Oryzomys peninsulae
Encyclopedia
Oryzomys peninsulae, also known as the Lower California Rice Rat, is a species of rodent
from western Mexico. Restricted to the southern tip of the Baja California peninsula
, it is a member of the genus Oryzomys
of family Cricetidae
. Only about twenty individuals, collected around 1900, are known, and subsequent destruction of its riverine habitat may have driven the species to extinction
.
Medium in size for its genus, it was first described as a separate species, but later lumped into other widespread species until it was reinstated as separate in 2009. It is distinctive in fur color—grayish brown on the forequarters and reddish brown on the hindquarters—and in some dimensions of its skull, with a high braincase, robust zygomatic arch
es (cheekbones), and long incisive foramina
(perforations of the palate
between the incisor
s and the molars
).
described it in 1897 as a full species of Oryzomys
. It was retained as a distinct species related to O. couesi and O. palustris until 1971, when Philip Hershkovitz
swept it, and other outlying populations of the same species group, as subspecies under an expanded concept of O. palustris. Raymond Hall concurred in the second edition (1981) of Mammals of North America, arguing that O. peninsulae differed less from mainland Oryzomys populations (currently classified as O. couesi mexicanus) than some other forms he included in O. palustris differed from each other. After studies of the contact zone between North American O. palustris and Central American O. couesi in southern Texas and northeastern Tamaulipas
(by Benson and Gehlbach in 1979 and Schmidt and Engstrom in 1994) made clear that the two are distinct from each other, O. peninsulae remained as a subspecies
of O. couesi. In 2009, Michael Carleton and Joaquín Arroyo-Cabrales reviewed the classification of western Mexican Oryzomys and used morphological
and morphometrical data to characterize four distinct Oryzomys species in the region. O. peninsulae and another isolated population, O. nelsoni from the Islas Marías
, were both retained as separate species, as was O. albiventer
from montane
mainland Mexico. They kept the population in the coastal lowlands as a subspecies, O. couesi mexicanus, of Oryzomys couesi.
The genus Oryzomys currently includes about eight species distributed from the eastern United States (O. palustris) into northwestern South America (O. gorgasi
). O. peninsulae is part of the O. couesi section, which is centered around the widespread Central American O. couesi and also includes various other species with more limited and peripheral distributions. Many aspects of the systematics
of this section remain unclear and it is likely that the current classification underestimates the group's true diversity. Oryzomys was previously a much larger genus, but most species were progressively removed in various studies, culminating in contributions by Marcelo Weksler and coworkers in 2006 that excluded more than forty species from the genus. Oryzomys and many of the species removed from it are 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.
In the skull
, the braincase is high, the zygomatic arch
es (cheekbones) are broad and squared, and the incisive foramina
, which perforate the palate
between the incisor
s and the molars
, are long and broad. The upper incisors are orthodont, with their cutting edge nearly vertical. Morphometrically, the skull of O. peninsulae is sharply distinct from other western Mexican Oryzomys.
In fourteen specimens measured by Carleton and Arroyo-Cabrales, total length was 227 to 305 mm (8.9 to 12.0 in), averaging 265.6 mm (10.46 in); head and body length was 113 to 152 mm (4.4 to 6.0 in), averaging 128.9 mm (5.07 in); tail length was 114 to 156 mm (4.5 to 6.1 in), averaging 136.8 mm (5.39 in); hindfoot length was 29 to 34 mm (1.1 to 1.3 in), averaging 32.0 mm (1.26 in); and occipitonasal length (a measure of total skull length) was 27.8 to 34.3 mm (1.09 to 1.35 in), averaging 31.5 mm (1.24 in).
in 1896 by D. Coolidge, and Edward William Nelson
and Edward Alphonso Goldman
obtained fifteen additional individuals in 1906 at San José del Cabo
. The two localities, which are about 13 km (8 mi) apart, were both located along the Río San José, a river in southernmost Baja California Sur
, near the southern tip of the Baja California peninsula
. Like other Oryzomys, O. peninsulae is a semiaquatic species, spending much of its time in the water, but suitable habitat for such a species is estimated to be no more than 13 km2 (5.0 sq mi) on the arid Baja California peninsula.
