Akodon Caenosus

''Akodon caenosus'' is a rodent in the genus ''Akodon'' found in northwestern Argentina and south-central Bolivia. Since its description in 1918, it has been alternatively classified as a separate species or a subspecies of ''Akodon lutescens'' (formerly ''Akodon puer''). The species ''Akodon aliquantulus'', described from some very small Argentine specimens in 1999, is now recognized as a synonym of ''A. caenosus''.

''Akodon caenosus'' is very small, averaging in weight, and variable in coloration, but generally brown. The underparts are sharply different in color from the upperparts. The skull has a short rostrum (front part), broad interorbital region (between the eyes), and narrow braincase. The karyotype includes 34 chromosomes. ''A. caenosus'' mostly occurs in Yungas vegetation and breeds mainly during the winter. It shares its range with many other sigmodontine rodents, including three other species of ''Akodon''.

Taxonomy

E. Budin collected the first specimen of the species on August 21, 1917, in Jujuy Province, northwestern Argentina, and the next year Oldfield Thomas used the animal as the holotype of a new subspecies of ''Akodon puer'', a Bolivian species. He described the new subspecies ''Akodon puer cænosus'' as darker and duller in color than the Bolivian form, but otherwise identical. In 1920, Thomas recognized additional differences between the two after examining more specimens and classified the Argentine form as a separate species, ''Akodon cænosus''. Most subsequent authors followed this arrangement, but since the 1980s some have placed the form (now spelled ''caenosus'') in ''A. puer'' again.Jayat et al., 2010, p. 25 In 1990, Philip Myers and others reviewed the '' Akodon boliviensis'' group, which includes ''A. puer'' and ''A. caenosus'', and again considered ''caenosus'' as a subspecies of ''puer''.Myers et al., 1990, p. 66 They retained ''caenosus'' as a separate subspecific name for the Argentine populations of ''puer'' because of its small size, dark fur,Myers et al., 1990, p. 73 and distinctive karyotype.Myers et al., 1990, p. 74 Myers and colleagues had included the name ''lutescens'' J.A. Allen, 1901, as a subspecies of ''Akodon puer'' Thomas, 1902, and in 1997 Sydney Anderson noted that the older name ''lutescens'' should instead be used for the species because of the Principle of Priority; therefore, he utilized the combination ''Akodon lutescens caenosus'' for the Argentine subspecies. Through the 1990s and 2000s, authors continued to differ on the classification of ''caenosus'' as either a full species or a subspecies or ''puer'' (=''lutescens'').

Two small ''Akodon'' collected in 1993 in Tucumán Province, northwestern Argentina, were given the name ''Akodon diminutus'' in 1994, but that name is a ''nomen nudum'' and therefore not available for use under the International Code of Zoological Nomenclature. In 1999, Mónica Díaz and others described these animals more fully as a new species, ''Akodon aliquantulus'', which they considered closely related to ''A. puer caenosus''.Díaz et al., 1999, p. 786 The specific name means "how little" or "how few" in Latin and refers to the small size of the species and the small sample Díaz and colleagues could use.Díaz et al., 1999, p. 794 In the 2005 third edition of ''Mammal Species of the World'', Guy Musser and Michael Carleton termed the differentiation between ''A. aliquantulus'' and ''A. lutescens'' (=''puer'') "unimpressive" and recommended further taxonomic research. Common names proposed for ''A. aliquantulus'' include "Diminutive Akodont" and "Tucumán Grass Mouse".

In 2010, Pablo Jayat and colleagues reviewed the members of the ''Akodon boliviensis'' group in Argentina. On the basis of sequences from the mitochondrial cytochrome ''b'' gene,Jayat et al., 2010, p. 5 they found ''A. caenosus'' to be closest to ''A. lutescens'' and '' A. subfuscus'', forming a clade that was the sister group to a clade of the remaining species in the ''A. boliviensis'' group—''A. boliviensis'', '' A. spegazzinii'', '' A. sylvanus'', and '' A. polopi''. They classified ''A. caenosus'' as a species separate from ''A. lutescens'' because the two forms did not form a single clade (''A. caenosus'' was instead closer to ''A. subfuscus''), and because the difference between the cytochrome ''b'' sequences of ''A. lutescens'' and ''A. caenosus'' was relatively high at 3.5%. ''A. aliquantulus'' was reduced to a synonym of ''A. caenosus'', because they found no substantial morphometrical differentiation between the two and could not replicate the characters Díaz and colleagues had noted as diagnostic for ''A. aliquantulus''.

