasiliensis. The third was caught at a stream near Bluefields
Bluefields
Bluefields is the capital of the municipality of the same name, and of Región Autónoma del Atlántico Sur in Nicaragua. It was also capital of the former Zelaya Department, which was divided into North and South Atlantic Autonomous Regions...
. Reid suggested that the species is semiaquatic, spending some time in the water, like other Oryzomys.
Conservation status
The 2009 IUCN Red ListIUCN Red List
The IUCN Red List of Threatened Species , founded in 1963, is the world's most comprehensive inventory of the global conservation status of biological species. The International Union for Conservation of Nature is the world's main authority on the conservation status of species...
assessed the conservation status of Oryzomys dimidiatus as "Least Concern
Least Concern
Least Concern is an IUCN category assigned to extant taxon or lower taxa which have been evaluated but do not qualify for any other category. As such they do not qualify as threatened, Near Threatened, or Conservation Dependent...
", noting that its distribution may be larger than currently known. Its population is presumed to be large and no threats to its habitat are known.
Literature cited
- Carleton, M.D. and Arroyo-Cabrales, J. 2009. Review of the Oryzomys couesi complex (Rodentia: Cricetidae: Sigmodontinae) in western Mexico. Bulletin of the American Museum of Natural History 331:94–127.
- Duff, A. and Lawson, A. 2004. Mammals of the World: A checklist. New Haven, Connecticut: Yale University Press, 312 pp. ISBN 071366021X
- Genoways, H.H. and Jones, J.K., Jr. 1971. Second specimen of Oryzomys dimidiatus. Journal of Mammalogy 52:833–834.
- Hershkovitz, P. 1944. A systematic review of the Neotropical water rats of the genus Nectomys (Cricetinae). Miscellaneous Publications, Museum of Zoology, University of Michigan 58:1–88.
- Hershkovitz, P. 1948. Mammals of northern Colombia. Preliminary report No. 3: Water rats (genus Nectomys), with supplemental notes on related forms. Proceedings of the United States National Museum 98:49–56.
- Hershkovitz, P. 1970. Supplementary notes on Neotropical Oryzomys dimidiatus and Oryzomys hammondi (Cricetinae) (subscription required). Journal of Mammalogy 51(4):789–794.
- Jones, J.K., Jr. and Engstrom, M.D. 1986. Synopsis of the rice rats (genus Oryzomys) of Nicaragua. Occasional Papers, The Museum, Texas Tech University 103:1–23.
- Musser, 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 reference. 3rd ed. Baltimore: The Johns Hopkins University Press, 2 vols., 2142 pp. ISBN 978-0-8018-8221-0
- Reid, F. 1997. A Field Guide to the Mammals of Central America and Southeast Mexico. 1st edition. Oxford University Press US, 334 pp. ISBN 9780195064018
- Reid, F. 2009. A Field Guide to the Mammals of Central America and Southeast Mexico. 2nd edition. Oxford University Press US, 346 pp. ISBN 9780195343229
- Sánchez H., J., Ochoa G., J. and Voss, R.S. 2001. Rediscovery of Oryzomys gorgasi (Rodentia: Muridae) with notes on taxonomy and natural history (subscription required). Mammalia 65:205–214.
- Thomas, O. 1905. New Neotropical Molossus, Conepatus, Nectomys, Proechimys, and Agouti, with a note on the genus Mesomys. Annals and Magazine of Natural History (7)15:584–591.
- Timm, R. and Reid, F. 2008. . In IUCN. IUCN Red List of Threatened Species. Version 2009.2. <www.iucnredlist.org>. Downloaded on March 23, 2010.
- Voss, R.S. 1991. An introduction to the neotropical muroid rodent genus Zygodontomys. Bulletin of the American Museum of Natural History 210:1–113.
- Weksler, M., Percequillo, A.R. and Voss, R.S. 2006. Ten new genera of oryzomyine rodents (Cricetidae: Sigmodontinae). American Museum Novitates 3537:1–29.