Lower California Rice Rat
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''Oryzomys peninsulae'', also known as the Lower California rice rat, is a species of
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 ...
from western
Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
. Restricted to the southern tip of the Baja California peninsula, it is a member of 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 ...
'' 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 ...
. Only about twenty individuals, collected around 1900, are known, and subsequent destruction of its riverine habitat may have driven the species to
extinction Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
. 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 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 ...
, robust
zygomatic arch In anatomy, the zygomatic arch (colloquially known as the cheek bone), is a part of the skull formed by the zygomatic process of temporal bone, zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the ...
es (cheekbones), and long
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
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 ...
between the
incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...
s and the
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, '' ...
).


Taxonomy

''Oryzomys peninsulae'' was first collected in 1896 and
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 ...
described it in 1897 as a full species 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 ...
''.Thomas, 1897, p. 548 It was retained as a distinct species related to '' O. couesi'' and '' O. palustris'' until 1971, when
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 ...
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 Classified may refer to: General *Classified information, material that a government body deems to be sensitive *Classified advertising or "classifieds" Music *Classified (rapper) (born 1977), Canadian rapper * The Classified, a 1980s American ro ...
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 Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...
and northeastern
Tamaulipas Tamaulipas, officially the Free and Sovereign State of Tamaulipas, is a state in Mexico; one of the 31 states which, along with Mexico City, comprise the 32 federal entities of Mexico. It is divided into 43 municipalities. It is located in nor ...
(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 In Taxonomy (biology), biological classification, subspecies (: subspecies) is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (Morphology (biology), morpholog ...
of ''O. couesi''.Carleton and Arroyo-Cabrales, 2009, p. 122; Benson and Gehlbach, 1979, p. 227; Schmidt and Engstrom, 1994, p. 914 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 The Islas Marías ("Mary Islands") make up an archipelago in Mexico, consisting of four islands. They are located in the Pacific Ocean, some off the coast of the state of Nayarit and about southeast of the tip of Baja California. They are part ...
, were both retained as separate species, as was '' O. albiventer'' from
montane Montane ecosystems are found on the slopes of mountains. The alpine climate in these regions strongly affects the ecosystem because temperatures lapse rate, fall as elevation increases, causing the ecosystem to stratify. This stratification is ...
mainland Mexico. They kept the population in the coastal lowlands as a subspecies, ''O. couesi mexicanus'', of ''Oryzomys couesi''.Carleton and Arroyo-Cabrales, 2009, p. 94 The genus ''Oryzomys'' currently includes about eight species distributed from the eastern
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
(''O. palustris'') into northwestern
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...
('' O. gorgasi'').Carleton and Arroyo-Cabrales, 2009, p. 106 ''O. peninsulae'' is part of the ''O. couesi'' section, which is centered on the widespread Central American ''O. couesi'' and also includes various other species with more limited and peripheral distributions.Carleton and Arroyo-Cabrales, 2009, p. 117 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 this section remain unclear and it is likely that the current classification underestimates the group's true diversity.Carleton and Arroyo-Cabrales, 2009, p. 107 ''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 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, 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.


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.Merriam, 1901, p. 274 In the
skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...
, the
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 ...
is high, the
zygomatic arch In anatomy, the zygomatic arch (colloquially known as the cheek bone), is a part of the skull formed by the zygomatic process of temporal bone, zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the ...
es (cheekbones) are broad and squared, and 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 ...
, which perforate 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 ...
between the
incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...
s and the
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, '' ...
, are long and broad. The upper incisors are
orthodont In rodents, incisor procumbency refers to the orientation of the upper incisor, defined by the position of the cutting edge of the incisor relative to the vertical plane of the incisors. Proodont (or procumbent) incisors have the cutting edge in f ...
, 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 , averaging ; head and body length was , averaging ; tail length was , averaging ; hindfoot length was , averaging ; and occipitonasal length (a measure of total skull length) was , averaging .


