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Oligoryzomys flavescens - Yellow Pygmy Rice Rat

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

Physical Description:

Oligoryzomys flavescens is a small rodent with a pointed muzzle, oval-shaped ears and a long, slender, naked tail.  Its tail, which is longer than its body, ranges from 85-155mm in length.  The body length ranges from 70-110 mm and it generally weighs anything from 9 to 40 grams (Nowak 1999).

Redford and Eisenberg (1992) describes O. flavescens’ body coloration in some detail.  The dorsal part of its head and body is brown with yellow and black tipped hairs mixed into the fur.  Continuing down the sides and rump, the brownish fur is washed with an orange color.  Finally, its venter is a whitish-yellow color that occasionally contains hints of gray fur mixed in.  Also, the fur of O. flavescens is very soft compared to other rodents.


    O. flavescens also has a very delicate skull which lacks supraoccipital ridges (Dickerman and Yates Feb. 1995).

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


O. flavescens is found from southern Brazil through Uruguay into parts of northern Argentina at least through to the Buenos Aires Province (Redford and Eisenberg 1992).

Ontogeny and Reproduction:

Very little is known about the reproduction of O. flavescens reproduction.  However, studies have shown that their reproductive peak is during the months of April and May with average litters containing about 5 young, with as little as three and as much as 7 offspring  produced.

Ecology and Behavior:

The genus Oligoryzomys and Oryzomys are very closely related in phylogeny.  One thing that is common with both genuses is that they tend to occupy three types of habitats.  They can arboreal, terrestrial or live near the water and be semi-aquatic.  Because of their versatility, sharing of territory among species exists (MacDonald 2001).

O. flavescens can utilize all three of these habitats.  This mammal typically occupies pasture fields surrounded by weedy margins found in crop land regions throughout its range (Busch et. al. 2001).  Sometimes these fields border rivers, streams and wetlands.  O. flavescens can also be found in the tall marsh grasses near these bodies of water (Redford and Eisenberg 1992).  Also, the yellow pygmy rice rat has the ability to climb trees in the area.  This could be a possible adaptation to escape predators (Nowak 1999).

O. flavescens typically is a solitary, nocturnal rodent that does not leave its home range and habitat much.  Trapping studies have shown that the species tends to be residential in its habitat (Busch and Kravetz 1992).

O. flavescens is a very generalized herbivore.  It’s a very opportunistic species, feeding on the grasses and green plants that surround it in its habitat (Ellis et. al. 1998).  Some cases of omnivory have been found in the wild.  Some individuals that lived near aquatic habitats had stomachs that contained invertebrate material (Redford and Eisenberg 1992).

Today, the numbers of O. flavescens being found is decreasing.  This is a general trend found both in Oligoryzomys and Oryzomys.  Human impacts and the introduction of domestic cats and murid rats and mice have been keeping the population numbers very low (MacDonald 2001).  This could be one of the reasons why information about these rodents is hard to find.


One of the most common things that O. flavescens is associated with is Hantavirus Pulmonary Syndrome (HPS).  HPS is an infectious disease typically characterized by fever, myalgia and headache followed by hypotension or shock.  These viruses can be fatal.  They are mainly transmitted to humans through the inhalation of infectious rodent feces, urine, or saliva, including O. flavescens (Bayard et. al. 2004).

Uruguay is one country that has had a growing number of hantavirus cases in recent years.  38 cases have been reported since April 1997 with a fatality rate of 21% (Delfraro et. al. 2003).

Surveys have been conducted to understand the origin of HPS in Uruguay.  Rodent traps were set out in places humans occupy and where HPS has been found before, including peridomestic areas, cropland fields, and road borders.  The survey found that 85.4% of all rodents captured where found in area disturbed by humans, which include the three areas listed above.  Yellow Pygmy Rice Rats were among the most frequently captured species in the survey.  It also was the only species that showed evidence of HPS antibodies with 2.6% prevalence (Delfraro et. al. 2003).

This study was only conducted in the southern part of Uruguay where HPS has been found in humans.  Studies are being conducted to determine whether morphological differences between O. flavescens species in Uruguay and Argentina exist and that HPS antibodies are present in those identified species.

Literature Cited:

Bayard, Vincent et. al.  Sept. 2004 Outbreak of Hantavirus Pulmonary Syndrome,   Los Santos, Panama, 1999-2000.  Emerging Infectious Diseases.  Vol. 10. No. 10.  Pg. 1635-1642.

Busch, M. et. al.  2001.  Habitat Selection by Akodon azarae and Calomys laucha in Pampean Argoecosystems.  Mammalia.  Vol. 65.  Issue 1.  Pg.        29.

Busch, M. and Kravetz, F.O.  1992.  Competitive Interactions Among Rodents in a Two Habitat System:  The Effect of Species Removal.  Mammalia.  Vol.             56.  Issue 4.  Pg. 541.

Delfraro, Adriana et. al. July 2003 Yellow Pygmy Rice Rat (Oligoryzomys flavescens) and Hantavirus Pulmonary Syndrome in Uruguay.  Emerging Infectious Diseases.  Vol. 9.  No. 7. Pg. 846-852.

Dickerson, A.W.  and Yates, T.L.  Feb. 1995.  Journal of Mammalogy.  Vol. 76. Issue 1.  Pg. 172.

Ellis, Barbara A.  et. al.  1998.  Dietary Habits of the Common Rodents in an Argoecosystem in Argentina.  Journal of Mammalogy.  Vol. 79.  Issue 4.            Pg. 1203.

MacDonald, David.  2001.  The Encyclopedia of Mammals.  Andromeda Oxford Limited.  Pg. 623.

Nowak, Ronald M.  1999.  Walker’s Mammals of the World.  6th edition. Vol. 2.  The John’s Hopkins University Press- Baltimore and London.

Redford, Kent H. and Eisenberg, John F.  1992.  Mammals of the Neotropics: The Southern Cone.  The University of Chicago Press-Chicago and London.  Vol. 2.  Pg. 266.

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

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