Rice Rat

Oryzomyini is a tribe of rodents in the subfamily Sigmodontinae of the family Cricetidae. It includes about 120 species in about thirty genera,Weksler et al., 2006, table 1 distributed from the eastern United States to the southernmost parts of South America, including many offshore islands. It is part of the clade Oryzomyalia, which includes most of the South American Sigmodontinae.

The name ''Oryzomyini'' derives from that of its type genus, ''Oryzomys'', which means "rice rat" or "rice mouse". Many species are also known as rice rats.

Taxonomy

Contents of Oryzomyini

An oryzomyine group was first envisaged by Oldfield Thomas in the early 20th century. He defined it to include pentalophodont species, which have a mesoloph(id) on the upper and lower molars, with a long palate (extending past the third molars). Thomas included ''Oligoryzomys'', '' Oecomys'', and ''Oryzomys'' (which included many species now in other genera), as well as '' Rhagomys'', which is currently classified in the related tribe Thomasomyini instead. In 1944, Hershkovitz proposed a more extended definition of the group, excluding ''Rhagomys'', but including ''Nectomys'' (then including '' Sigmodontomys''), '' Neacomys'', and '' Scolomys''. Some subsequent authors did not separate the oryzomyines from the thomasomyines, which were distinguished from them by having a short palate, including Vorontsov, who in 1959 was the first to use Oryzomyini as a formal family-group name. He included most current oryzomyines as well as the thomasomyines and tylomyines, which are now known to be more distantly related.Weksler, 2006, p. 4

The genera ''Holochilus'' (including ''Lundomys'' at the time), ''Pseudoryzomys'', and ''Zygodontomys'' were not included at the time because of their tetralophodont molars (lacking complete mesoloph(id)s); instead, ''Holochilus'' was considered to be a sigmodont, related to '' Sigmodon'' and ''Reithrodon'', and ''Pseudoryzomys'' and ''Zygodontomys'' were considered to be members of Phyllotini, another large South American tribe. Although lacking mesoloph(id)s, these genera share other characters with oryzomyines, and a series of papers by Robert Voss and coworkers in the early 1990s established their membership in Oryzomyini.Weksler, 2006, p. 5

In a 1993 paper, Voss and Carleton proposed the first cladistic diagnosis of Oryzomyini. They included twelve genera and proposed five synapomorphies for the tribe: presence of a pair of mammae on the chest; a long palate marked by posterolateral palatal pits, perforations near the third molar; absence of an alisphenoid strut, which in some sigmodontines separates two foramina (openings) in the skull; absence of a suspensory process of the squamosal bone attached to the roof of the tympanic cavity, the tegmen tympani; and absence of a gall bladder.Voss and Carleton, 1993, p. 31 Some of these have been reversed in some oryzomyines; for example, an alisphenoid strut is present in several oryzomyines, including ''Eremoryzomys''.Weksler, 2006, p. 38

The contents of Oryzomyini have been largely stable since, but the allocation of some animals has been contentious. '' Megaoryzomys'', an extinct giant rat from the Galápagos Islands, has been allocated to both Oryzomyini and Thomasomyini, but its correct classification currently remains unclear.Musser and Carleton, 2005 The genus '' Scolomys'' has been excluded from Oryzomyini on the basis of studies of the mitochondrial cytochrome ''b'' gene, but the nuclear IRBP gene provides evidence for its membership in Oryzomyini.Weksler, 2006, p. 124 '' Handleyomys fuscatus'' was placed in the thomasomyine genus '' Aepeomys'' before its close relationship to '' H. intectus'' was recognized in 2002. '' Microakodontomys transitorius'' was described as a transitional form between oryzomyines and akodontines, but later allied with Oryzomyini and even summarily dismissed as an anomalous ''Oligoryzomys''.

In the early 2000s, the advent of molecular phylogenetics has led to progress in the understanding of the relationships of oryzomyines. They are currently classified in the family Cricetidae, which includes the voles, hamsters, deermice, and many other species, chiefly in the Americas and Eurasia. Within this family, they are part of the subfamily Sigmodontinae, which is mainly distributed in South America but also extends into southern North America. Sigmodontinae includes several tribes, most of which cluster into a clade now known as Oryzomyalia, which includes Oryzomyini, Akodontini, Phyllotini, Thomasomyini, and other, smaller groups, but not the cotton rats (''Sigmodon'') and the Ichthyomyini.

