Emmons's Rice Rat
Encyclopedia
Euryoryzomys emmonsae, also known as Emmons's Rice Rat or Emmons' Oryzomys, is a rodent
from Amazonian 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 occurs 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.
Euryoryzomys emmonsae is a relatively large rice rat, weighing 46 to 78 g (1.6 to 2.8 oz), with a distinctly long tail and relatively long, tawny brown fur. The skull is slender and the incisive foramina
(openings in the bone of the palate
) are broad. The animal has 80 chromosome
s and its karyotype
is similar to that of other Euryoryzomys. Its conservation status
is assessed as "Data Deficient
", but deforestation may pose a threat to this species.
, Michael Carleton, Eric Brothers, and Alfred Gardner reviewed the taxonomy
of species previously lumped under "Oryzomys capito" (now classified in the genera Hylaeamys
, Euryoryzomys
, and Transandinomys
). They described the new species Oryzomys emmonsae on the basis of 17 specimens from three locations in the state of Pará
in northern Brazil; these animals had been previously identified as Oryzomys macconnelli (now Euryoryzomys macconnelli) and then as Oryzomys nitidus (now Euryoryzomys nitidus). The specific name honors Louise H. Emmons, who, among other contributions to Neotropical mammalogy, collected three of the known examples of the species in 1986, including the holotype
. The new species was placed in what they termed the “Oryzomys nitidus group", which also included O. macconelli, O. nitidus, and O. russatus.
In 2000, James Patton, Maria da Silva
, and Jay Malcolm reported on mammals collected at the Rio Juruá in western Brazil. In this report, they provided further information on the Oryzomys species reviewed by Musser and colleagues, including sequence
data from the mitochondrial
cytochrome b
gene. Their analysis reaffirmed that O. emmonsae was a distinct species and found that it was closest to O. macconnelli and O. russatus, differing from both by about 12% in the cytochrome b sequence; O. nitidus was more distantly related, differing by 14.7%. The average sequence difference between the three O. emmonsae studied was 0.8%.
In 2006, an extensive morphological and molecular phylogenetic analysis by Marcelo Weksler showed that species then placed in the genus Oryzomys
did not form a single, cohesive (monophyletic
) group; for example, O. macconnelli, O. lamia (placed under O. russatus by Musser and colleagues) and O. russatus clustered together in a single natural group (clade
), but were not closely related to the type species
of Oryzomys, the marsh rice rat
(O. palustris). Later in 2006, Weksler and colleagues described several new genera to accommodate species previously placed in Oryzomys, among which was Euryoryzomys
for the "O. nitidus complex", including O. emmonsae.
Thus, the species is now known as Euryoryzomys emmonsae. As a species of Euryoryzomys, it is classified within the tribe Oryzomyini
("rice rats"), which includes over a hundred species, mainly from South and Central America. Oryzomyini in turn is part of the subfamily Sigmodontinae
of family Cricetidae
, along with hundreds of other species of mainly small rodents.
Compared to E. nitidus and E. macconnelli, the skull is relatively small and slender. It has broad and short incisive foramina
(perforations of the palate
between the incisor
s and the molars
) and lacks sphenopalatine vacuities
which perforate the mesopterygoid fossa, the gap behind the end of the palate. The animal is similar to other members of the genus in the pattern of the arteries
of the head. The alisphenoid strut
, an extension of the alisphenoid bone which separates two foramina
(openings) in the skull (the masticatory-buccinator foramen and the foramen ovale accessorius) is rarely present; its presence is more frequent in E. nitidus. The capsular process
, a raising of the bone of the mandible
(lower jaw) behind the third molar, houses the back end of the lower incisor in most Euryoryzomys, but is absent in E. emmonsae and E. macconnelli. Traits of the teeth are similar to those of E. nitidus and other Euryoryzomys.
The karyotype
includes 80 chromosome
s with a total of 86 major arms (2n = 80; FN = 86). The X chromosome
is subtelocentric (with one pair of long arms and one pair of short arms) and the Y chromosome
is acrocentric (with only one pair of arms, or with a minute second pair). Among the autosome
s (non-sex chromosomes), the four metacentric or submetacentric (with two pairs of arms as long as or not much shorter than the other) pairs of chromosomes are small, and the 35 pairs of acrocentrics range from large to small. Some of those have a minute second pair of arms and could also be classified as subtelocentric, which would raise FN to 90. This karyotype is similar to other known karyotypes of members of Euryoryzomys.