Río San José no longer exists, having fallen prey to irrigation projects, and touristic development of its estuary has resulted in pollution. Biologists working in the area in 1979 and from 1991 to 1993 failed to find O. peninsulae, casting doubt on its continued existence. The lack of records for over a century, small distribution, and destruction of the only known habitat led Carleton and Arroyo-Cabrales to consider the conservation status
of O. peninsulae as "critically endangered, if not extinct". They noted that the status of the species had previously been obscured because it had been lumped for decades under O. couesi, a widely distributed and secure species.
from another part of Mexico in a shipment of farm products, but this hypothesis is disproved by the clear differentiation from other western Mexican Oryzomys that the species exhibits.
The species's range may be relict
ual in nature: although Oryzomys currently does not extend further north along the eastern coast of the Gulf of California
than coastal southern Sonora
, the past distribution of the genus may have extended further north along the shores of the Gulf of California, perhaps even into the southwestern United States, and from there south into the Baja California peninsula. When Oryzomys disappeared from the northern regions, O. peninsulae would have become isolated on the peninsula. This possibility is supported by the relatively close resemblance between O. peninsulae and O. couesi mexicanus, from coastal western Mexico.
Alternatively, the ancestor of Oryzomys peninsulae may have arrived during the late Miocene
, about six million years ago, when the southern tip of the Baja California peninsula was an island located near what is now Nayarit
and Jalisco
in western Mexico. Some plants and birds from the area may have a similar biogeographic
heritage.
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 western Mexico. Restricted to the southern tip of the Baja California peninsula
Baja California Peninsula
The Baja California peninsula , is a peninsula in northwestern Mexico. Its land mass separates the Pacific Ocean from the Gulf of California. The Peninsula extends from Mexicali, Baja California in the north to Cabo San Lucas, Baja California Sur in the south.The total area of the Baja California...
, it is a member of 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 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...
. Only about twenty individuals, collected around 1900, are known, and subsequent destruction of its riverine habitat may have driven the species to extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
.
Medium in size for its genus, it was first described as a separate species, but later lumped into other widespread species until it was reinstated as separate in 2009. It is distinctive in fur color—grayish brown on the forequarters and reddish brown on the hindquarters—and in some dimensions of its skull, with a high braincase, robust zygomatic arch
Zygomatic arch
The zygomatic arch or cheek bone is formed by the zygomatic process of temporal bone and the temporal process of the zygomatic bone , the two being united by an oblique suture; the tendon of the Temporalis passes medial to the arch to gain insertion into the coronoid process...
es (cheekbones), and long 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"....
).
Taxonomy
Oryzomys peninsulae was first collected in 1896 and 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...
described it in 1897 as a full species 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...
. It was retained as a distinct species related to O. couesi and O. palustris until 1971, when 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...
swept it, and other outlying populations of the same species group, as subspecies under an expanded concept of O. palustris. Raymond Hall concurred in the second edition (1981) of Mammals of North America, arguing that O. peninsulae differed less from mainland Oryzomys populations (currently classified as O. couesi mexicanus) than some other forms he included in O. palustris differed from each other. After studies of the contact zone between North American O. palustris and Central American O. couesi in southern Texas and northeastern Tamaulipas
Tamaulipas
Tamaulipas officially Estado Libre y Soberano de Tamaulipas is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 43 municipalities and its capital city is Ciudad Victoria. The capital city was named after Guadalupe Victoria, the...
(by Benson and Gehlbach in 1979 and Schmidt and Engstrom in 1994) made clear that the two are distinct from each other, O. peninsulae remained as a subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
of O. couesi. In 2009, Michael Carleton and Joaquín Arroyo-Cabrales reviewed the classification of western Mexican Oryzomys and used morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
and morphometrical data to characterize four distinct Oryzomys species in the region. O. peninsulae and another isolated population, O. nelsoni from the Islas Marías
Islas Marías
The Islas Marías are an archipelago of four islands that belong to Mexico. They are located in the Pacific Ocean, some off the coast of the state of Nayarit. They are part of the municipality of San Blas, Nayarit...
, were both retained as separate species, as was O. albiventer
Oryzomys albiventer
Oryzomys albiventer is a rodent in the genus Oryzomys of family Cricetidae from interior western Mexico, in the states of Jalisco, Guanajuato, and Michoacán. First described in 1901 as a separate species, it was later lumped under O. couesi and the marsh rice rat until it was reinstated as a...
from montane
Montane
In biogeography, montane is the highland area located below the subalpine zone. Montane regions generally have cooler temperatures and often have higher rainfall than the adjacent lowland regions, and are frequently home to distinct communities of plants and animals.The term "montane" means "of the...
mainland Mexico. They kept the population in the coastal lowlands as a subspecies, O. couesi mexicanus, of Oryzomys couesi.