Internal classification

The relationships among the oryzomyine genera have long been obscure, although several studies provided insights into the relations of some genera.Weksler, 2006, p. 7 The single most significant problem in oryzomyine taxonomy has been the definition of the type genus, ''Oryzomys'',Weksler, 2006, p. 10 which in one classification included all animals then recognized as oryzomyines. Many groups were later excluded from the genus, but even so it included forty species that did not form a monophyletic group.

In the 2000s, Marcelo Weksler published several studies in which he used evidence from IRBP, a nuclear gene, and morphology to assess the relationships among the members of Oryzomyini. He provided support for several intergeneric relationships and clarified the scale of the ''Oryzomys'' problem, as species of ''Oryzomys'' appeared in about ten separate clades.Weksler, 2006, figs. 34–39 In a 2006 publication, he and coworkers described ten new genera for species previously placed in ''Oryzomys'' and transferred some others to ''Handleyomys'', leaving only about six species in ''Oryzomys''.

Weksler's analyses suggested that oryzomyines fall into four major clades, which were largely congruent across his analyses of morphology and IRBP, but support for all of those was limited and the placement of some genera remained unclear. He dubbed these clades "clade A" through "clade D". Some analyses supported a relationship between clades C and D, which in turn were related to clade B, with clade A at a basal position, but other analyses could not resolve the relationships among the major clades. The four clades are as follows:
*Clade A includes only '' Scolomys'' and ''Zygodontomys'', but support for a relationship between those morphologically and ecologically dissimilar genera was not strong.Weksler, 2006, p. 75
*Clade B includes at least '' Oecomys'', ''Handleyomys'', ''Euryoryzomys'', '' Transandinomys'', '' Hylaeamys'', and ''Nephelomys'', the latter four of which were included in ''Oryzomys'' until 2006.Weksler et al., 2006 ''Amphinectomys'' and '' Mindomys'' were also recovered in the clade in some analyses, but the former, for which most morphological characters are unknown, is more likely related to ''Nectomys'' within clade DWeksler, 2006, p. 133 and the relations of the latter, a poorly known yet unique animal, are obscure as some analyses suggest it is a basal member of Oryzomyini.Weksler et al., 2006, p. 17 There is little support for all intergeneric relationships within the clade.
*Clade C includes ''Oligoryzomys'', '' Neacomys'', '' Microryzomys'', and '' Oreoryzomys'', which was placed in ''Oryzomys'' until 2006. No intergeneric relationships within this clade received substantial support.
*Clade D includes ''Drymoreomys'',Percequillo et al., 2011, p. 372 ''Eremoryzomys'', '' Cerradomys'', ''Sooretamys'', ''Oryzomys'', ''Lundomys'', ''Pseudoryzomys'', ''Holochilus'', ''Aegialomys'', '' Nesoryzomys'', '' Melanomys'', '' Sigmodontomys'', ''Nectomys'', most likely ''Amphinectomys'', and the extinct '' Megalomys'',Weksler et al., 2006, p. 18 '' Agathaeromys'', and ''Pennatomys''. ''Eremoryzomys'', ''Cerradomys'', ''Sooretamys'', and ''Aegialomys'' were included in ''Oryzomys'' before 2006. There is some support for a basal placement of ''Eremoryzomys'' within this clade and for two major groups of related genera, one including ''Holochilus'', ''Lundomys'', and ''Pseudoryzomys'' and the other ''Nectomys'', ''Amphinectomys'', ''Sigmodontomys'', ''Melanomys'', ''Aegialomys'', and ''Nesoryzomys'',Weksler, 2006, figs. 37–39 with the extinct ''Megalomys'' and ''Pennatomys''. The ''Holochilus''–''Lundomys''–''Pseudoryzomys'' group also includes the extinct ''Noronhomys'' and '' Carletonomys''. ''Oryzomys'' may be its closest relative, but it is possible that this arrangement is the result of a convergent development of adaptations to a life in the water in the two groups.Weksler, 2006, p. 131 Within the group of ''Nectomys'' and related genera, there is some support for a core group that excludes ''Aegialomys'' and ''Nesoryzomys'' and for two subclades within the core group, one including ''Nectomys'' and ''Amphinectomys'' and the other ''Melanomys'' and ''Sigmodontomys''.