Distribution, ecology, and status

Twenty-one specimens of ''O. peninsulae'' are known: six were caught at Santa Anita in 1896 by D. Coolidge, and
Edward William Nelson Edward William Nelson (May 8, 1855 – May 19, 1934) was an American naturalist and ethnologist. A collector of specimens and field naturalist of repute, he became a member of several expeditions to survey the fauna and flora. He was part o ...
and
Edward Alphonso Goldman Edward Alphonso Goldman (July 7, 1873 – September 2, 1946) was an American field naturalist and mammalogist. He worked extensively in Mexico with Edward William Nelson and described and revised many groups of mammals. He was considered a leading ...
obtained fifteen additional individuals in 1906 at
San José del Cabo San José del Cabo (, ''Saint Joseph of the Cape'') is a coastal city located on the Gulf of California coast, near the southern tip of Baja California Sur (state), México. San José del Cabo is situated on the edge of a shallow bay, some northea ...
. The two localities, which are about apart, were both located along the Río San José, a river in southernmost
Baja California Sur Baja California Sur, officially the Free and Sovereign State of Baja California Sur, is a state in Mexico. It is the 31st and last state to be admitted, in 1974. It is also the second least populated Mexican state and the ninth-largest state by ...
, near the southern tip of the Baja California peninsula. Like other ''Oryzomys'' species, ''O. peninsulae'' 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 much of its time in the water, but suitable habitat for such a species is estimated to be no more than 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 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 ...
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 with ''O. couesi'', a widely distributed and secure species.


Origin

It is uncertain how ''Oryzomys peninsulae'' arrived at its recent locale in Baja California Sur. In 1922, Nelson suggested that it may have been introduced 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 relictual in nature: while ''Oryzomys'' is currently found along the eastern coast of the
Gulf of California The Gulf of California (), also known as the Sea of Cortés (''Mar de Cortés'') or Sea of Cortez, or less commonly as the Vermilion Sea (''Mar Vermejo''), is a marginal sea of the Pacific Ocean that separates the Baja California peninsula from ...
only as far north as coastal southern
Sonora Sonora (), officially Estado Libre y Soberano de Sonora (), is one of the 31 states which, along with Mexico City, comprise the Administrative divisions of Mexico, Federal Entities of Mexico. The state is divided into Municipalities of Sonora, 72 ...
,Carleton and Arroyo-Cabrales, 2009, fig. 7 the past distribution of the genus may have extended further northward, perhaps even into the southwestern
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
, and from there south into Baja California. Subsequent disappearance of ''Oryzomys'' from the northern regions would have led to its observed
disjunct distribution In biology, a taxon with a disjunct distribution is one that has two or more groups that are related but considerably separated from each other geographically. The causes are varied and might demonstrate either the expansion or contraction of a s ...
, with ''O. peninsulae'' isolated on the peninsula. This possibility is supported by the relatively close resemblance between ''O. peninsulae'' and ''O. couesi mexicanus'', from coastal western Mexico.Carleton and Arroyo-Cabrales, 2009, pp. 107–108 Alternatively, the ancestor of ''Oryzomys peninsulae'' may have arrived by
rafting Rafting and whitewater rafting are recreational outdoor activities which use an inflatable raft to navigate a river or other body of water. This is often done on whitewater or different degrees of rough water. Dealing with risk is often a ...
during the late
Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...
, about six million years ago, when the southern tip of the Baja California peninsula was an island located near what is now
Nayarit Nayarit, officially the Free and Sovereign State of Nayarit, is one of the 31 states that, along with Mexico City, comprise the Political divisions of Mexico, Federal Entities of Mexico. It is divided in Municipalities of Nayarit, 20 municipalit ...
and
Jalisco Jalisco, officially the Free and Sovereign State of Jalisco, is one of the 31 states which, along with Mexico City, comprise the 32 Political divisions of Mexico, Federal Entities of Mexico. It is located in western Mexico and is bordered by s ...
in western Mexico. Some plants and birds from the area may have a similar
biogeographic Biogeography is the study of the distribution of species and ecosystems in geographic space and through geological time. Organisms and biological communities often vary in a regular fashion along geographic gradients of latitude, elevation, ...
heritage.Carleton and Arroyo-Cabrales, 2009, p. 108


Footnotes


References


Literature cited

* * * * * * * * * * * * {{Featured article peninsulae Endemic mammals of Mexico Endemic fauna of the Baja California Peninsula Mammals described in 1897 Taxa named by Oldfield Thomas