Description

''Akodon caenosus'' is the smallest of the Argentine members of the ''A. boliviensis'' group–indeed, among the smallest of all species of ''Akodon''.Díaz et al., 1999, p. 795 The upperparts are uniformly colored, but their tone is variable: generally ochraceous brown, but approaching yellow, red, or olivaceous in some individuals. Reddish tones occur mostly in lactating females. High-altitude animals are generally lighter, but there is also conspicuous variation within populations.Jayat et al., 2010, p. 24 The ears are similar to the upperparts, but some individuals have the sides more rich and clear in color. The underparts are clearly different in color, varying from light gray to yellowish or reddish. There are yellowish rings around the eyes,Jayat et al., 2010, p. 18 which are more highly developed in high-altitude populations. There are white to yellowish hairs on the fore- and hindfeet. The tail is variably covered with hair and is dark brown above and white to buffy below.

In the skull, the rostrum (front part) is short, the interorbital region (between the eyes) is broad and hourglass-shaped, and the braincase is small. The zygomatic plate, the flattened front part of the zygomatic arch, is narrow, with poorly developed zygomatic notches at their front, but there is considerable variation in the features of the plate. The incisive foramina (openings in the front part of the palate) extend back to between the first molars. The mesopterygoid fossa, the openings behind the bony palate, is very narrow. In the mandible (lower jaw), the masseteric ridges, which anchor some of the chewing muscles, extend to near the front margin of the first molar. The capsular process, raising in the back part of the mandibular bone that accommodates the root of the incisor, is poorly developed. The upper incisors are orthodont (with the chewing edge in the horizontal plane) to slightly opisthodont (with the chewing edge inclined backward). The molars show some accessory crests and other features, such as the anteroloph on the first upper molar and the mesoloph on the first and second upper molar.

In twelve adult Argentine ''A. caenosus'', total length is , averaging ; tail length is , averaging ; hindfoot length is , averaging ; ear length is , averaging ; and weight is , averaging . The karyotype includes 34 chromosomes with a fundamental number of 40 major arms (2n = 34, FN = 40). The autosomes includes three large and one very small pairs of metacentrics, with two long arms, and twelve small to medium-sized acrocentric pairs, which have a long and a very short arm. The X chromosome is medium-sized and subtelocentric, with a long and a short arm, and the Y chromosome is very small and is acrocentric in Jujuy specimens, but metacentric in those from Tucumán. The karyotype is separated from that of ''A. lutescens'' by three Robertsonian translocations.

Members of the ''Akodon boliviensis'' group, including ''A. caenosus'', are generally similar and difficult to separate, but they differ in relative cranial measurements and some other characters. ''A. spegazzinii'' is larger than ''A. caenosus''; ''A. sylvanus'' is darker and has less contrast between the upper- and underparts and less well-developed eye-rings; ''A. polopi'' has a squared interorbital region and more well-developed ridges on its skull; and ''A. boliviensis'' is paler and has more densely furred ears.

Distribution and ecology

''Akodon caenosus'' is found from northwestern Argentina into south-central Bolivia. In Bolivia, it occurs in Tarija and Chuquisaca Departments.Anderson, 1997, p. 422 Its Argentine distribution extends from far northern Salta to southern Catamarca at altitudes ranging from . It is mostly found in Yungas, but also in the highest levels of the Chaco and the lowest of the Andean mountain grasslands. It occurs together with ''A. boliviensis'', ''A. sylvanus'', '' A. simulator'', and species of ''Oxymycterus'', '' Calomys'', ''Phyllotis'', ''Oligoryzomys'', ''Necromys'', '' Andinomys'', '' Graomys'', and ''Abrothrix''. Breeding occurs throughout the year, but mostly from November to January, during the summer. Molting occurs mostly during the winter and autumn. The oestrid fly ''Cuterebra apicalis'' and the flea '' Hectopsylla gracilis'' have been recorded from ''A. caenosus''. The mites '' Androlaelaps fahrenholzi'', ''Androlaelaps rotundus'', and ''Eulaelaps stabularis'' have been found on ''A. aliquantulus''.Lareschi et al., 2003, p. 60