In thirteen specimens measured by Musser, head and body length ranges from 120 to 142 mm (4.7 to 5.6 in), tail length (12 specimens only) from 130 to 160 mm (5.1 to 6.3 in), hindfoot length from 32 to 35 mm (1.3 to 1.4 in), ear length (three specimens only) from 23 to 24 mm (0.91 to 0.94 in), and body mass from 46 to 78 g (1.6 to 2.8 oz).
in the state of Pará, between the Xingu and Tocantins rivers, but the limits of its range remain inadequately known. No other South American rainforest muroid rodent is known to have a similar distribution. Musser and colleagues reported it from three locations and Patton and others added a fourth; in some of those it occurs together with E. macconnelli or Hylaeamys megacephalus.
Specimens of E. emmonsae for which detailed habitat data are available were caught in "viny forest", a microhabitat that often included much bamboo. All were captured on the ground, some in bamboo thickets and another under a log. Musser and colleagues speculated that E. emmonsae may be scansorial, spending time both on the ground and climbing in vegetation, like the similarly long-tailed rice rat Cerradomys subflavus.
" because it is so poorly known. It may be threatened by deforestation and logging, but occurs in at least one protected area
, the Floresta Nacional de Tapirape-Aquiri.
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....
from Amazonian Brazil in the genus Euryoryzomys
Euryoryzomys
Euryoryzomys is a genus of rodents in the tribe Oryzomyini of family Cricetidae. It includes six species, which are distributed in South America. Until 2006, its members were included in the genus Oryzomys, but they are not closely related to the type species of that genus, and therefore they were...
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...
. 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 occurs only in a limited area south of the Amazon River
Amazon River
The Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined...
in the state of Pará
Pará
Pará is a state in the north of Brazil. It borders the Brazilian states of Amapá, Maranhão, Tocantins, Mato Grosso, Amazonas and Roraima. To the northwest it also borders Guyana and Suriname, and to the northeast it borders the Atlantic Ocean. The capital is Belém.Pará is the most populous state...
, a distribution that is apparently unique among the muroid rodents of the region.
Euryoryzomys emmonsae is a relatively large rice rat, weighing 46 to 78 g (1.6 to 2.8 oz), with a distinctly long tail and relatively long, tawny brown fur. The skull is slender and 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:...
(openings in the bone 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...
) are broad. The animal has 80 chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s and its karyotype
Karyotype
A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
is similar to that of other Euryoryzomys. 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 assessed as "Data Deficient
Data Deficient
Data Deficient is a category applied by the IUCN, other agencies, and individuals to a species when the available information is not sufficient for a proper assessment of conservation status to be made...
", but deforestation may pose a threat to this species.
Taxonomy
In 1998, Guy MusserGuy Musser
Guy Graham Musser is an American zoologist. His main research field is the subfamily Murinae, in which he has described many new species.Musser was born in Salt Lake City, Utah...
, Michael Carleton, Eric Brothers, and Alfred Gardner reviewed the taxonomy
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...
of species previously lumped under "Oryzomys capito" (now classified in the genera Hylaeamys
Hylaeamys
Hylaeamys is a genus of South American oryzomyine rodents found principally in humid forested areas east of the Andes. The species in this genus have historically been placed in Oryzomys. They are most closely related to Euryoryzomys, Transandinomys, Nephelomys, Oecomys, and Handleyomys, and most...
, Euryoryzomys
Euryoryzomys
Euryoryzomys is a genus of rodents in the tribe Oryzomyini of family Cricetidae. It includes six species, which are distributed in South America. Until 2006, its members were included in the genus Oryzomys, but they are not closely related to the type species of that genus, and therefore they were...
, and Transandinomys
Transandinomys
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...
). They described the new species Oryzomys emmonsae on the basis of 17 specimens from three locations in the state of Pará
Pará
Pará is a state in the north of Brazil. It borders the Brazilian states of Amapá, Maranhão, Tocantins, Mato Grosso, Amazonas and Roraima. To the northwest it also borders Guyana and Suriname, and to the northeast it borders the Atlantic Ocean. The capital is Belém.Pará is the most populous state...
in northern Brazil; these animals had been previously identified as Oryzomys macconnelli (now Euryoryzomys macconnelli) and then as Oryzomys nitidus (now Euryoryzomys nitidus). The specific name honors Louise H. Emmons, who, among other contributions to Neotropical mammalogy, collected three of the known examples of the species in 1986, including 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...