The genus Oryzomys currently includes about eight species distributed from the eastern United States (O. palustris) into northwestern South America (O. gorgasi
Oryzomys gorgasi
Oryzomys gorgasi, also known as Gorgas's Oryzomys or Gorgas's Rice Rat, is a rodent in the genus Oryzomys of family Cricetidae. First collected as a living animal in 1967, it is known from only a few localities, including a freshwater swamp in the lowlands of northwestern Colombia and a mangrove...
). O. peninsulae is part of the O. couesi section, which is centered around the widespread Central American O. couesi and also includes various other species with more limited and peripheral distributions. 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 this section remain unclear and it is likely that the current classification underestimates the group's true diversity. Oryzomys was previously a much larger genus, but most species were progressively removed in various studies, culminating in contributions by Marcelo Weksler and coworkers in 2006 that excluded more than forty species from the genus. Oryzomys and many of the species removed from it are 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 peninsulae is a medium-sized member of the genus, smaller than O. albiventer but larger than O. couesi mexicanus. Its fur is grayish-brown on the forequarters, but reddish-brown on the hindquarters; this coloration pattern is unique among western Mexican Oryzomys. The underparts are a dirty white, the feet white above, and the tail dark or brownish above and dirty white below.In the skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...
, the braincase is high, the zygomatic arch
Zygomatic arch
The zygomatic arch or cheek bone is formed by the zygomatic process of temporal bone and the temporal process of the zygomatic bone , the two being united by an oblique suture; the tendon of the Temporalis passes medial to the arch to gain insertion into the coronoid process...
es (cheekbones) are broad and squared, 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:...
, which perforate 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"....
, are long and broad. The upper incisors are orthodont, with their cutting edge nearly vertical. Morphometrically, the skull of O. peninsulae is sharply distinct from other western Mexican Oryzomys.
In fourteen specimens measured by Carleton and Arroyo-Cabrales, total length was 227 to 305 mm (8.9 to 12.0 in), averaging 265.6 mm (10.46 in); head and body length was 113 to 152 mm (4.4 to 6.0 in), averaging 128.9 mm (5.07 in); tail length was 114 to 156 mm (4.5 to 6.1 in), averaging 136.8 mm (5.39 in); hindfoot length was 29 to 34 mm (1.1 to 1.3 in), averaging 32.0 mm (1.26 in); and occipitonasal length (a measure of total skull length) was 27.8 to 34.3 mm (1.09 to 1.35 in), averaging 31.5 mm (1.24 in).
Distribution, ecology, and status
Twenty-one specimens of O. peninsulae are known: six were caught at Santa AnitaSanta Anita, Baja California Sur
Santa Anita is a village in Los Cabos Municipality, Baja California Sur, western Mexico, on the right bank of a river, the Arroyo San José. In 1968, it was described as a "little farming village". It is the type locality of the rice rat Oryzomys peninsulae, now possibly extinct...
in 1896 by D. Coolidge, and Edward William Nelson
Edward William Nelson
Edward William Nelson was an American naturalist and ethnologist. He was born in Manchester, New Hampshire. In 1871 together with his family, he became homeless due to the Chicago Fire....
and Edward Alphonso Goldman
Edward Alphonso Goldman
Edward Alphonso Goldman was an American zoologist. He worked extensively in Mexico with Edward William Nelson and described and revised many groups of mammals....
obtained fifteen additional individuals in 1906 at San José del Cabo
San José del Cabo
The city of San José del Cabo is located in Baja California Sur Mexico and is the seat of the municipality of Los Cabos at the south end of the Baja California peninsula. In the 2010 census it had a population of 69,788. Together with neighboring Cabo San Lucas it forms a major tourist destination...
. The two localities, which are about 13 km (8 mi) apart, were both located along the Río San José, a river in southernmost Baja California Sur
Baja California Sur
Baja California Sur , is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. Before becoming a state on October 8, 1974, the area was known as the South Territory of Baja California. It has an area of , or 3.57% of the land mass of Mexico and comprises...
