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Oligoryzomys microtis - Small-eared Pygmy Rice Rat

Photo by Christopher Yahnke, collection of the Field Museum of Natural History

Physical Description:

Oligorhyzomys microtis is the smallest of 15 species in the genus.  Its weakly bicolored tail is long, slender and naked, and equals roughly 128% of the head and body length.  It has relatively long hind feet.  The average head and body length for O. microtis is 146mm, tail length is 188mm, hind feet are 24mm, and the average ear length is 14mm.  On average each weighs about 18g (Redford and Eisenberg 1992).  The four longer hind toes have tufts of silvery grey bristles that expand beyond the claws.  The pelage is coarse but not spiny or bristly.  Its back is yellow-brown or gray mixed with black and it is white or gray underneath with less black.  Its throat is variably white to gray.  The cheeks are orangish brown or gray, the muzzle is pointed and the ears are oval and densely furred inside (Nowak 1999).  The whiskers are short, sometimes reaching the ear tip but never the shoulder (Emmons 1990).

The skull is small but stout and the rostrum is relatively broad and stocky.  However, these identifying features are common to the genus and may not always be used to properly identify the species in the field.

 Photo by Christopher Yahnke, collection of the Field Museum of Natural History


O. microtis is found in the lowlands of eastern Peru and Bolivia extending east to southern Brazil and south to northern Argentina into the province of Corrientes.  It also occurs throughout eastern Paraguay and into the Chaco (Redford and Eisenberg 1992).

Small-eared pygmy rice rats are found in many different habitats depending on geographic location.  Overall this species seems to prefer dense vegetation.  They also occupy houses and camps.  In Paraguay and Argentina they inhabit marshes and wet grasslands.  In Brazil they occupy the edges of gallery forests and also secondary growth habitats.  In the Chaco they prefer wet and dry marshy habitats (Redford and Eisenberg 1992).

Ontogeny and Reproduction:

Females have four pairs of mammae.  The average litter size is 5 but ranges from 2 -11 young per litter.  Breeding occurs year round and mature females produce about 6 litters per year.  Nests are made of woven grasses and placed in a slight depression in the ground (Nowak 1999).    

Ecology and Behavior:

Oligoryzomys microtis are nocturnal, solitary, and terrestrial.  While they are not characterized as arboreal, they are good climbers.  These animals feed on seeds, fruits and insects.  They can be found feeding in rice fields and storage barns (Nowak 1999).


O. microtis has some very important implications for humans.  It is an agricultural pest and can be found feeding in rice fields and storage barns.  It is also a household pest that may take up residence in homes and camps (Emmons 1990).  While humans across the world share their homes and barns with many rodent species, O. microtis could have a more negative effect on humans than many other rodent species.  Its most important role in relation to humans is that it may be a vector of many diseases.  Studies done on this species have resulted in the first successful isolation of Rio Mamore virus and the first evidence for the existence of hantavirus in Peru (Powers 1999).  Other areas that have reported cases of hantavirus from Oligoryzomys species include Paraguay, the Chaco, Argentina, Chile, Bolivia, Brazil, Venezuela and Panama (Chu 2003).  In most cases, the viruses associated with human infections in these areas are closely related to Oligoryzomys-borne viruses (Padula 2000).  These diseases apparently do not harm the rodent host but can be fatal to humans.  Transmission may occur by rodent bites or contact, but occurs most frequently from breathing virus-contaminated aerosols of rodent excreta.  Because the disease is apparently so easy to obtain from infected rodents, knowledge and control of O. microtis is very important for human health (Schmaljohn 1997).

While the Small-eared pygmy rice rat is referred to as Oligoryzomys microtis throughout this paper, the actual species name is under some discrepancy among scientists.  Allen gave it the name O. microtis in 1916 but it has since been referred to as O. delicates, O. longicaudatus, O. fornesi, O. mattogrosssae, and O. chaparensis (Carleton 1989).  

Literature Cited:

Carleton, Michael D., and G.G. Musser.  1989.  Systematic Studies of Oryzomyine Rodents (Muridae, Sigmodontince): A Synopsis of Microryzomys.  Bulletin of the American Museum of Natural History.  191:71.

Chu, Yong-Kyu, R.D. Owen, L.M. Gonzalez, and C.B. Jonsson.  2003.  The Complex Ecology of Hantavirus in Paraguay.  The American Journal of Tropical Medicine and Hygiene.  69(3):263-268.

Emmons, Louise H.  Neotropical Rainforest Mammal: A Field Guide.  1990.  University of Chicago Press.  180.

Nowak, Ronald M. Walker’s Mammals of the World. 6th Edition, Volume 2. 1999. John Hopkins University Press. 1366-1369.

Padula, P.J., S.B Colavecchia, V.P. Marinez, M.O. Gonzalez Della Valle, A. Edelstein, S.D.L. Miguel, J. Russi, J. Mora Riquelme, N. Colucci, M. Almiron, and R.D. Rabinovich.  2000.  Genetic Diversity, Distribution, and Serological Features of Hantavirus Infection in Five Countries in South America. Journal of Clinical Microbiology.  38(8):3029-3035.

Powers, A.M., D.R. Mercer, D.M. Watts, H. Guzman, C.F. Fulhorst, V.L. Popov, and R. B. Tesh.  1999.  Isolation and genetic characterization of a hantavirus (Bunyaviridae: Hantavirus) from a rodent, Oligoryzomys microtis (Muridae), collected in northeastern Peru.  The American Journal of Tropical Medicine and Hygiene.  61(1):92-98.

Redford, K.H., and J.F. Eisenberg.  Mammals of the Neotropics: The Southern Cone.  Volume 2.  1992.  University of Chicago Press.  266-267.

Schmaljohn, Connie and B. Hjelle.  1997.  Hantaviruses: A Global Disease Problem.  Emerging Infectious Diseases.  3(2).


Reference written by Amy Schumacher, Biology 378 (Mammalogy), University of Wisconsin – Stevens Point.  Edited by Christopher Yahnke. Page last updated August 15, 2005.

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