. The new species was placed in what they termed the “Oryzomys nitidus group", which also included O. macconelli, O. nitidus, and O. russatus.
In 2000, James Patton, Maria da Silva
Maria Nazareth F. da Silva
Maria Nazareth Ferreira da Silva is a zoologist from Manaus, BrazilDa Silva has, for many years, specialised in the study of Amazonian mammals and has described several new species, including the Tefe Spiny Rat and Gardner's Spiny Rat....
, and Jay Malcolm reported on mammals collected at the Rio Juruá in western Brazil. In this report, they provided further information on the Oryzomys species reviewed by Musser and colleagues, including sequence
DNA sequence
The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...
data from the mitochondrial
Mitochondrial DNA
Mitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
cytochrome b
Cytochrome b
Cytochrome b/b6 is the main subunit of transmembrane cytochrome bc1 and b6f complexes. In addition, it commonly refers to a region of mtDNA used for population genetics and phylogenetics.- Function :...
gene. Their analysis reaffirmed that O. emmonsae was a distinct species and found that it was closest to O. macconnelli and O. russatus, differing from both by about 12% in the cytochrome b sequence; O. nitidus was more distantly related, differing by 14.7%. The average sequence difference between the three O. emmonsae studied was 0.8%.
In 2006, an extensive morphological and molecular phylogenetic analysis by Marcelo Weksler showed that species then placed 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...
did not form a single, cohesive (monophyletic
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
) group; for example, O. macconnelli, O. lamia (placed under O. russatus by Musser and colleagues) and O. russatus clustered together in a single natural group (clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
), but were 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...
of Oryzomys, 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...
(O. palustris). Later in 2006, Weksler and colleagues described several new genera to accommodate species previously placed in Oryzomys, among which was Euryoryzomys
Euryoryzomys
Euryoryzomys is a genus of rodents in the tribe Oryzomyini of family Cricetidae. It includes six species, which are distributed in South America. Until 2006, its members were included in the genus Oryzomys, but they are not closely related to the type species of that genus, and therefore they were...
for the "O. nitidus complex", including O. emmonsae.
Thus, the species is now known as Euryoryzomys emmonsae. As a species of Euryoryzomys, it is classified within 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"), which includes over a hundred species, mainly from South and Central America. Oryzomyini in turn is part of 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
Euryoryzomys emmonsae is a fairly large, long-tailed rice rat with long, soft fur. The hairs on the back are 8 to 10 mm (0.3 to 0.4 in) long. It generally resembles E. nitidus in these and other characters, but has a longer tail. E. macconnelli is slightly larger and has longer and duller fur. In E. emmonsae, the upperparts are tawny brown, but a bit darker on the head because many hairs have black tips. The hairs of the underparts are gray at the bases and white at the tips; overall, the fur appears mostly white. In most specimens, there is a patch on the chest where the gray bases are absent. The longest of the vibrissae (whiskers) of the face extend slightly beyond the ears. The eyelids are black. The ears are covered with small, yellowish brown hairs and appear dark brown overall. The feet are covered with white hairs above and brown below. There are six pads on the plantar surface, but the hypothenar is reduced. The ungual tufts, tufts of hair which surround the bases of the claws, are well-developed. The tail is like the body in color above, and mostly white below, but in the 10 mm (0.4 in) nearest the tail tip it is brown below.Compared to E. nitidus and E. macconnelli, the skull is relatively small and slender. It has broad and short 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 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...
between the incisor
Incisor
Incisors are the first kind of tooth in heterodont mammals. They are located in the premaxilla above and mandible below.-Function:...
s and the 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 lacks 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...
which perforate the mesopterygoid fossa, the gap behind the end of the palate. The animal is similar to other members of the genus in the pattern of the arteries
Artery
Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....
of the head. The alisphenoid strut
Alisphenoid strut
In some rodents, the alisphenoid strut is an extension of the alisphenoid bone that separates two foramina in the skull, the masticatory–buccinator foramen and the foramen ovale accessorius...
, an extension of the alisphenoid bone which separates two foramina
Foramen
In anatomy, a foramen is any opening. Foramina inside the body of humans and other animals typically allow muscles, nerves, arteries, veins, or other structures to connect one part of the body with another.-Skull:...
(openings) in the skull (the masticatory-buccinator foramen and the foramen ovale accessorius) is rarely present; its presence is more frequent in E. nitidus. The capsular process
Capsular process
In rodents, the capsular process or projection is a bony capsule that contains the root of the lower incisor. It is visible on the labial side of the mandible as a raising in the bone...