The affinities of some species remain unclear. Many oryzomyines are known from the Lesser Antilles, including "'' Ekbletomys hypenemus''" and species of ''Megalomys'' and ''Oligoryzomys'', but most remain undescribed.

Description

Most oryzomyines are nondescript rodents that look like common house mice and rats, but the tribe also includes some spectacularly specialized forms. The smallest members, mainly in clade C, may have a head-body length of as little as and mass of , but the largest living oryzomyine, ''Nectomys'', reaches head and body lengths over and mass of about ; ''Lundomys'' and ''Holochilus'' are only a little smaller. Some of the extinct species from the Lesser Antilles, such as "''Ekbletomys hypenemus''" and ''Megalomys desmarestii'', were even larger.

Distribution and ecology

Oryzomyines range from New Jersey in the north, where the Marsh Rice Rat (''Oryzomys palustris'') is found, to Tierra del Fuego in the south, where ''Oligoryzomys magellanicus'' occurs. Extinct species are known from Jamaica ('' Oryzomys antillarum''), the Galápagos Islands (''Nesoryzomys'' and '' Aegialomys galapagoensis''), Fernando de Noronha (''Noronhomys''), and the Lesser Antilles north to Anguilla (''Megalomys'', ''Oligoryzomys victus'', and several unidentified genera). They are abundant in many environments, from rainforest to grassland.Weksler, 2006, p. 3 Most live in the forest, but ''Zygodontomys'', ''Lundomys'', ''Pseudoryzomys'', ''Aegialomys'', and ''Nesoryzomys'' live exclusively in open vegetation and some other genera include both forest and non-forest forms. Most oryzomyines are relatively unspecialized animals that live on the ground, but ''Oecomys'' is specialized to live in trees and various members of clade D, including ''Holochilus'', ''Oryzomys'', and ''Nectomys'', are semiaquatic, spending at least some of their time in the water.Weksler, 2006, pp. 78–79

References

Literature cited

*Musser, G.G. and Carleton, M.D. 2005. Superfamily Muroidea. Pp. 894–1531 in Wilson, D.E. and Reeder, D.M. (eds.)
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Baltimore: The Johns Hopkins University Press, 2 vols., 2142 pp. 
*Pardiñas, U.F.J. 2008
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(subscription required). ''Journal of Mammalogy'' 89(5):1270–1278.
*Pardiñas, U.F.J., D'Elía, G. and Ortiz, P.E. 2002
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*Percequillo, A.R., Weksler, M., and Costa, L.P. 2011
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(subscription required). ''Zooilogical Journal of the Linnean Society'' 161(2):357–390.
*Ray, C.E. 1962. The Oryzomyine Rodents of the Antillean Subregion. Doctor of Philosophy thesis, Harvard University, 211 pp.
*Turvey, S.T. 2009. Holocene Extinctions. Oxford University Press US, 359 pp. 
*Turvey, S.T., Weksler, M., Morris, E.L., and Nokkert, M. 2010
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(subscription required). ''Zoological Journal of the Linnean Society'' 160:748–772.
*Voss, R.S. and Carleton, M.D. 1993
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*Voss, R.S., Gómez-Laverde, M. and Pacheco, V. 2002
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*Voss, R.S. and Myers, P. 1991
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*Weksler, M. 2006
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*Weksler, M., Percequillo, A.R. and Voss, R.S. 2006
Ten new genera of oryzomyine rodents (Cricetidae: Sigmodontinae)
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*Zijlstra, J.S., Madern, P.A. and Hoek Ostende, L.W. van den. 2010
New genus and two new species of Pleistocene oryzomyines (Cricetidae: Sigmodontinae) from Bonaire, Netherlands Antilles
(subscription required). ''Journal of Mammalogy'' 91(4):860–873.

{{Taxonbar, from=Q2410209

Sigmodontinae
Stored-product pests
Mammal tribes