, near the southern tip of the Baja California peninsula
Baja California Peninsula
The Baja California peninsula , is a peninsula in northwestern Mexico. Its land mass separates the Pacific Ocean from the Gulf of California. The Peninsula extends from Mexicali, Baja California in the north to Cabo San Lucas, Baja California Sur in the south.The total area of the Baja California...
. Like other Oryzomys, O. peninsulae is a semiaquatic species, spending much of its time in the water, but suitable habitat for such a species is estimated to be no more than 13 km2 (5.0 sq mi) on the arid Baja California peninsula.
Río San José no longer exists, having fallen prey to irrigation projects, and touristic development of its estuary has resulted in pollution. Biologists working in the area in 1979 and from 1991 to 1993 failed to find O. peninsulae, casting doubt on its continued existence. The lack of records for over a century, small distribution, and destruction of the only known habitat led Carleton and Arroyo-Cabrales to consider the 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...
of O. peninsulae as "critically endangered, if not extinct". They noted that the status of the species had previously been obscured because it had been lumped for decades under O. couesi, a widely distributed and secure species.
Origin
It is uncertain how Oryzomys peninsulae arrived at its current distribution in Baja California Sur. In 1922, Nelson suggested that it may have been introducedIntroduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
from another part of Mexico in a shipment of farm products, but this hypothesis is disproved by the clear differentiation from other western Mexican Oryzomys that the species exhibits.
The species's range may be relict
Relict
A relict is a surviving remnant of a natural phenomenon.* In biology a relict is an organism that at an earlier time was abundant in a large area but now occurs at only one or a few small areas....
ual in nature: although Oryzomys currently does not extend further north along the eastern coast of the Gulf of California
Gulf of California
The Gulf of California is a body of water that separates the Baja California Peninsula from the Mexican mainland...
than coastal southern Sonora
Sonora
Sonora officially Estado Libre y Soberano de Sonora is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided into 72 municipalities; the capital city is Hermosillo....
, the past distribution of the genus may have extended further north along the shores of the Gulf of California, perhaps even into the southwestern United States, and from there south into the Baja California peninsula. When Oryzomys disappeared from the northern regions, O. peninsulae would have become isolated on the peninsula. This possibility is supported by the relatively close resemblance between O. peninsulae and O. couesi mexicanus, from coastal western Mexico.
Alternatively, the ancestor of Oryzomys peninsulae may have arrived during the late Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
, about six million years ago, when the southern tip of the Baja California peninsula was an island located near what is now Nayarit
Nayarit
Nayarit officially Estado Libre y Soberano de Nayarit is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is divided in 20 municipalities and its capital city is Tepic.It is located in Western Mexico...
and Jalisco
Jalisco
Jalisco officially Estado Libre y Soberano de Jalisco is one of the 31 states which, with the Federal District, comprise the 32 Federal Entities of Mexico. It is located in Western Mexico and divided in 125 municipalities and its capital city is Guadalajara.It is one of the more important states...
in western Mexico. Some plants and birds from the area may have a similar biogeographic
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
heritage.
Literature cited
- Alvarez-Castañeda, S.T. 1994. Current status of the rice rat, Oryzomys couesi peninsularis (subscription required). The Southwestern Naturalist 39(2):99–100.
- Benson, D.E. and Gehlbach, F.R. 1979. Ecological and taxonomic notes on the rice rat (Oryzomys couesi) in Texas (subscription required). Journal of Mammalogy 60(1):225–228.
- 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.
- Goldman, E.A. 1918. The rice rats of North America. North American Fauna 43:1–100.
- Hall, E.R. 1981. Mammals of North America. Volume II. Caldwell, New Jersey: The Blackburn Press, pp. 601–1181, 1–90 (index). ISBN 1-930665-31-8
- International Commission on Zoological Nomenclature. 1999. International Code of Zoological Nomenclature. 4th ed. London: The International Trust for Zoological Nomenclature. ISBN 0-85301-006-4
- Merriam, C.H. 1901. Synopsis of the rice rats (genus Oryzomys) of the United States and Mexico. Proceedings of the Washington Academy of Sciences 3:273–295.
- 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
- Schmidt, C.A. and Engstrom, M.D. 1994. Genic variation and systematics of rice rats (Oryzomys palustris species group) in southern Texas and northeastern Tamaulipas, Mexico (subscription required). Journal of Mammalogy 75(4):914–928.
- Thomas, O. 1897. Descriptions of new rats and rodents from America. Annals and Magazine of Natural History (6)20:544–553.
- 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.