Conservation status

The IUCN currently assesses ''A. aliquantulus'' as "Data Deficient" because so little is known about it, but notes that ranching and fire may threaten it.Pardinas and Jayat, 2008 ''Akodon lutescens'', including ''A. caenosus'', is assessed as "Least Concern" because of its wide distribution, large population, and ability to persist in disturbed habitats. However, habitat loss may threaten Yungas populations.Dunnum et al., 2008

Footnotes

References

Literature cited

*Anderson, S. 1997
Mammals of Bolivia, taxonomy, and distribution
Bulletin of the American Museum of Natural History 231:1–652.
*Díaz, M.M., Barquez, R.M., Braun, J.K. and Mares, M.A. 1999
A new species of ''Akodon'' (Muridae: Sigmodontinae) from northwestern Argentina
(subscription required). Journal of Mammalogy 80(3):786–798.
*Duff, A. and Lawson, A. 2004. Mammals of the World: A checklist. New Haven: A & C Black, 312 pp.
*Dunnum, J., Vargas, J., Bernal, N., Zeballos, H., Vivar, E., Patterson, B., Jayat, J. and D'Elia, G. 2008. . In IUCN. IUCN Red List of Threatened Species. Version 2009.2.
www.iucnredlist.org
. Downloaded on April 3, 2010.
*International Commission on Zoological Nomenclature. 1999
International Code of Zoological Nomenclature
Fourth edition. London: The International Trust for Zoological Nomenclature.
*Jayat, J.P., Ortiz, P.E., Salazar-Bravo, J., Pardiñas, U.F.J. and D'Elía, G. 2010

( ttp://www.mapress.com/zootaxa/2010/f/z02409p061f.pdf abstract. Zootaxa 2409:1–61.
*Lareschi, M., Autino, A.G., Díaz, M.M., and Barquez, R.M. 2003
New host and locality records for mites and fleas associated with wild rodents from northwestern Argentina
Revista de la Sociedad Entomológica de Argentina 62(3–4):60–64.
*Lareschi, M., Sanchez, J.P., Ezquiaga, M.C., Autino, A.G., Díaz, M.M. and Barquez, R.M. 2010
Fleas associated with mammals from northwestern Argentina, with new distributional reports
(subscription required). Comparative Parasitology 77(2):207–213.
*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.
*Myers, P., Patton, J.L. and Smith, M.F. 1990
A review of the ''boliviensis'' group of ''Akodon'' (Muridae: Sigmodontinae) with emphasis on Perú and Bolivia
Miscellaneous Publications of the Museum of Zoology, University of Michigan 177:1–89.
*Pardinas, U. and Jayat, J.P. 2008. . In IUCN. IUCN Red List of Threatened Species. Version 2009.2.
www.iucnredlist.org
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*Pinto, C.M. and Claps, G.L. 2005. First record of ''Cuterebra almeidai'' (Guimaraes and Carrera) from Argentina, new host records for ''Cuterebra apicalis'' Guerin-Meneville, and list of ''Cuterebra'' (Diptera: Oestridae) in the collection of the Instituto-Fundacion Miguel Lillo, Tucuman, Argentina. Proceedings of the Entomological Society of Washington 107(3):572–575.
*Thomas, O. 1918
On small mammals from Salta and Jujuy collected by Mr. E. Budin
Annals and Magazine of Natural History (9)1:186–193.
*Thomas, O. 1920
A further collection of mammals from Jujuy
Annals and Magazine of Natural History (9)5:188–196.

{{Good article

Akodon
Mammals of Argentina
Mammals described in 1918
Taxa named by Oldfield Thomas