, a raising of the bone of the mandible
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
(lower jaw) behind the third molar, houses the back end of the lower incisor in most Euryoryzomys, but is absent in E. emmonsae and E. macconnelli. Traits of the teeth are similar to those of E. nitidus and other Euryoryzomys.
The karyotype
Karyotype
A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...
includes 80 chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s with a total of 86 major arms (2n = 80; FN = 86). The X chromosome
X chromosome
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...
is subtelocentric (with one pair of long arms and one pair of short arms) and the Y chromosome
Y chromosome
The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
is acrocentric (with only one pair of arms, or with a minute second pair). Among the autosome
Autosome
An autosome is a chromosome that is not a sex chromosome, or allosome; that is to say, there is an equal number of copies of the chromosome in males and females. For example, in humans, there are 22 pairs of autosomes. In addition to autosomes, there are sex chromosomes, to be specific: X and Y...
s (non-sex chromosomes), the four metacentric or submetacentric (with two pairs of arms as long as or not much shorter than the other) pairs of chromosomes are small, and the 35 pairs of acrocentrics range from large to small. Some of those have a minute second pair of arms and could also be classified as subtelocentric, which would raise FN to 90. This karyotype is similar to other known karyotypes of members of Euryoryzomys.
In thirteen specimens measured by Musser, head and body length ranges from 120 to 142 mm (4.7 to 5.6 in), tail length (12 specimens only) from 130 to 160 mm (5.1 to 6.3 in), hindfoot length from 32 to 35 mm (1.3 to 1.4 in), ear length (three specimens only) from 23 to 24 mm (0.91 to 0.94 in), and body mass from 46 to 78 g (1.6 to 2.8 oz).
Distribution and ecology
The known distribution of Euryoryzomys emmonsae is limited to a portion of the Amazon Rainforest south of the Amazon RiverAmazon River
The Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined...
in the state of Pará, between the Xingu and Tocantins rivers, but the limits of its range remain inadequately known. No other South American rainforest muroid rodent is known to have a similar distribution. Musser and colleagues reported it from three locations and Patton and others added a fourth; in some of those it occurs together with E. macconnelli or Hylaeamys megacephalus.
Specimens of E. emmonsae for which detailed habitat data are available were caught in "viny forest", a microhabitat that often included much bamboo. All were captured on the ground, some in bamboo thickets and another under a log. Musser and colleagues speculated that E. emmonsae may be scansorial, spending time both on the ground and climbing in vegetation, like the similarly long-tailed rice rat Cerradomys subflavus.
Conservation status
The IUCN currently lists Euryoryzomys emmonsae as "Data DeficientData Deficient
Data Deficient is a category applied by the IUCN, other agencies, and individuals to a species when the available information is not sufficient for a proper assessment of conservation status to be made...
" because it is so poorly known. It may be threatened by deforestation and logging, but occurs in at least one protected area
Protected area
Protected areas are locations which receive protection because of their recognised natural, ecological and/or cultural values. There are several kinds of protected areas, which vary by level of protection depending on the enabling laws of each country or the regulations of the international...
, the Floresta Nacional de Tapirape-Aquiri.
Literature cited
- Musser, G.G., Carleton, M.D., Brothers, E.M. and Gardner, A.L. 1998. Systematic studies of oryzomyine rodents (Muridae: Sigmodontinae): diagnoses and distributions of species formerly assigned to Oryzomys "capito". Bulletin of the American Museum of Natural History 236:1–376.
- 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
- Patton, J.L., da Silva, M.N.F. and Malcolm, J.R. 2000. Mammals of the Rio Juruá and the evolutionary and ecological diversification of Amazonia. Bulletin of the American Museum of Natural History 244:1–306.
- Percequillo, A., Wecksler, M. and Patton, J.L. 2008. . In IUCN. IUCN Red List of Threatened Species. Version 2009.2. <www.iucnredlist.org>. Downloaded on March 14, 2010.
- Weksler, M. 2006. Phylogenetic relationships of oryzomyine rodents (Muroidea: Sigmodontinae): separate and combined analyses of morphological and molecular data. Bulletin of the American Museum of Natural History 296:1–149.
- Weksler, M., Percequillo, A.R. and Voss, R.S. 2006. Ten new genera of oryzomyine rodents (Cricetidae: Sigmodontinae). American Museum Novitates 3537:1–29.